This provides a list of OOPs FAQs-4 which are of "Difference between" kind

1. 1.Difference between String and StringBuilder
S.No String StringBuilder
1 String class belongs to the StringBuilder class belongs to the
namespace System. namespace System.Text.
2 String class is immutable. StringBuilder class is mutable.
Immutable means that the string Consider the following example:
cannot be changed. Consider the class sampleClass {
following example: public static void Main() {
class sampleClass { StringBuilder sampleSB = new
public static void Main() { StringBuilder("Hai",10);
string sampleStr = "Hai"; sampleSB = new
sampleStr = "Hello"; StringBuilder("Hello");
Console.WriteLine(sampleStr); Console.WriteLine(sampleSB);
} }
} }
Output of this code will be: Output of this code will be:
Hello Hello
In this example, you have created In this example, you are doing the
a string called sampleStr. You same thing. But the string "Hai" will
have initially assigned the value be overwritten as "Hello" and no new
"Hai". And then you try to strings will be created in the memory.
overwrite its value with "Hello".
You get the overwritten value as
output. But the problem lies in the
number of strings that get created
in memory. When you create the
string as "Hai", this string gets
created in the memory. When you
try to change the value to "Hello",
instead of overwriting the existing
"Hai" string it will create a new
string in the memory and assign
this new string "Hello" to
sampleStr.
3 You can directly assign a string to You cannot directly assign a string to
string class instance. For example, StringBuilder instance. For example,
String sampleStr = "Hai" is valid. StringBuilder sampleSB = "Hai" will
lead to the following error:
"cannot implicitly convert type 'string'
to 'System.Text.StringBuilder' "
You can assign a string to
StringBuilder using the following
statement:
StringBuilder sampleSB = new
StringBuilder("Hai");
4 String concatenation is done using String concatenation is done using

2. + operator. Here is an example: Append method. Here is an example:
class sampleClass { class sampleClass {
public static void Main() { public static void Main() {
string sampleStr = "Hello!"; StringBuilder sampleSB = new
sampleStr += " Good Day!"; StringBuilder("Hello!");
Console.WriteLine(sampleStr); sampleSB.Append("Good
} Day!");
} Console.WriteLine(sampleSB);
Output of this code will be: }
Hello! Good Day! }
Here you have used += operator Output of this code will be:
to perform both concatenation and Hello! Good Day!
assignment using single operator.
You can also use + and =
separately as shown below:
sampleStr = sampleStr + " Good
Day!";
5 During string concatenation, During string concatenation, additional
additional memory will be memory will be allocated if and only if
allocated. the string buffer's capacity is reached.
6 During string concatenation, If the number of concatenations to be
additional memory will be done is random or not known, then it is
allocated if and only if the string recommended to use stringBuilder
buffer's capacity is reached.
7 You cannot set a limit (specifying You can set a limit to StringBuilder
how many strings can be using the member called capacity
concatenated) to a string object which will by default have the value
using string class. 16. You can override it to any number.
The maximum value acceptable is
equivalent to MaxValue of Int32. If
you feel that you do not want to
reserve 16 as the capacity then you can
very well redefine it. However the
capacity will dynamically grow based
on the number of strings that you
append.
Here is an example demonstrating the
usage of capacity:
class sampleClass {
public static void Main() {
StringBuilder sampleSB = new
StringBuilder();
Console.WriteLine(
sampleSB.Capacity);
sampleSB.Capacity = 1;
Console.WriteLine(
sampleSB.Capacity);

3. sampleSB.Append("str1");
sampleSB.Append("str2");
Console.WriteLine(
sampleSB.Capacity);
}
}
Output of this code will be:
16
1
8
2.Difference between Delegate and Interface
S.No Delegate Interface
1 Delegates can only be methods. Interface can include both properties
Here is an example: and methods.
delegate void sampleDelegate(); Here is an example for an interface:
interface ItestInterface
{
int paraml {
get; set; }
void sampleMethod();
}
2 Delegate can be applied to only When a class implements an interface,
one method at a time it can implement all the methods
associated with it
3 You can use a delegate that is You can use an interface only when
visible in your scope your class or struct implements it
4 Within a class, you can implement Within a class, you can implement an
the same delegate any number of interface method only once. In
times. Assume that either Example2, interface ITestInterface has
sampleClass1 or sampleClass2 of a method called sampleMethod().
Examplel includes a method When sampleClass1 implements
called sampleMethod2( ) with the ITestInterface it implements
same signature as that of delegate, sampleMethod() only once. If not, then
then the same delegate can be it will end up in error.
used to access both
sampleMethod() as well as
sampleMethod2( )
5 Delegate can implement any When an interface method is
method that shares the same implemented, same method name and
signature as that of the delegate signature has to be overridden
6 Delegate is mainly used for Interfaces are not used for handling
handling events events

4. 7 You need not bother about the When a class implements an interface,
other methods available in the though the class requires only one
class.You are concerned about method it has to implement all the
only the method that matches methods of the interface
delegate signature.
8 To access a method using To access the method, you need an
delegate, you need not require any instance of the class which implements
access to the instance of the class the interface or you need an interface
where the method is defined reference pointing to the method
implemented by the class
9 You can access anonymous You cannot access anonymous
methods using delegates methods.Only named methods
declared in interface can be accessed
by the implementing class.
10 When you call a method using a When you are calling a method using
delegate, all the method pointers interface reference, you are directly
associated with the delegate will accessing the method of the class that
be scanned through before the implements the interface. This is a
method execution. This is not a direct method call and it doesn't have
direct method call as you assume. any overhead.
It has a considerable performance
overhead.
11 Delegates can wrap methods of Accessing sealed types is not
sealed classes.Sealed classes are permissible in interface.
those which cannot be inherited.
12 Delegates can wrap any method Class can implement any number of
matching its signature irrespective interfaces and it should override only
of which ever class the method the methods belonging to those
belongs to interfaces
13 Delegates can wrap static This provision is not available with
methods. Examplel discussed interfaces .
above has used the delegate to
wrap a static method called
sampleMethod()
14 Delegate cannot involve in Interface can inherit other interfaces.
inheritance. When a class implements that
interface, it has to implement all the
methods belonging to the interface and
its inherited interfaces as well.
Here is an example of an interface
inheriting from other interfaces:
interface IInterface: IInterface,1
IInterface2
{
void sampleMethod1();
void sampleMethod2();

5. s}
Example1: Using Delegate
delegate void sampleDelegate( );
class sampleClass1{
static void sampleMethod( ) {
Console.WriteLine(“Executing sampleMethod of sampleClass1”);
}
}
class sampleClass2 {
static void Main( ) {
sampleDelegate dele1 = new sampleDelegate(sampleClass1.sampleMethod);
dele1();
}
}
Example2: Using Interface
interface ITestInterface{
void sampleMethod( );
}
class sampleClass1 : ITestInterface {
void sampleMethod( ) {
Console.WriteLine(“Executing sampleMethod of sampleClass1”);
}
}
class sampleClass2 {
static void Main( ) {
sampleClass1 obj1 = new sampleClass1( );
obj1.sampleMethod( );
}
}
3.Difference between Virtual and Abstract keywords in .NET
S.No Virtual Abstract
1 If you feel that the derived class But if you want to enforce that derived
may or may not override the base class must override the base class
class method, then you will define method then you will define the base
the base class method as virtual. class method as abstract.
Consider the following example: namespace Application1 {
namespace Application1 { public abstract class abstractClass {
public class virtualClass { public abstract void abstractMethod();
public virtual void }
virtualMethod(){ public class
Console.WriteLine("Virtual derivedClass:abstractClass{
Method.."); public override void abstractMethod(){
} Console.WriteLine("Overridden..");

6. } }
public class }
derivedClass:virtualClass{ public class testClass {
public override void public static void Main() {
virtualMethod(){ derivedClass obj = new
Console.WriteLine("Overridden.." derivedClass();
); obj.abstractMethod();
} }
} }
public class testClass { }
public static void Main() { Output of this code will be:
virtualClass obj = new Overridden..
virtualClass();
obj.virtualMethod();
derivedClass dObj = new
derivedClass();
dObj.virtualMethod();
}
}
}
Output of this code will be:
Virtual Method..
Overridden..
2 Virtual methods need not be Abstract methods should compulsorily
compulsorily overridden. In the be overridden by the derived class. In
above example if the derived class the above example, if the derivedClass
does not override the method does not override abstractMethod, then
virtualMethod, then again the during compilation you will get the
code will work. following error:
'Application1.derivedClass' does not
implement inherited abstract member
'Application1.abstractClass.abstractMe
thod()'
3 To define a base class method to If you want to define a method as
be virtual, you need not include abstract in the base class then the base
any new definition for the base class should also be marked as
class. In the earlier example, you abstract. Consider the following
can see that the class virtualClass example:
just includes an access modifier namespace Application1 {
followed by the class name. No public class abstractClass {
other additional modifiers are public abstract void abstractMethod();
required. }
}
In this example, abstractClass has an
abstract method. Hence the class in
itself has to be marked abstract. But it
is not done in the above example.
Therefore during compilation, you will
end up in the following error:
'Application1.abstractClass.abstractMe

7. thod()' is abstract but it is contained in
nonabstract class
'Application1.abstractClass'
4 Virtual method can have a method Abstract methods have only the
body. In the earlier example, signature. It cannot have method body.
virtualMethod of virtualClass has However the abstract class can include
method definition like any of the non-abstract methods and these
other methods that you define and methods can have method body
it is perfectly legal. defined. Consider the following
example:
namespace Application1 {
public abstract class abstractClass {
public abstract void abstractMethod(){
Console.WriteLine("Abstract
method..");
}
}
}
Here you are trying to include method
body for an abstract method. This is
not permissible. Hence during
compilation you will end up in the
following error:
"'Application1.abstractClass.abstractM
ethod()' cannot declare a body because
it is marked abstract"
5 Class containing virtual method Class containing abstract method
can be instantiated. Here is an cannot be instantiated. It can only be
example: inherited. Consider the following
namespace Application1 { example:
public class virtualClass { namespace Application1 {
public virtual void public abstract class abstractClass {
virtualMethod(){ public abstract void abstractMethod();
Console.WriteLine("Virtual }
Method.."); public class testClass {
} public static void Main() {
} abstractClass obj = new
Public class testClass { abstractClass();
public static void Main() { }
virtualClass obj = new }
virtualClass(); }
obj.virtualMethod(); Here you are trying to create an
} instance of abstract class. This is not
} possible. Hence during compilation
} you will end up in the following error:
Output of this code will be: "cannot create an instance of the
Virtual Method… abstract class or interface
Application1.abstractClass"
6 Not just methods, virtual modifier Apart from class and method, the

8. can also be used with properties. modifer abstract can be associated with
properties, events and indexers.
7 There is a restriction when using Abstract modifiers also have a
virtual modifer. You cannot use restriction. They cannot be used along
this modifer along with static or with static or virtual or override
abstract or override modifiers. modifiers.
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