Tutorial c#

.Net Framework and C#
By: Vikas Srivastava, Department of Computer Applications, JSSATEN 1
1. What is .NET?
 It is a platform neutral framework.
 It is a layer between the operating system and the programming language.
 It supports many programming languages, including VB.NET, C# etc.
 .NET provides a common set of class libraries, which can be accessed from any .NET
based programming language. There will not be separate set of classes and libraries for
each language. If you know any one .NET language, you can write code in any .NET
language!!
 In future versions of Windows, .NET will be freely distributed as part of operating system
and users will never have to install .NET separately.
2. What is Not?
 .NET is not an operating system.
 .NET is not a programming language.
3. ".NET is a framework"
Confused with this definition?
 We cannot define .NET as a 'single thing'.
 It is a new, easy, and extensive programming platform.
 It is not a programming language, but it supports several programming languages.
 By default .NET comes with few programming languages including C# (C Sharp),
VB.NET, J# and managed C++.
 .NET is a common platform for all the supported languages. It gives a common class
library, which can be called from any of the supported languages.
 So, developers need not learn many libraries when they switch to a different language.
Only the syntax is different for each language.
 When you write code in any language and compile, it will be converted to an
'Intermediate Language' (Microsoft Intermediate Language - MSIL).
 So, your compiled executable contains the IL and not really executable machine
language.
 When the .NET application runs, the .NET framework in the target computer take care of
the execution. (To run a .NET application, the target computer should have .NET
framework installed.)
 The .NET framework converts the calls to .NET class libraries to the corresponding APIs
of the Operating system.
 Whether you write code in C# or VB.NET, you are calling methods in the same .NET
class libraries.

 The same .NET framework executes the C# and VB.NET applications.
 So, there won't be any performance difference based on the language you write code.
3.1 Is it platform independent?
Many people ask this question "Java is platform independent, what about .NET?”.
The answer is "Yes" and "No”!
 The code you write is platform independent, because whatever you write is getting
compiled into MSIL.
 There is no native code, which depends on your operating system or CPU. But when you
execute the MSIL, the .NET framework in the target system will convert the MSIL into
native platform code.
 So, if you run your .NET exe in a Windows machine, the .NET framework for Windows
will convert it into Windows native code and execute.
 If you run your .NET application in Unix or Linux, the .NET framework for Unix/Linux will
convert your code into Unix/Linux native code and execute.
 So, your code is purely platform independent and runs anywhere!
 But wait, we said it wrong... there is no .NET framework for UNIX or Linux available now.
 Microsoft has written the .NET framework only for Windows.
 If you or some one else write a .NET framework for other platforms in future, your code
will run there too. So, let us wait until someone write .NET framework for Linux before
you run your .NET code in Linux.
3.2 Major Issues before .NET
 Registration of COM components.
 Unloading COM components
 Versioning Problem (DLL Hell)
The .NET Platform
The .Net platform is a set of technologies. Microsoft .NET platform simplify software development
(Windows or WEB) by building applications of XML Web services.
The .NET platform consists of the following core technologies which are refer as components of
.NET: -
 The .NET Framework
 The .NET Enterprise Servers
 Building block services
 Visual Studio .NET
A programming model (.NET framework) enables developers to build Extensible Markup
Language (XML) Web Services and applications.

The .NET Platform Architecture
3.3 Components of .NET Framework
The .Net Framework consists of:
 Common Language Runtime
 Class Libraries
 Support for Multiple Programming Language
Components of .NET Framework
.NET Compliant
Languages
(VC++, VB.NET,
ASP.NET, C# and
other third party
languages)
Common Language Runtime
(Memory Management, Common Type
System, Garbage Collector)
Windows Forms
Web Forms
Web Services
.NET Framework Base Class Library
(ADO.NET, XML, Threading, Diagnostics, IO,
Security, etc.)
Developer Tools.NET Framework
Application
Libraries
CLR
.NET
Enterprise
Servers
.NET
Building
Block
Services
C#, Visual Basic .NET
Visual Studio .NET
Windows OS (Win 32)

4. Application Development and Execution
.NET is a multilingual platform then any .NET based language can be chosen to develop
applications.
4.1 Choosing a Compiler
According to the language we can choose its run time aware compiler for .NET platform. Because
it is a multilingual execution environment, the runtime supports a wide variety of data types and
language features.
4.2 Compiling to MSIL
Source code to native code and code execution
When compiling source code, the compiler translates it into an intermediate code represented in
MSIL. Before code can be run, MSIL code must be converted to CPU-specific code, usually by a
just-in-time (JIT) compiler. When a compiler produces MSIL, it also produces metadata. Metadata
includes following information: -
Source Code
Compiler
Class Libraries
(IL & Metadata)
EXE/DLL (IL &
Metadata)
Class loader
Security
checks
Managed
Native Code
Execution
JIT Compiler
Runtime Engine
Call to an un-
compiled
method
Trusted pre-
JIT code only

 Description of the types in your code, including the definition of each type.
 The signatures of each type’s members,
 The members that your code references.
 Other data that the runtime uses at execution time.
The MSIL and metadata are contained in a portable executable (PE) file that is based on and
extends the published Microsoft PE and Common object file format (COFF) used historically for
executable content. The file format, which accommodates MSIL or native code as well as
metadata, enables the operating system to recognize common language runtime images.
4.3 Compiling MSIL to Native Code
Before you can run Microsoft intermediate language (MSIL), it must be compiled against the
common language runtime to native code for the target machine architecture. The .NET
Framework provides two ways to perform this conversion:
•A .NET Framework just-in-time (JIT) compiler.
•The .NET Framework Ngen.exe (Native Image Generator).
4.4 Compilation by the JIT Compiler
JIT compilation converts MSIL to native code on demand at application run time, when the
contents of an assembly are loaded and executed. Because the common language runtime
supplies a JIT compiler for each supported CPU architecture, developers can build a set of MSIL
assemblies that can be JIT-compiled and run on different computers with different machine
architectures. However, if your managed code calls platform-specific native APIs or a platform-
specific class library, it will run only on that operating system.
JIT compilation takes into account the possibility that some code might never be called during
execution. Instead of using time and memory to convert all the MSIL in a PE file to native code, it
converts the MSIL as needed during execution and stores the resulting native code in memory so
that it is accessible for subsequent calls in the context of that process. The loader creates and
attaches a stub to each method in a type when the type is loaded and initialized. When a method
is called for the first time, the stub passes control to the JIT compiler, which converts the MSIL for
that method into native code and modifies the stub to point directly to the generated native code.
Therefore, subsequent calls to the JIT-compiled method go directly to the native code.
Install-Time Code Generation Using NGen.exe
Because the JIT compiler converts an assembly's MSIL to native code when individual methods
defined in that assembly are called, it affects performance adversely at run time. In most cases,
that diminished performance is acceptable. More importantly, the code generated by the JIT
compiler is bound to the process that triggered the compilation. It cannot be shared across
multiple processes. To allow the generated code to be shared across multiple invocations of an
application or across multiple processes that share a set of assemblies, the common language
runtime supports an ahead-of-time compilation mode. This ahead-of-time compilation mode uses
the Ngen.exe (Native Image Generator) to convert MSIL assemblies to native code much like the
JIT compiler does. However, the operation of Ngen.exe differs from that of the JIT compiler in
three ways:
•It performs the conversion from MSIL to native code before running the application instead of
while the application is running.
•It compiles an entire assembly at a time, instead of one method at a time.
•It persists the generated code in the Native Image Cache as a file on disk.

4.5 Summary of Managed Code Execution Process
The process of compiling and executing managed code is given below: -
1. When you compile a program developed in a language that targets the CLR, instead of
compiling the source code into machine-level code, the compiler translates it into
Microsoft Intermediate Language (MSIL) or Intermediate language (IL). This ensures
language interoperability.
2. In addition to translating the code into IL, the compiler also produces metadata about the
program during the process of compilation. Metadata contains the description of the
program, such as classes and interfaces, the dependencies and the versions of the
components used in the program.
3. The IL and the metadata are linked in assembly.
4. The compiler creates the .EXE or .DLL file.
5. When you execute the .EXE or .DLL file, the code (converted to IL) and all the other
relevant information from the base class library is sent to the class loader. The class
loader loads the code in the memory.
6. Before the code can be executed, the .NET framework needs to convert the IL into native
or CPU-specific code. The Just-in-time (JIT) compiler translates the code from IL to
managed native code. The CLR supplies a JIT compiler for each supported CPU
architecture. During the process of compilation, the JIT compiler compiles only the code
that is required during execution instead of compiling the complete IL code. When an
uncompiled method is invoked during execution, the JIT compiler converts the IL for that
method into native code. This process saves the time and memory required to convert
the complete IL into native code.
7. During JIT compilation, the code is also checked for type safety. Type safety ensures that
objects are always accessed in a compatible way. Therefore, if you try to pass an 8-byte
value to a method that accepts a 4-byte value as a parameter, the CLR will detect and
trap such an attempt. Type safety also ensures that objects are safely isolated from each
other and are therefore safe from any malicious corruption.
8. After translating the IL into native code, the converted code is sent to the .NET runtime
manager.
9. The .NET runtime manager executes the code. While executing the code, a security
check is performed to ensure that the code has the appropriate permissions for
accessing the available resources.
Common Language Infrastructure (CLI)
The Common Language Infrastructure (CLI) is an open specification developed by Microsoft that
describes the executable code and runtime environment that allows multiple high-level languages
to be used on different computer platforms without being rewritten for specific architectures.
The common language Infrastructure (CLI) is a theoretical model of a development platform that
provides a device and language independent way to express data and behavior of applications.
The CLI specification describes the following four aspects: -
 The Common Type System (CTS)
The language interoperability and .NET Class Framework are not possible without all the
language sharing the same data type. CTS is an important part of the runtime support for cross-
language integration. The CTS performs the following functions:
 Establishes a framework that enables cross-language integration, type safety and high
performance code execution.

 Provides an object-oriented model that supports the complete implementation of many
programming languages.
The CTS supports two general categories of types: -
1. Value Types
Value types directly contain their data, and instances of value types are either allocated on the
stack or allocated inline in a structure. Value types can be built-in, user-defined or enumerations
types.
2. Reference Types
Reference types store a reference to the value’s memory address, and are allocated on the heap.
Reference types can be self-describing types, pointers type or interface types. The type of a
reference type can be determined from values of self-describing types. Self-describing types are
further split into arrays and class types are user-defined classes, boxed value types, and
delegates.
A set of data types and operations that are shared by all CTS-compliant programming
languages.
 Metadata
Information about program structure is language-agnostic, so that it can be referenced between
languages and tools, making it easy to work with code written in a language you are not using.
 Common Language Specification (CLS)
A set of base rules to which any language targeting the CLI should conform in order to
interoperate with other CLS-compliant languages. The CLS rules define a subset of the Common
Type System.
 Virtual Execution System (VES)
The VES loads and executes CLI-compatible programs, using the metadata to combine
separately generated pieces of code at runtime.
The Common Language Runtime
The CLR is one of the most essential components of the .NET framework. The CLR or the
runtime provides functionality such as exception handling, security, debugging, and versioning
support to any language that targets it. The CLR can execute programs written any language.
You can use the compilers to write the code that runs in the managed execution environment
provided by the CLR. The code that is developed with a language compiler that targets the CLR
is managed code. On the other hand, the code that is developed without considering the
conventions and requirements of the common language run time is called unmanaged code.
CLR activates objects, performs security checks, lays them out in memory, executes them and
garbage collects these objects as well.
The CLR is a runtime engine that loads required classes, performs just in time compilations, and
enforces security checks and a bunch of other runtime functions.

The CLR executables are either exe or DLL files that consist mostly of metadata and code. These
executables must adhere to a file format called the Portable Executable (PE) file format.
Features Provided by CLR
Some of the features provided by the CLR are as follows: -
 Automatic Memory Management: The CLR provides the garbage collection feature for
managing the lifetime of an object. This process relieves a programmer of the task of
manual memory management by deallocating the blocks of memory associated with
objects that are no longer being used. The objects whose lifetime is managed by the
garbage collection process are called managed data.
 Standard Type System: The CLR implements a formal specification called Common Type
System (CTS). The CTS is an important part of the support provided by the CLR for
cross-language integration because it provides a type system that is common across all
programming languages. It also defines the rules that ensure that objects written in
different languages can interact with each other.
 Language Interoperability: Language interoperability is the ability of an application to
interact with another application written in a different programming language. Language
interoperability helps maximize code reuse. For example, you can write a class in Visual
Basic and inherit it in a code written in Visual C++.
 Platform Independence: When you compile a program developed in language that targets
the CLR, the compiler translates the code into an intermediate language. This language
is CPU-independent. This means that the code can be executed from any platform that
supports the .NET CLR.
 Security Management The traditional operating system security model provides
permissions to access resources, such as memory and data, based on user accounts. In
.NET platform security is achieved through the Code Access Security (CAS) model. The
CAS model specifies what the code can access instead of specifying who can access
resources.
 Type Safety: This feature ensures that objects are always accessed in compatible ways.
Therefore the CLR will prohibit a code from assigning a 10-byte value to an object that
occupies 8 bytes.
Advantages of the .NET Framework
 Consistent programming model
 Multi-platform applications
 Multi-Language integration
 Automatic Resource Management
 Ease of deployment

OOPS & C#
The skeleton of object - oriented programming is of course the concepts of class. The C# on
OOPS explains classes and their importance in implementation of object oriented principles.
Any language can be called object oriented if it has data and method that use data
encapsulated in items named objects. An object oriented programming method has many
advantages; some of them are flexibility and code reusability.
Key Concepts of Object Orientation
 Abstraction
 Encapsulation
 Inheritance
 Polymorphism
Abstraction is the ability to generalize an object as a data type that has a specific set of
characteristics and is able to perform a set of actions.
Object-oriented languages provide abstraction via classes. Classes define the properties and
methods of an object type.
Examples:
You can create an abstraction of a dog with characteristics, such as color, height, and weight,
and actions such as run and bite. The characteristics are called properties, and the actions are
called methods.
A Recordset object is an abstract representation of a set of data.
Classes are blueprints for Object.
Objects are instance of classes.
Object References
When we work with an object we are using a reference to that object. On the other hand, when
we are working with simple data types such as Integer, we are working with the actual value
rather than a reference.
When we create a new object using the New keyword, we store a reference to that object in a
variable. For instance:
Draw MyDraw = new Draw;
This code creates a new instance of Draw. We gain access to this new object via the MyDraw
variable. This variable holds a reference to the object.
Now we have a second variable, which also has a reference to that same object. We can use
either variable interchangeably, since they both reference the exact same object. The thing we
need to remember is that the variable we have is not the object itself but, rather, is just a
reference or pointer to the object itself.
Early binding means that our code directly interacts with the object, by directly calling its
methods. Since the compiler knows the object's data type ahead of time, it can directly compile
code to invoke the methods on the object. Early binding also allows the IDE to use IntelliSense to
aid our development efforts; it allows the compiler to ensure that we are referencing methods that
do exist and that we are providing the proper parameter values.

Late binding means that our code interacts with an object dynamically at run-time. This provides
a great deal of flexibility since our code literally doesn't care what type of object it is interacting
with as long as the object supports the methods we want to call. Because the type of the object
isn't known by the IDE or compiler, neither IntelliSense nor compile-time syntax checking is
possible but we get unprecedented flexibility in exchange.
If we enable strict type checking by using Option Strict On at the top of our code modules, then
the IDE and compiler will enforce early binding behavior. By default, Option Strict is turned off and
so we have easy access to the use of late binding within our code.
Access Modifiers
Access Modifiers are keywords used to specify the declared accessibility of a member of a type.
Public is visible to everyone. A public member can be accessed using an instance of a class, by
a class's internal code, and by any descendants of a class.
Private is hidden and usable only by the class itself. No code using a class instance can access
a private member directly and neither can a descendant class.
Protected members are similar to private ones in that they are accessible only by the containing
class. However, protected members also may be used by a descendant class. So members that
are likely to be needed by a descendant class should be marked protected.

Internal/Friend is public to the entire application but private to any outside applications. Internal
is useful when you want to allow a class to be used by other applications but reserve special
functionality for the application that contains the class. Internal is used by C# and Friend by VB
.NET.
Protected Internal may be accessed only by a descendant class that's contained in the same
application as its base class. You use protected internal in situations where you want to deny
access to parts of a class functionality to any descendant classes found in other applications.

Composition of an OBJECT
We use an interface to get access to an object's data and behavior. The object's data and
behaviors are contained within the object, so a client application can treat the object like a black
box accessible only through its interface. This is a key object-oriented concept called
Encapsulation. The idea is that any programs that make use of this object won't have direct
access to the behaviors or data-but rather those programs must make use of our object's
interface.
There are three main parts of Object:
1. Interface
2. Implementation or Behavior
3. Member or Instance variables
Interface
The interface is defined as a set of methods (Sub and Function routines), properties (Property
routines), events, and fields (variables or attributes) that are declared Public in scope.
Implementation or Behavior
The code inside of a method is called the implementation. Sometimes it is also called behavior
since it is this code that actually makes the object do useful work.
Client applications can use our object even if we change the implementation-as long as we don't
change the interface. As long as our method name and its parameter list and return data type
remain unchanged, we can change the implementation all we want.
So Method Signature depends on:
 Method name

 Data types of parameters
 Either Parameter is passed ByVal or ByRef.
 Return type of method
It is important to keep in mind that encapsulation is a syntactic tool-it allows our code to continue
to run without change. However, it is not semantic-meaning that, just because our code continues
to run, doesn't mean it continues to do what we actually wanted it to do.
Member or Instance Variables
The third key part of an object is its data, or state. Every instance of a class is absolutely identical
in terms of its interface and its implementation-the only thing that can vary at all is the data
contained within that particular object.
Member variables are those declared so that they are available to all code within our class.
Typically member variables are Private in scope-available only to the code in our class itself.
They are also sometimes referred to as instance variables or as attributes. The .NET Framework
also refers to them as fields.
We shouldn't confuse instance variables with properties. A Property is a type of method that is
geared around retrieving and setting values, while an instance variable is a variable within the
class that may hold the value exposed by a Property.
Interface looks like a class, but has no implementation.
The only thing it contains is definitions of events, indexers, methods and/or properties. The
reason interfaces only provide definitions is because they are inherited by classes and structs,
which must provide an implementation for each interface member defined.
Defining an Interface: MyInterface.cs
interface IMyInterface
{
void MethodToImplement();
}
Above listing shows defines an interface named IMyInterface.
All the methods of Interface are public by default and no access modifiers (like private, public) are
allowed with any method of Interface.
Using an Interface: InterfaceImplementer.cs
class InterfaceImplementer : IMyInterface
{
public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}
The InterfaceImplementer class in above listing implements the IMyInterface interface. Indicating
that a class inherits an interface is the same as inheriting a class. In this case, the following
syntax is used:
class InterfaceImplementer : IMyInterface

Note that this class inherits the IMyInterface interface; it must implement its all members. While
implementing interface methods all those needs to be declared public only. It does this by
implementing the MethodToImplement() method. Notice that this method implementation has the
exact same signature, parameters and method name, as defined in the IMyInterface interface.
Any difference will cause a compiler error.
Inheritance is the idea that one class, called a subclass, can be based on another class,
called a base class. Inheritance provides a mechanism for creating hierarchies of objects.
Inheritance is an important object-oriented concept. It allows you to build a hierarchy of related
classes, and to reuse functionality defined in existing classes.
Inheritance is the ability to apply another class's interface and code to your own class.
Normal base classes may be instantiated themselves, or inherited. Derived classes can inherit
base class members marked with protected or greater access. The derived class is specialized to
provide more functionality, in addition to what its base class provides. Inheriting base class
members in derived class is not mandatory.
C# supports two types of Inheritance mechanisms: -
1) Implementation Inheritance
2) Interface Inheritance
What is Implementation Inheritance?
- When a class (type) is derived from another class(type) such that it inherits all the members of
the base type it is Implementation Inheritance
What is Interface Inheritance?
- When a type (class or a struct) inherits only the signatures of the functions from another type it
is Interface Inheritance
In general, Classes can be derived from another class, hence support Implementation inheritance
At the same time Classes can also be derived from one or more interfaces Hence they support
Interface inheritance Structs can derive from one more interface, hence support Interface
Inheritance Structs cannot be derived from another class they are always derived from
SystemValueType
Types of Inheritance
1. Single Inheritance
2. Multilevel Inheritance
3. Multiple Inheritance (Implementation is possible through Interface)
4. Hierarchical Inheritance

Example: -
Single Inheritance: - Multilevel Inheritance: - Hierarchical Inheritance: -
public class A public class A public class A
{ } { } { }
public class B : A public class B : A public class B : A
{ } { } { }
public class C : B public class C : A
Multiple Inheritance: - { } { }
public class A public class D : A
{ } { }
public class B
{ }
public class C : A, B
{ }
Polymorphism
Polymorphism is the ability to define a method or property in a set of derived classes with
matching method signatures but provide different implementations and then distinguish the
objects' matching interface from one another at runtime when you call the method on the base
class.
It is a feature to use one name in many forms. It can be achieved in following ways: -
 Method Overloading
 Method Overriding
 Method Hiding
Class B
Class B
Single Inheritance
Class A
Class DClass C
Class A
Hierarchical Inheritance
Class A
Class B
Class C
Class A Class B
Class C
Multiple InheritanceMultilevel Inheritance

Method overriding and hiding makes use of the following three method keywords –
1. new
2. virtual
3. override
1. When a derived class inherits from a base class, it gains all the methods, fields, properties and
events of the base class. To change the data and behavior of a base class, you have two
choices: you can replace the base member with a new derived member, or you can override a
virtual base member.
Replacing a member of a base class with a new derived member requires the new keyword. If a
base class defines a method, field, or property, the new keyword is used to create a new
definition of that method, field, or property on a derived class. The new keyword is placed before
the return type of a class member that is being replaced. For example:
public class BaseClass
{
public void DoWork() { }
public int WorkField;
public int WorkProperty
{
get { return 0; }
}
}
public class DerivedClass : BaseClass
{
public new void DoWork() { }
public new int WorkField;
public new int WorkProperty
{
get { return 0; }
}
}
DerivedClass B = new DerivedClass();
B.DoWork(); // Calls the new method.
BaseClass A = (BaseClass)B;
A.DoWork(); // Calls the old method.
2,3. In order for an instance of a derived class to completely take over a class member from a
base class, the base class has to declare that member as virtual. This is accomplished by adding
the virtual keyword before the return type of the member. A derived class then has the option of
using the override keyword, instead of new, to replace the base class implementation with its
own. For example:
public class BaseClass
{
public virtual void DoWork() { }
public virtual int WorkProperty
{
get { return 0; }
}
}

public class DerivedClass : BaseClass
{
public override void DoWork() { }
public override int WorkProperty
{
get { return 0; }
}
}
DerivedClass B = new DerivedClass();
B.DoWork(); // Calls the new method.
BaseClass A = (BaseClass)B;
A.DoWork(); // Also calls the new method.
Remarks about Virtual
 When a virtual method is invoked, the run-time type of the object is checked for an
overriding member. The overriding member in the most derived class is called, which
might be the original member, if no derived class has overridden the member.
 By default, methods are non-virtual. You cannot override a non-virtual method.
 You cannot use the virtual modifier with the static, abstract, private or override modifiers.
 Virtual properties behave like abstract methods, except for the differences in declaration
and invocation syntax.
 It is an error to use the virtual modifier on a static property.
 A virtual inherited property can be overridden in a derived class by including a property
declaration that uses the override modifier.
Remarks about Override
 The override modifier is required to extend or modify the abstract or virtual
implementation of an inherited method, property, indexer, or event.
 An override method provides a new implementation of a member inherited from a base
class. The method overridden by an override declaration is known as the overridden base
method. The overridden base method must have the same signature as the override
method.
 You cannot override a non-virtual or static method. The overridden base method must be
virtual, abstract, or override.
 An override declaration cannot change the accessibility of the virtual method. Both the
override method and the virtual method must have the same access level modifier.
 You cannot use the modifiers new, static, virtual, or abstract to modify an override
method.
 An overriding property declaration must specify the exact same access modifier, type,
and name as the inherited property, and the overridden property must be virtual, abstract,
or override.

Class and Objects
Classes
A class is a construct that enables you to create your own custom types by grouping together
variables of other types, methods and events. A class is like a blueprint. It defines the data and
behavior of a type. If the class is not declared as static, client code can use it by creating objects
or instances which are assigned to a variable. The variable remains in memory until all references
to it go out of scope. At that time, the CLR marks it as eligible for garbage collection. If the class
is declared as static, then only one copy exists in memory and client code can only access it
through the class itself, not an instance variable.
Declaring Class
public class Customer
{
//Fields, properties, methods and events go here...
}
The class keyword is preceded by the access level. Because public is used in this case, anyone
can create objects from this class. The name of the class follows the class keyword.
Objects
An object is basically a block of memory that has been allocated and configured according to the
blueprint. A program may create many objects of the same class. Objects are also called
instances, and they can be stored in either a named variable or in an array or collection.
Creating Objects
A class and an object are different things. A class defines a type of object, but it is not an object
itself. An object is a concrete entity based on a class, and is sometimes referred to as an instance
of a class.
Objects can be created by using the new keyword followed by the name of the class that the
object will be based on, like this:
Customer object1 = new Customer();
When an instance of a class is created, a reference to the object is passed back to the
programmer. In the previous example, object1 is a reference to an object that is based on
Customer.
Class Modifiers
A class-declaration can optionally include a sequence of class modifiers:
class-modifiers:
class-modifier
class-modifiers class-modifier
class-modifier:
new, public, protected, internal, private, abstract, sealed

The new modifier is permitted on nested classes. The new modifier can be used to modify a
nested type if the nested type is hiding another type.
The public, protected, internal, and private modifiers control the accessibility of the class.
Depending on the context in which the class declaration occurs, some of these modifiers may not
be permitted
The abstract modifier is used to indicate that a class is incomplete and that it is intended to be
used only as a base class. An abstract class differs from a non-abstract class in the following
ways:
 An abstract class cannot be instantiated directly, and it is a compile-time error to use the
new operator on an abstract class. While it is possible to have variables and values
whose compile-time types are abstract, such variables and values will necessarily either
be null or contain references to instances of non-abstract classes derived from the
abstract types.
 An abstract class is permitted (but not required) to contain abstract methods and
members.
 An abstract class cannot be sealed.
Features of Abstract Methods:
 An abstract method is implicitly a virtual method.
 Abstract method declarations are only permitted in abstract classes.
 Because an abstract method declaration provides no actual implementation, there is no
method body; the method declaration simply ends with a semicolon and there are no
braces ({ }) following the signature. For example:
Copypublic abstract void MyMethod();
 The implementation is provided by an overriding method, which is a member of a non-
abstract class.
 It is an error to use the static or virtual modifiers in an abstract method declaration.
Abstract properties behave like abstract methods, except for the differences in declaration and
invocation syntax.
 It is an error to use the abstract modifier on a static property.
 An abstract inherited property can be overridden in a derived class by including a
property declaration that uses the override modifier.
An abstract class must provide implementation for all interface members.
Example: -
// abstract_keyword.cs
// Abstract Classes
using System;
abstract class MyBaseC // Abstract class
{
protected int x = 100;
protected int y = 150;
public abstract void MyMethod(); // Abstract method

public abstract int GetX // Abstract property
{
get;
}
public abstract int GetY // Abstract property
{
get;
}
}
class MyDerivedC: MyBaseC
{
public override void MyMethod()
{
x++;
y++;
}
public override int GetX // overriding property
{
get
{
return x+10;
}
}
public override int GetY // overriding property
{
get
{
return y+10;
}
}
public static void Main()
{
MyDerivedC mC = new MyDerivedC();
mC.MyMethod();
Console.WriteLine("x = {0}, y = {1}", mC.GetX, mC.GetY);
}
}
The sealed modifier is used to prevent derivation from a class. A compile-time error occurs if a
sealed class is specified as the base class of another class.
A sealed class cannot also be an abstract class.
The sealed modifier is primarily used to prevent unintended derivation, but it also enables certain
run-time optimizations. In particular, because a sealed class is known to never have any derived
classes, it is possible to transform virtual function member invocations on sealed class instances
into non-virtual invocations.
Example: use of Sealed modifier
// cs_sealed_keyword.cs

// Sealed classes
using System;
sealed class MyClass
{
public int x;
public int y;
}
class MainClass
{
{
MyClass mC = new MyClass();
mC.x = 110;
mC.y = 150;
Console.WriteLine("x = {0}, y = {1}", mC.x, mC.y);
}
}
Output: x=110, y=150
Constructors
Whenever a class or struct is created, its constructor is called. A class or struct may have multiple
constructors that take different arguments.
Constructors allow the programmer to set default values, limit instantiation, and write code that is
flexible and easy to read.
 Constructor is used to initialize an object (instance) of a class.
 Constructor is a like a method without any return type.
 Constructor has same name as class name.
 Constructor follows the access scope (Can be private, protected, public, Internal and
external).
 Constructor can be overloaded.
Constructors generally following types:
 Default Constructor
 Parameterized constructor
 Private Constructor
 Static Constructor
 Copy Constructor
Default Constructor
A constructor that takes no parameters is called a default constructor.
When a class is initiated default constructor is called which provides default values to different
data members of the class.
You need not to define default constructor it is implicitly defined.

Example: -
class Program
{
class C1
{
int a, b;
public C1()
{
this.a = 10;
this.b = 20;
}
public void display()
{
Console.WriteLine("Value of a: {0}", a);
Console.WriteLine("Value of b: {0}", b);
}
}
static void Main(string[] args)
{
C1 ob1 = new C1();
ob1.display();
Console.ReadLine();
}
}
Output: - Value of a: 10
Value of b: 20
Parameterized constructor
Constructor that accepts arguments is known as parameterized constructor. There may be
situations, where it is necessary to initialize various data members of different objects with
different values when they are created. Parameterized constructors help in doing that task.
class Program
{
class C1
{
int a, b;
public C1(int x, int y)
{
this.a = x;
this.b = y;
}
{

}
}
{ // Here when you create instance of the class
// parameterized constructor will be called
C1 ob1 = new C1(10,20);
ob1.display();
Console.ReadLine();
}
}
Output: - Value of a: 10
Value of b: 20
Private Constructor
Private constructors are used to restrict the instantiation of object using 'new' operator. A private
constructor is a special instance constructor. It is commonly used in classes that contain static
members only.
 If you don't want the class to be inherited we declare its constructor private.
 We can't initialize the class outside the class or the instance of class can't be created
outside if its constructor is declared private.
 We have to take help of nested class (Inner Class) or static method to initialize a class
having private constructor.
Example: -
class Program
{
class C1
{
int a, b;
public C1(int x, int y)
{
this.a = x;
this.b = y;
}
public static C1 create_instance()
{ return new C1(12, 20); }
{
int z = a + b;
Console.WriteLine(z);
}
}
{ // Here the class is initiated using a static method of the
class than only you can use private constructor

C1 ob1 = C1.create_instance();
ob1.display();
Console.ReadLine();
}
}
Static Constructors
C# supports two types of constructor, a class constructor static constructor and an instance
constructor (non-static constructor).
Static constructors might be convenient, but they are slow. The runtime is not smart enough to
optimize them in the same way it can optimize inline assignments. Non-static constructors are
inline and are faster.
Static constructors are used to initializing class static data members.
Point to be remembered while creating static constructor:
1. There can be only one static constructor in the class.
2. The static constructor should be without parameters.
3. It can only access the static members of the class.
4. There should be no access modifier in static constructor definition.
Static members are preloaded in the memory. While instance members are post loaded into
memory.
Static methods can only use static data members.
Example:
class Program
{
public class test
{
static string name;
static int age;
static test()
{
Console.WriteLine("Using static constructor to initialize
static data members");
name = "John Sena";
age = 23;
}
public static void display()
{
Console.WriteLine("Using static function");
Console.WriteLine(name);
Console.WriteLine(age);
}
}
{
test.display();
Console.ReadLine();
}}

Output:
Using static constructor to initialize static data members
Using static function
John Sena
23
Copy Constructor
If you create a new object and want to copy the values from an existing object, you use copy
constructor.
This constructor takes a single argument: a reference to the object to be copied.
Example:
class Program
{
class c1
{
int a, b;
public c1(int x, int y)
{
this.a = x;
this.b = y;
}
// Copy construtor
public c1(c1 a)
{
this.a = a.a;
this.b = a.b;
}
{
int z = a + b;
Console.WriteLine(z);
}
}
{
c1 ob1 = new c1(10, 20);
ob1.display();
// Here we are using copy constructor. Copy
constructor is
using the values already defined with ob1
c1 ob2 = new c1(ob1);
ob2.display();
Console.ReadLine();
}
}
Output:
30
30

Destructors
The .NET framework has an in built mechanism called Garbage Collection to de-allocate memory
occupied by the un-used objects. The destructor implements the statements to be executed
during the garbage collection process. A destructor is a function with the same name as the name
of the class but starting with the character ~.
Example:
class Complex
{
public Complex()
{
// constructor
}
~Complex()
{
// Destructor
}
}
 Remember that a destructor can't have any modifiers like private, public etc. If we declare
a destructor with a modifier, the compiler will show an error.
 Also destructor will come in only one form, without any arguments.
 There is no parameterized destructor in C#.
Destructors are invoked automatically and can't be invoked explicitly. An object becomes eligible
for garbage collection, when it is no longer used by the active part of the program. Execution of
destructor may occur at any time after the instance or object becomes eligible for destruction.
Operator Overloading
Operator overloading permits user-defined operator implementations to be specified for
operations where one or both of the operands are of a user-defined class or struct type.
In another way, Operator overloading is a concept in which operator can define to work with the
user defined data types such as structs and classes in the same way as the pre-defined data
types.
There are many operators which can not be overloaded, which are listed below: -
Conditional Operator &&, ||
Compound Assignment +=, -=, *=, /=, %=
Other Operators [], ( ), =, ?:, ->, new, sizeof, typesof.

public class Item
{
public int i;
public Item(int j)
{ i = j; }
public static Item operator +(Item x, Item y)
{
Console.WriteLine("OPerator +" + x.i + "" + y.i);
Item z = new Item(x.i + y.i);
return z;
}
}
class Program
{
{
Item a = new Item(10);
Item b = new Item(5);
Item c;
c = a + b;
Console.WriteLine(c.i);
Console.Read();
}
}
Output: Operator + 10 5
15
 In C#, a special function called operator function is used for overloading purpose.
 These special function or method must be public and static.
 They can take only value arguments.
 The ref and out parameters are not allowed as arguments to operator functions.
The general form of an operator function is as follows.
public static return_type operator op (argument list)
Where the op is the operator to be overloaded and operator is the required keyword.
Example: Overloading of Unary operator
class Complex
{
private int x;
private int y;
public Complex()
{}
public Complex(int i, int j)
{

x = i;
y = j;
}
public void ShowXY()
{
Console.WriteLine("{0}t{1}",x,y);
}
public static Complex operator -(Complex c) //PASSING OBJECT
{
Complex temp = new Complex();
temp.x = -c.x;
temp.y = -c.y;
return temp;
}
}
class Program
{
{
Complex c1 = new Complex(10, 20);
c1.ShowXY(); // displays 10 & 20
Complex c2 = new Complex();
c2.ShowXY(); // displays 0 & 0
c2 = -c1; //overloading unary operator
c2.ShowXY(); // diapls -10 & -20
Console.Read();
}
}
Output:
10 20
0 0
-10 -20
Namespace:
Namespaces are C# program elements designed to help you organize your programs. They also
provide assistance in avoiding name clashes between two sets of code.
In Microsoft .Net, Namespace is like containers of objects. They may contain unions, classes,
structures, interfaces, enumerators and delegates. Main goal of using namespace in .Net is for
creating a hierarchical organization of program. In this case, you need not to worry about the
naming conflicts of classes, functions, variables etc., inside a project.
In Microsoft .Net, every program is created with a default namespace. This default namespace is
called as global namespace. But the program itself can declare any number of namespaces, each
of them with a unique name. The advantage is that every namespace can contain any number of
classes, functions, variables and also namespaces etc., whose names are unique only inside the

namespace. The members with the same name can be created in some other namespace
without any compiler complaints from Microsoft .Net.
To declare namespace C# .Net has a reserved keyword namespace. If a new project is created in
Visual Studio .NET it automatically adds some global namespaces. These namespaces can be
different in different projects. But each of them should be placed under the base namespace
System. The names space must be added and used through the using operator, if used in a
different project.
A namespace has the following properties:
 They organize large code projects.
 They are delimited with the . operator.
 The using directive means you do not need to specify the name of the namespace for
every class.
 The global namespace is the "root" namespace: global::system will always refer to the
.NET Framework namespace System.
Now have a look at the example of declaring some namespace:
namespace SampleNamespace
{
class SampleClass{}
interface SampleInterface{}
struct SampleStruct{}
enum SampleEnum{a,b}
delegate void SampleDelegate(int i);
namespace SampleNamespace.Nested
{
class SampleClass2{}
}
}
Within a namespace, you can declare one or more of the following types:
 another namespace
 class
 interface
 struct
 enum
 delegate
Namespaces implicitly have public access and this is not modifiable.
It is possible to define a namespace in two or more declarations. For example, the following
example defines two classes as part of the MyCompany namespace:
namespace MyCompany.Proj1
{
class MyClass
{
}
}

namespace MyCompany.Proj1
{
class MyClass1
{
}
}
Example: The following example shows how to call a static method in a nested
namespace:
using System;
namespace SomeNameSpace
{
public class MyClass
{
static void Main()
{
Nested.NestedNameSpaceClass.SayHello();
}
}
// a nested namespace
namespace Nested
{
public class NestedNameSpaceClass
{
public static void SayHello()
{
Console.WriteLine("Hello");
}
}
}
}
Output
Hello
Example: Calling Nested Namespace Members
// Namespace Declaration
using System;
namespace Ex_nestedNamespace
{
namespace tutorial
{
class example
{
public static void MyPrint1()
{
Console.WriteLine("First Example of calling another namespace member.");
}
}
}

namespace Ex_NameSpace
{
class Program
{
{
tutorial.example.MyPrint1();
tutorial.example1.MyPrint2();
Console.Read();
}
}
}
}
namespace Ex_nestedNamespace.tutorial
{
class example1
{
public static void MyPrint2()
{
Console.WriteLine("Second Example of calling another namespace member.");
}
}
}
Output:
First Example of calling another namespace member.
Second Example of calling another namespace member.
Interface
An Interface is a reference type and it contains only abstract members. Interface's members can
be Events, Methods, Properties and Indexers. But the interface contains only declaration for its
members. Any implementation must be placed in class that realizes them. The interface can not
contain constants, data fields, constructors, destructors and static members. All the member
declarations inside interface are implicitly public and they cannot include any access modifiers.
An interface has the following properties:
 An interface is like an abstract base class: any non-abstract type that implements the
interface must implement all its members.
 An interface cannot be instantiated directly.
 Interfaces can contain events, indexers, methods, and properties.
 Interfaces contain no implementation of methods.
 Classes and structs can implement more than one interface.
 An interface itself can inherit from multiple interfaces.
interface IPoint
{

int x
{ get; set; }
int y
{ get; set; }
}
namespace Ex_Interface
{
class MyPoint:IPoint
{
private int myX;
private int myY;
public MyPoint(int x, int y)
{
myX= x;
myY=y;
}
public int x
{
get
{
return myX;
}
set
{
myX=value;
}
}
public int y
{
get
{
return myY;
}
set
{
myY=value;
}
}
}
class Program
{
private static void PrintPoint(IPoint P)
{
Console.WriteLine("x={0}, y={1}",P.x,P.y);
}
{
MyPoint P = new MyPoint(2, 3);
Console.Write("My Point::");
PrintPoint(P);
Console.Read();
}
}

}
Output:
My Point::x=2, y=3
Another Example of Interface by Casting Interface methods:
interface add
{ int sum();}
interface Multiply
{ int mul();}
class Calculate : add, Multiply
{
int a, b;
public Calculate(int x, int y)
{
a = x;
b = y;
}
public int sum()
{ return (a + b);}
public int mul()
{ return a * b; }
}
namespace Ex_MultipleInterface
{
class Program
{
{
Calculate cal = new Calculate(5, 10);
add A = (add)cal;
Console.WriteLine("Sum::" + A.sum());
Multiply M = (Multiply)cal;
Console.WriteLine("Multiplication::" + M.mul());
Console.Read();
}
}
}
Output:
Sum::15
Multiplication::50

Delegates
In .NET, you use delegates to call event procedure. Delegates are objects that you use to call the
methods of other objects. Delegates are said to be object-oriented function pointers since they
allow a function to be invoked indirectly by using a reference to the function.
However, unlike function pointers, the delegates in .NET are reference types, based on the class
System.Delegate. In addition, delegates in .NET can reference both shared and instance
methods.
In another way, a delegate can be defined as a type safe function pointer. You use delegates
to call the methods of other objects. They are object-oriented function pointers since they allow a
function to be invoked indirectly by using a reference to the function.
Where are Delegates used?
The most common example of using delegates is in events.
You define a method that contains code for performing various tasks when an event (such as a
mouse click) takes place.
This method needs to be invoked by the runtime when the event occurs. Hence this method, that
you defined, is passed as a parameter to a delegate.
Starting Threads/Parallel Processing:
You defined several methods and you wish to execute them simultaneously and in parallel to
whatever else the application is doing. This can be achieved by starting new threads. To start a
new thread for your method you pass your method details to a delegate.
Generic Classes: Delegates are also used for generic class libraries which have generic
functionality defined. However the generic class may need to call certain functions defined by the
end user implementing the generic class. This can be done by passing the user defined functions
to delegates.
Creating and Using Delegates:
Using delegates is a two step process-
..........1) Define the delegate to be used
..........2) Create one or more instances of the delegate
Syntax for defining a delegate:
delegate string reviewStatusofARegion();
to define a delegate we use a key word delegate followed by the method signature the delegate
represents. In the above example string reviewStatusofARegion(); represents any method that
returns a string and takes no parameters.
Syntax for creating an instance of the delegate:

reviewStatusofARegion = new reviewStatusofARegion(myClass.getEurope);
private string getEurope()
{
return “Doing Great in Europe”;
}
To create an instance of the delegate you call its constructor. The delegate constructor takes one
parameter which is the method name.
The method signature should exactly match the original definition of the delegate. If it does not
match the compiler would raise an Error.
C# provides support for Delegates through the class called Delegate in the System
namespace. Delegates are of two types.
 Single-cast delegates
 Multi-cast delegates
A Single-cast delegate is one that can refer to a single method whereas a Multi-cast
delegate can refer to and eventually fire off multiple methods that have the same signature.
The signature of a delegate type comprises are the following.
 The name of the delegate
 The arguments that the delegate would accept as parameters
 The return type of the delegate
A delegate is either public or internal if no specifier is included in its signature. Further, you
should instantiate a delegate prior to using the same.
The following is an example of how a delegate is declared.
Listing 1: Declaring a delegate
public delegate void TestDelegate(string message);
The return type of the delegate shown in the above example is "void" and it accepts a string
argument. Note that the keyword "delegate" identifies the above declaration as a delegate to a
method. This delegate can refer to and eventually invoke a method that can accept a string
argument and has a return type of void, i.e., it does not return any value.
Listing 2: Instantiating a delegate
TestDelegate t = new TestDelegate(Display);
Implementing Delegates in C#
This section illustrates how we can implement and use delegates in C#.This section illustrate how
we can implement and use delegates in C#.

Example 1: Single Cast Delegate
namespace Ex_Delegate
{
delegate int Operation(int x, int y); //declaration
class Metaphor
{
public static int Add(int a, int b)
{ return a + b; }
public static int Sub(int a, int b)
{ return a - b; }
public static int Mul(int a, int b)
{ return a * b; }
}
class Program
{
{ // Delegate instances
Operation opr1 = new Operation(Metaphor.Add);
Operation opr2 = new Operation(Metaphor.Sub);
Operation opr3 = new Operation(Metaphor.Mul);
//invoking of delegates
int ans1 = opr1(200, 100);
int ans2 = opr2(200, 100);
int ans3 = opr3(20, 10);
Console.WriteLine("n Addition:" + ans1);
Console.WriteLine("n Subtract:" + ans2);
Console.WriteLine("n Multiplication:" + ans3);
Console.Read();
} }}
Example 2: Single Cast Delegate
namespace Ex_SingleCastDelegate
{ //Declare the delegate
public delegate void TestDelegate(string message);
class Program
{
public static void Display(string message)
{Console.WriteLine("The string entered is : " + message);}
{ //Initiate the delegate
TestDelegate t = new TestDelegate(Display);
Console.WriteLine("Please enter a string::");
string message = Console.ReadLine();
t(message);
Console.ReadLine();
}}}
Multicast Delegate
A multi-cast delegate is basically a list of delegates or a list of methods with the same signature.
A multi-cast delegate can call a collection of methods instead of only a single method.

Example: Multicast Delegate
namespace Ex_MulticastDelegate
{
public delegate void TestDelegate();
class Program
{
public static void Display1()
{
Console.WriteLine("This is first method");
}
public static void Display2()
{
Console.WriteLine("This is second method");
}
{
TestDelegate t1 = new TestDelegate(Display1);
TestDelegate t2 = new TestDelegate(Display2);
t1 = t1 + t2; // Make t1 a multi-cast delegate
t1(); //Invoke delegate
Console.Read();
} } }
In another way, You can also assign the references of multiple methods to a delegate and use it
to invoke multiple methods. Such a delegate is called a multi-cast delegate as multiple method
references are cast to it and then the delegate is used to invoke these methods.
What are Attributes?
An Attribute is a declarative tag which can be used to provide information to the compiler about
the behaviour of the C# elements such as classes and assemblies.
C# provides convenient technique that will handle tasks such as performing compile time
operations , changing the behaviour of a method at runtime or maybe even handle unmanaged
code.
C# Provides many Built-in Attributes.
Some Popular ones are
 Obsolete
 DllImport
 Conditional
 WebMethod
It is also possible to create new ones by extending the System.Attribute class.
For example:
using System;
[CLSCompliant(true)]
Public class myClass
{ // class code }
Web services also make use of attributes. The attribute [WebMethod] is used to specify that a
particular method is to be exposed as a web service.

Why Attributes ?
The reason attributes are necessary is because many of the services they provide would be very
difficult to accomplish with normal code. You see, attributes add what is called metadata to your
programs. When your C# program is compiled, it creates a file called an assembly, which is
normally an executable or DLL library. Assemblies are self-describing because they have
metadata written to them when they are compiled. Via a process known as reflection, a program's
attributes can be retrieved from its assembly metadata. Attributes are classes that can be written
in C# and used to decorate your code with declarative information. This is a very powerful
concept because it means that you can extend your language by creating customized declarative
syntax with attributes.
How it is used in C#?
Attributes are elements that allow you to add declarative information to your programs. This
declarative information is used for various purposes during runtime and can be used at design
time by application development tools. For example, there are attributes such as
DllImportAttribute that allow a program to communicate with the Win32 libraries. Another
attribute, ObsoleteAttribute, causes a compile-time warning to appear, letting the developer know
that a method should no longer be used. When building Windows forms applications, there are
several attributes that allow visual components to be drag-n-dropped onto a visual form builder
and have their information appear in the properties grid. Attributes are also used extensively in
securing .NET assemblies, forcing calling code to be evaluated against pre-defined security
constraints. These are just a few descriptions of how attributes are used in C# programs.
Predefined .NET
Attribute
Valid Targets Description
AttributeUsage Class Specifies the valid usage of another attribute
class.
CLSCompliant All Indicates whether a program element is
compliant with the Common Language
Specification (CLS).
DllImport Method Specifies the DLL location that contains the
implementation of an external method.
MTAThread Method (Main) Indicates that the default threading model for an
application is multithreaded apartment (MTA).
NonSerialized Field Applies to fields of a class flagged as
Serializable; specifies that these fields won’t be
serialized.
Obsolete All except Assembly, Module,
Parameter, and Return
Marks an element obsolete—in other words, it
informs the user that the element will be
removed in future versions of the product.
ParamArray Parameter Allows a single parameter to be implicitly treated
as a params (array) parameter.
Serializable Class, struct, enum, delegate Specifies that all public and private fields of this
type can be serialized.
STAThread Method (Main) Indicates that the default threading model for an
application is STA.
ThreadStatic Field (static) Implements thread-local storage (TLS)—in other
words, the given static field isn’t shared across
multiple threads and each thread has its own
copy of the static field.

Predefined attributes
Example:
Pre-defined attributes are used to store external information into metadata. For example, consider
the following piece of code:
public class testAttribute {
[DllImport("sampleDLL.dll")]
public static extern sampleFunction(int sampleNo, string sampleString );
public static void Main( ) {
string strVar;
sampleFunction(10, “Test Attribute”);
}
}
Using the example code above, you can import a method called sampleFunction from
sampleDLL.dll and use it in your program as if it’s your own method. This is achieved using the
pre-defined attribute “DllImport”.

Multi-Threading
Multithreading forms a subset of multitasking. Instead of having switch between programs this
feature switches between different parts of the same program. For example when you are writing
words in Ms-word then spell checking is going on background.
Thread - A thread (or "thread of execution") is a sort of context in which code is running. Any one
thread follows program flow for wherever it is in the code, in the obvious way.
A thread is a unit of processing, and multitasking is the simultaneous execution of multiple
threads. Multitasking comes in two flavors: cooperative and preemptive. Very early versions of
Microsoft Windows supported cooperative multitasking, which meant that each thread was
responsible for relinquishing control to the processor so that it could process other threads.
However, Microsoft Windows NT-and, later, Windows 95, Windows 98, and Windows 2000-
support the same preemptive multitasking that OS/2 does. With preemptive multitasking, the
processor is responsible for giving each thread a certain amount of time in which to execute-a
timeslice. The processor then switches among the different threads, giving each its timeslice, and
the programmer doesn't have to worry about how and when to relinquish control so that other
threads can run. .NET will only work only on preemptive multitasking operating systems.
1. Starting Thread
Object thread is obtained from System.Threading namespace. With the use object of this class
we can create a new thread, delete, pause, and resume threads. Simple a new thread is created
by Thread class and started by Thread.Start().
eg. Thread th = new Thread (new ThreadStart (somedata));
th.Start();
2. Pausing Thread
Some time the requirement to pause a thread for certain time of interval; you can attain the same
by using Sleep (n) method. This method takes an integer value to determine how long a thread
should pause or Sleep.
eg. th.Sleep(2000);
Note:
To pause or sleep a thread for an in determine time, just call the sleep () method as: [make sure
you have added System.Threading namespace] Thread.Sleep(TimeOut.Infinite).
To Resume or interrupt this call : Thread.Interrupt () method.
3. Suspending Thread
Of course, there is a Suspend () method which suspends the thread. It is suspended until a
Resume () method called.
eg. if (th.ThreadState = = ThreadState.Running)
th.Suspended();
4. Resuming Thread
To Resume a suspended thread, there is a Resume () method, thread resumes if earlier
suspended if not so then there is no effect of Resume () method on the thread.
eg. if (th.ThreadState = = ThreadState.Suspended)
th.Resume();

5. Killing Thread
You can call Abort () method to kill a thread, before calling the same method, make sure thread is
alive.
eg. if (th.IsAlive)
th.Abort();
Suspend and Resume in Threading
It is similar to sleep and Interrupt. Suspend allows you to block a thread until another thread calls
Thread.Resume ().The difference between sleep and suspend is that the later does no
immediately place a thread in the wait state. The thread does not suspend until the .Net runtime
determines that it is in a safe place to suspend it. Sleep will immediately place a thread in a wait
state.
Important:
You can change thread priority for that just supply : th.Priority = ThreadPriority.Highest. [th –
Thread name]. Priority sets the sequence of thread in which they are running. You can set the
following priority to thread(s):
1. ThreadPriority.Highest
2. ThreadPriority.AboveNormal
3. ThreadPriority.Normal
4. ThreadPriority.BelowNormal
5. ThreadPriority.Lowest
Code Example of Multithreading
using System.Threading;
namespace Ex_ThreadExample
{
class SimpleThread
{
private Thread thread1;
private Thread thread2;
private void Method1()
{
for (int i =0; i<10;i++)
{
Console.WriteLine ("i = " +i);
Thread.Sleep (400); // 200 miliseconds pause
}
}
private void Method2()
{
for (int i =0;i<10;i++)
{
Console.WriteLine ("i = " + 100 * i);
Thread.Sleep (100); // 100 miliseconds pause
}
}
{
SimpleThread app = new SimpleThread ();

app.thread1 = new Thread (new ThreadStart (app.Method1));
// thread start Delegate Method
app.thread2 = new Thread (new ThreadStart (app.Method2));
app.thread1.Start ();
app.thread2.Start ();
Console.WriteLine ();
Console.ReadLine();
}
}
}
-----------x----------------x--------------------------x-------------------------x----------------x---------------x----------
Exception Handling
Overview of Exception Handling
Exceptions are error conditions that arise when the normal flow of a code path-that is, a series of
method calls on the call stack-is impractical. Exception handling is an in built mechanism in .NET
framework to detect and handle run time errors. The exceptions are anomalies that occur during
the execution of a program. They can be because of user, logic or system errors. If a user
(programmer) do not provide a mechanism to handle these anomalies, the .NET run time
environment provide a default mechanism, which terminates the program execution. C# provides
three keywords try, catch and finally to do exception handling. The try encloses the statements
that might throw an exception whereas catch handles an exception if one exists. The finally can
be used for doing any clean up process.
The general form try-catch-finally in C# is shown below:
try
{
// Statement which can cause an exception.
}
catch(Type x)
{
// Statements for handling the exception
}
finally
{
//Any cleanup code
}
If any exception occurs inside the try block, the control transfers to the appropriate catch block
and later to the finally block.
But in C#, both catch and finally blocks are optional. The try block can exist either with one or
more catch blocks or a finally block or with both catch and finally blocks.
If there is no exception occurred inside the try block, the control directly transfers to finally block.
We can say that the statements inside the finally block is executed always. Note that it is an error
to transfer control out of a finally block by using break, continue, return or goto.
In C#, exceptions are nothing but objects of the type Exception. The Exception is the ultimate
base class for any exceptions in C#. The C# itself provides couple of standard exceptions. Or
even the user can create their own exception classes, provided that this should inherit from either

Exception class or one of the standard derived classes of Exception class like
DivideByZeroExcpetion ot ArgumentException etc.
The modified form of the above program with exception handling mechanism is as follows: -
//C#: Exception Handling
using System;
class MyClient
{
{
int x = 0; int div = 0;
try
{ div = 100/x;
Console.WriteLine("Not executed line");
}
catch(DivideByZeroException de)
{ Console.WriteLine("Exception occured"); }
finally
{ Console.WriteLine("Finally Block"); }
Console.WriteLine("Result is {0}",div);
}
}
Multiple Catch Blocks
A try block can throw multiple exceptions, which can handle by using multiple catch blocks.
Remember that more specialized catch block should come before a generalized one. Otherwise
the compiler will show a compilation error.
//C#: Exception Handling: Multiple catch
using System;
class MyClient
{
{
int x = 0;
int div = 0;
try
{
div = 100/x;
}
catch(DivideByZeroException de)
{ Console.WriteLine("DivideByZeroException" ); }
catch(Exception ee)
{ Console.WriteLine("Exception" ); }
finally
{ Console.WriteLine("Finally Block"); }
}
}

Catching All Exception
By providing a catch block without a brackets or arguments, we can catch all exceptions occurred
inside a try block. Even we can use a catch block with an Exception type parameter to catch all
exceptions happened inside the try block since in C#, all exceptions are directly or indirectly
inherited from the Exception class.
//C#: Exception Handling: Handling all exceptions
using System;
class MyClient
{
{
int x = 0;
int div = 0;
try
{ div = 100/x;
}
catch
{ Console.WriteLine("oException" );}
}
}
The following program handles all exception with Exception object.
//C#: Exception Handling: Handling all exceptions
using System;
class MyClient
{
{
int x = 0;
int div = 0;
try
{
div = 100/x;
}
catch(Exception e)
{ Console.WriteLine("oException" );}
}
}
Throwing an Exception
In C#, it is possible to throw an exception programmatically. The 'throw' keyword is used for this
purpose. The general form of throwing an exception is as follows.
throw exception_obj;
For example the following statement throws an ArgumentException explicitly.

throw new ArgumentException("Exception");
Example
//C#: Exception Handling:
using System;
class MyClient
{
{
try
{ throw new DivideByZeroException("Invalid Division");}
catch(DivideByZeroException e)
{ Console.WriteLine("Exception" ); }
Console.WriteLine("LAST STATEMENT");
}
}
Standard Exceptions
There are two types of exceptions: exceptions generated by an executing program and
exceptions generated by the common language runtime. System.Exception is the base class for
all exceptions in C#. Several exception classes inherit from this class including
ApplicationException and SystemException. These two classes form the basis for most other
runtime exceptions. Other exceptions that derive directly from System.Exception include
IOException, WebException etc.
The common language runtime throws SystemException. The ApplicationException is thrown by
a user program rather than the runtime. The SystemException includes the
ExecutionEngineException, StaclOverFlowException etc. It is not recommended that we catch
SystemExceptions nor is it good programming practice to throw SystemExceptions in our
applications.
 System.OutOfMemoryException
 System.NullReferenceException
 Syste.InvalidCastException
 Syste.ArrayTypeMismatchException
 System.IndexOutOfRangeException
 System.ArithmeticException
 System.DevideByZeroException
 System.OverFlowException
User-Defined Exceptions
In C#, it is possible to create our own exception class. But Exception must be the ultimate base
class for all exceptions in C#. So the user-defined exception classes must inherit from either
Exception class or one of its standard derived classes.
//C#: Exception Handling: User defined exceptions
using System;
class MyException : Exception
{
public MyException(string str)
{ Console.WriteLine("User defined exception");}

}
class MyClient
{
{
try
{ throw new MyException("RAJESH"); }
catch(Exception e)
{Console.WriteLine("Exception caught here" + e.ToString()); }
Console.WriteLine("LAST STATEMENT");
}
}
------------------x-------------------------x------------------------------x-------------------------x-----------------------
File Handling in C#
File handling is an unmanaged resource in your application system. It is outside your application
domain (unmanaged resource). It is not managed by CLR.
Data is stored in two ways, persistent and non-persistent manner.
When you open a file for reading or writing, it becomes stream.
Stream: Stream is a sequence of bytes traveling from a source to a destination over a
communication path.
The two basic streams are input and output streams. Input stream is used to read and output
stream is used to write.
The System.IO namespace includes various classes for file handling.
The parent class of file processing is stream. Stream is an abstract class, which is used as the
parent of the classes that actually implement the necessary operations.
The primary support of a file as an object is provided by a .NET Framework class called File. This
static class is equipped with various types of (static) methods to create, save, open, copy, move,
delete, or check the existence of a file.

System.IO
System
System.IO
System.IO
Diagram to represent file-handling class hierarchy
The following table describes some commonly used classes in the System.IO namespace: -
Class Name Description
FileStream It is used to read from and write to any location within a file
BinaryReader It is used to read primitive data types from a binary stream
BinaryWriter It is used to write primitive data types in binary format
StreamReader It is used to read characters from a byte Stream
StreamWriter It is used to write characters to a stream
StringReader It is used to read from a string buffer
StringWriter It is used to write into a string buffer
DirectoryInfo It is used to perform operations on directories
FileInfo It is used to perform operations on files
Object
MarshalByref
Object
FileSystemInfo
FileInfo Directoryinfo File Path Directory DriveInfo

Reading and Writing in the text file
StreamWriter Class
The StreamWriter class in inherited from the abstract class TextWriter. The TextWriter class
represents a writer, which can write a series of characters.
The following table describes some of the methods used by StreamWriter class: -
Methods Description
Close Close the current StreamWriter object and underlying stream
Flush
Clears all buffers for the current writer and causes any buffered
data to be written to the underlying stream.
Write Writes to the Stream
WriteLine
Writes data specified by the overloaded parameters, followed by
end of line.
Program to write user input to a file using StreamWriter Class
using System;
using System.Text;
using System.IO;
namespace FileWriting_SW
{
class Program
{
class FileWrite
{
public void WriteData()
{
FileStream fs = new FileStream("c:test.txt", FileMode.Append, FileAccess.Write);
StreamWriter sw = new StreamWriter(fs);
Console.WriteLine("Enter the text which you want to write to the file");
string str = Console.ReadLine();
sw.WriteLine(str);
sw.Flush();
sw.Close();
fs.Close();
}
}
{
FileWrite wr = new FileWrite();

wr.WriteData();
}
}
}
StreamReader Class
The StreamReader class is inherited from the abstract class TextReader. The TextReader class
represents a reader, which can read series of characters.
The following table describes some methods of the StreamReader class: -
Methods Description
Close
Closes the object of StreamReader class and the underlying stream, and
release any system resources associated with the reader
Peek Returns the next available character but doesn't consume it
Read Reads the next character or the next set of characters from the stream
ReadLine Reads a line of characters from the current stream and returns data as a string
Seek Allows the read/write position to be moved to any position with the file
Program to read from a file using StreamReader Class
using System;
using System.IO;
namespace FileReading_SR
{
class Program
{
class FileRead
{
public void ReadData()
{
FileStream fs = new FileStream ("c:test.txt", FileMode.Open, FileAccess.Read);
StreamReader sr = new StreamReader (fs);
Console.WriteLine("Program to show content of test file");
sr.BaseStream.Seek(0, SeekOrigin.Begin);
string str = sr.ReadLine();
while (str != null)
{

Console.WriteLine(str);
str = sr.ReadLine();
}
Console.ReadLine();
sr.Close();
fs.Close();
}
}
{
FileRead wr = new FileRead();
wr.ReadData();
}
}
}
Unsafe Mode
When you use the new keyword to create a new instance of a reference type, you are asking the
CLR to set aside enough memory to use for the variable. The CLR allocates enough memory for
the variable and associates the memory with your variable. Under normal conditions, your code is
unaware of the actual location of that memory, as far as a memory address is concerned. After
the new operation succeeds, your code is free to use the allocated memory without knowing or
caring where the memory is actually located on your system.
Occasionally, however, you need to work with a specific memory address in your C# code. Your
code may need that extra ounce of performance, or your C# code may need to work with legacy
code that requires that you provide the address of a specific piece of memory. The C# language
supports a special mode, called unsafe mode, which enables you to work directly with memory
from within your C# code.
This special C# construct is called unsafe mode because your code is no longer safe from the
memory-management protection offered by the CLR. In unsafe mode, your C# code is allowed to
access memory directly, and it can suffer from the same class of memory-related bugs found in C
and C++ code if you're not extremely careful with the way you manage memory.
Generally, When we write any program in C#, we create managed code. Managed code is
executed under the control of CLR. CLR causes that programmer do not need to manage
memory and take care about memory’s allocation and deallocation. CLR also allows you to write
what is called ‘unsafe code’.
The CLR knows how to manipulate three kinds of pointers:
 Managed pointers: These pointers can point to data contained in the object heap
managed by the garbage collector. These pointers are not used explicitly by the C# code.
They are thus used implicitly by the C# compiler when it compiles methods with out and
ref arguments.
 Unmanaged function pointers: The pointers are conceptually close to the notion of
delegate.
 Unmanaged pointers: These pointers can point to any data contained in the user
addressing space of the process. The C# language allows to use this type of pointers in
zones of code considered unsafe.

Compilation options to allow unsafe code
Unsafe code must be used on purpose and you must also provide the /unsafe option to the
csc.exe compiler to tell it that you are aware that the code you wish to compile contains zones
which will be seen as unverifiable by the JIT compiler. Visual Studio offers the Build Allow unsafe
code project property to indicate that you wish to use this compiler option.
Declaring unsafe code in C#
In C#, the unsafe keyword lets the compiler know when you will use unsafe code. It can be used
in three situations:
Before the declaration of a class or structure. In this case, all the methods of the type can use
pointers.
Before the declaration of a method. In this case, the pointers can be used within the body of this
method and in its signature.
Within the body of a method (static or not). In this case, pointers are only allowed within the
marked block of code. For example:
unsafe
{
...
}
Using pointers in C#
Each object, whether it is a value or reference type instance, has a memory address at which it is
physically located in the process. This address is not necessarily constant during the lifetime of
the object as the garbage collector can physically move objects store in the heap.
To create a pointer you can use the following declaration:
Type* variable_name;
As a type may be used each type that is not a reference-type field. It can be only: sbyte, byte,
short, ushort, int, uint, long, ulong, char, float, double etc.
Following examples show different pointers declarations:
int* pi; // declaration a pointer to integer variable
float* pf, pq // two pointers to float variables
char* pz // pointer to char
Advantages of UNSAFE in C#
 Performance and flexibility, by using pointer you can access data and manipulate it in the
most efficient way possible.
 Compatibility, in most cases we still need to use old windows API’s, which use pointers
extensively, or third parties may supply DLLs that some of its functions need pointer
parameters. Although this can be done by writing the DLLImport declaration in a way that
avoids pointers, but in some cases it’s just much simpler to use pointer.
 Memory Addresses, there is no way to know the memory address of some data without
using pointers.

Disadvantages of UNSAFE in C#
 Complex syntax, to use pointers you need to go through more complex syntax than we
used to experience in C#.
 Harder to use, you need be more careful and logical while using pointers, miss-using
pointers might lead to the following: -
- Overwrite other variables
- Stack Overflow
- Access areas of memory that doesn’t contain any data as they do.
- Overwrite some information of the code for the .net runtime, which will suerly
lead your application to crash.
 Your code will be harder to debug. A simple mistake in using pointers might lead your
application to crash randomly and unpredictably.
 Type-safety, using pointers will cause the code to fail in the .net type-safety checks, and
of course if your security police don’t allow non type-safety code, then the .net framework
will refuse to execute your application.
ADO.NET
ADO.NET is a model used by .NET applications to communicate with a database for retrieving,
accessing and updating data.
Features of ADO.NET
Some of the features of ADO.NET are given below:
1. Disconnected data architecture – ADO.NET uses the disconnected data architecture.
Applications connect to the database only while retrieving and updating data. After data is
retrieved, the connection with the databse is closed. When the database needs to be
updated, the connection is re-established. Working with applications that do not follow a
disconnected architecure leads to a wastage of valuable system resources, since the
application connects to the database and keeps the connection open until it stops
running, but does not actually interact with the database except while retrieving and
updating data.
2. Data Cahed in datasets – A dataset is the most common method of accessing data since
it implements a disconnected architecture. Since ADO.NET is based on a disconnected
data structure, it is not possible for the application to interact with the database for
processing each record. Therefore, the data is retrieved and store in datasets. A dataset
is a chached set of database records.You can work with the records stored in a dataset
as you work with real data; the only difference being that the dataset is independent of
data source and you remain disconnected from the data source.
3. Data transfer in XML format – XML is the fundamental format for data transfer in
ADO.NET. Data is transferred from a databse into a dataset and from the dataset to
another component by using XML. You can use XML file as a data source and store data
from it in a dataset. Using XML as the data transfer language is beneficial as XML is an
industry standard for exchanging information between types of applications.
4. Interaction with the database is done through data commands – All operations on the
database are performed by using data commands. A data command can be a SQL
statement or a stored procedure. You can retrieve, insert, delete or modify data from a
database by executing data commands.
The ADO.NET Object Model
ADO.NET uses a structured process flow containing components. The structured process flow or
the object model is shown in following figure:

ADO.NET Object Model
Data Access in ADO.NET relies on two components: DataSet and Data Provider.
DataSet
The dataset is a disconnected, in-memory representation of data. It can be considered as a local
copy of the relevant portions of the database. The DataSet is persisted in memory and the data in
it can be manipulated and updated independent of the database. When the use of this DataSet is
finished, changes can be made back to the central database for updating. The data in DataSet
can be loaded from any valid data source like Microsoft SQL server database, an Oracle
database or from a Microsoft Access database.
Data Provider
The Data Provider is responsible for providing and maintaining the connection to the database. A
DataProvider is a set of related components that work together to provide data in an efficient and
performance driven manner.
The .NET Framework currently comes with two DataProviders: the SQL Data Provider which is
designed only to work with Microsoft's SQL Server 7.0 or later and the OleDb DataProvider which
allows us to connect to other types of databases like Access and Oracle. Each DataProvider
consists of the following component classes:
 The Connection object which provides a connection to the database
 The Command object which is used to execute a command
 The DataReader object which provides a forward-only, read only, connected recordset
 The DataAdapter object which populates a disconnected DataSet with data and performs
update
Connection
Data Reader
Command
Data Adapter
.NET
Application
DataSet
Database
Data Provider
Accessing
Retrieved data
Accessing retrieved data
Filling dataset w ith data
Establish connection
Executes a command
to retrieve data
Transfer data to the
dataset and reflects the
changes in dataset
Retrieve data in a read-
only, forwarded only

Component classes that make up the Data Providers
The Connection Object
The Connection object creates the connection to the database. Microsoft Visual Studio .NET
provides two types of Connection classes: the SqlConnection object, which is designed
specifically to connect to Microsoft SQL Server 7.0 or later, and the OleDbConnection object,
which can provide connections to a wide range of database types like Microsoft Access and
Oracle. The Connection object contains all of the information required to open a connection to the
database.
The Command Object
The Command object is represented by two corresponding classes: SqlCommand and
OleDbCommand. Command objects are used to execute commands to a database across a data
connection. The Command objects can be used to execute stored procedures on the database,
SQL commands, or return complete tables directly. Command objects provide three methods that
are used to execute commands on the database:
 ExecuteNonQuery: Executes commands that have no return values such as INSERT,
UPDATE or DELETE
 ExecuteScalar: Returns a single value from a database query
 ExecuteReader: Returns a result set by way of a DataReader object
The DataReader Object
The DataReader object provides a forward-only, read-only, connected stream recordset from a
database. Unlike other components of the Data Provider, DataReader objects cannot be directly
instantiated. Rather, the DataReader is returned as the result of the Command object's
ExecuteReader method. The SqlCommand.ExecuteReader method returns a SqlDataReader
object, and the OleDbCommand.ExecuteReader method returns an OleDbDataReader object.
The DataReader can provide rows of data directly to application logic when you do not need to
keep the data cached in memory. Because only one row is in memory at a time, the DataReader
provides the lowest overhead in terms of system performance but requires the exclusive use of
an open Connection object for the lifetime of the DataReader.
The DataAdapter Object
The DataAdapter is the class at the core of ADO .NET's disconnected data access. It is
essentially the middleman facilitating all communication between the database and a DataSet.
The DataAdapter is used either to fill a DataTable or DataSet with data from the database with it's
Fill method. After the memory-resident data has been manipulated, the DataAdapter can commit
the changes to the database by calling the Update method. The DataAdapter provides four
properties that represent database commands:
 SelectCommand
 InsertCommand
 DeleteCommand
 UpdateCommand
When the Update method is called, changes in the DataSet are copied back to the database and
the appropriate InsertCommand, DeleteCommand, or UpdateCommand is executed.

ADO.NET Archtecture
The following diagram illustrates the relationship between a .NET Framework data provider and a
DataSet
Basic steps involved in data access with ADO.NET in disconnected environment
Data access using ADO.NET involves following steps:
 Defining the connection string for the database server.
 Defingin the connection to the database using connection string.
 Defining the command or command string that contains the query.
 Defining the data adapter using the command string and command object.
 Creating a new dataset object.
 If the command SELECT, filling the dataset object with the result of the query through the
data adapter.
 Reading the records from the Data Tables in the datasets using the Data Row and Data
Column Objects.
 If the command is Update, Insert, or Delete, then updating the dataset through the data
Adapter.
 Accepting to save the changes in the data set to the database.
Connection methods:
 Open- Opens the connection to our database.
 Close- Closes the database connection.
 Dispose- Realeses the resources on the connection object, used to force garbage
collecting, ensuring no resources are being held after our connection is used. It
automatically call close method.
 State- Tells you what type of connection state your object is in used to check whether
your conection is still using any resources.
 If (ConnectionObject.Sate==Connectionstate.Open)

Command Object methods:
 Execute Reader- Simply executes the SQL query against the database using Read().
 ExecuteNonQuery- Used whenever you work with SQL stored procedures with
parameters.
 Execute Scalar-Returns a lightining fast signle value as an object from your database.
Ex: - object val = Command.ExecuteScalar();
 Prepare- Equivalent to ADO’s Command.Prepare= TRUE property. Useful in caching the
SQL command so it runs faster when called more than one.
Ex: - Commad.Prepare();
 Dispose- Release the resources on the command object.
Data Reader methods:
 Read-Means the record pointer to the first row, which allows the data to be read by
column Name of index position
Ex: - If (Reader.Read()==true);
 IsClosed- A method that can determine if the Datareader is closed
 If (DataReader.Isclosed==false);
 Next Result-Equivalent to ADO’s NextRecordset method, where a batch of SQL
statement are executed with this method before advancing to the next set of data results.
Ex: - DataReader.NextResult();
 Close- close the Data Reader.
Example of Use of Command Object
It supports a command Type:
1. Text A SQL command defining the statements to be executed at the data source.
2. Stored Procedure The name of SP. You can use parameter property of a command to
access input and output parameters and return values.
3. Table Direct the name of a table.
Example Code1: -
Static void GetSalesByCategory( String ConnectionString, String CategoryName)
{
Using (SqlConnection connection= New SqlConnection(ConnectionString))
{
SqlCommand command = new Sqlcommand();
Command.Connection=connection;
Command.CommandText=”SalesByCategory”;
Command.CommandType=CommandType.StoredProcedure;
SqlParameter parameter = New SqlParameter();
parameter.ParameterName=”@ CategoryName”;
parameter.SqlDbType=SqlDbType.Nvarchar;
parameter.Direction=ParameterDirection.Input;
parameter.Value= CategoryName;
Command.Parameters.Add(parameter);
Connection.Open();
SQLDataReader.reader=Command.ExecuteReader();
If (reader.HasRows)
{
While(reader.Read())

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