Vb net xp_04

Introducing Procedures and CommonDialog Classes
in Visual Basic .NET
Objectives
In this lesson, you will learn to:
Identify the different types of procedures in Visual
Basic .NET
Identify the importance of procedure overloading
Identify the importance of procedure overriding
Implement the MsgBox( ) function
Implement the CommonDialog classes

©NIIT Introducing Procedures and CommonDialog Classes in VB .NET/Lesson 4/Slide 1 of 34

Procedures
Are sets of one or more program statements that can be
executed by referring to the procedure name.
Are the key to modular programming. They simplify the task
of maintaining and debugging the application code.
Are useful for performing repetitive tasks, such as fetching
specific records from a database, and text and control
manipulation.
Are of three types:
Sub procedures
Function procedures
Property procedures

Sub Procedure
Is a block of code enclosed within the Sub and End Sub
statements.
Does not return a value. However, it can take arguments,
such as constants, variables, and expressions that are
passed to it by the calling code.
Example
Public Sub Check_Acct_Status (ByVal
CustAcct As Integer, ByVal Amount As Single)
’The statements of the Sub procedure are
given here.
End Sub


Sub Procedure (Contd.)
Can be defined in a module, a class, or a structure.
Can be created with one of the following access modifiers:
Public
Protected
Friend
Protected Friend
Private


Can take arguments. You declare each argument by
specifying the argument name and the data type.
Can have an optional argument. An optional argument
must be declared at the end of the argument list and must
have a default value.
Can be called by using the Call keyword and providing
values for all the arguments that are not optional. You can
pass arguments to a sub procedure either by value or by
reference.
Can be further categorized as:
General procedures
Event-handling procedures

General procedure
Is a block of code that performs a specific task.
Event-handling procedure
Is a block of code that is executed when a specific
event occurs, such as the click of a button or the
loading of a form in the memory.
Is a combination of the object name and the type of
event that has occurred.


Function Procedure
Is a block of code enclosed within the Function and End
Function statements.
Returns a value to the calling code.
Example
Public Function Check_Acct_Status (ByVal
CustAcct As Integer, ByVal Amount As Single)
As Integer
' The statements of the Sub procedure
are given here.
Return Amount
End Function

Function Procedure (Contd.)
Can be defined in a module, a class, or a structure.
Can be created with one of the following access modifiers:
Public, Protected, Friend, Protected Friend,
and Private.
Has Public access by default.
Uses the Return statement to return a value to the calling
procedure.
Return value can be trapped by calling a function and
assigning the return value of the function to a variable.
Example
TotalAmount =
Calculate_Amount(Total_Salary,months)

Property Procedure
Is a set of code statements that are used to assign or
retrieve the values of the properties declared within a
module, a class, or a structure.
Is a type of variable that stores the values of an object of a
class or a structure.
Can help you define a property as read-only, write-only, or
read/write type.
Is of two types:
Get procedures are used to retrieve the values from a
property.
Set procedures are used to assign values to a property.


Property Procedure (Contd.)
Is always invoked implicitly by the code that refers to the
property, that is, the code uses the name of the property and
provides values for all the arguments that are not optional.


Passing Arguments to a Procedure
You can pass arguments to a procedure:
By value using the ByVal keyword
By reference using the ByRef keyword
In Visual Basic .NET, the default argument passing
mechanism is ByVal.


Parameter Array
Can be used to pass an array of values for an argument of
a procedure.
Is defined by using the keyword ParamArray.
Rules:
You cannot use more than one parameter array in a
procedure, and it must be the last argument in the procedure
definition.
The parameter array must be passed by value.
The code within the procedure must use the parameter
array as a one-dimensional array. In addition, each element
of the array must be of the same data type as the data
type of ParamArray.

Parameter Array (Contd.)
The parameter array is optional. The default value of a
parameter array is an empty one-dimensional array.
The parameter array must be the lone optional argument in
the list of arguments for a procedure. All other
arguments preceding the parameter array must be used.


Procedure Overloading
Means defining multiple procedures using the same name
but different argument lists, also referred to as signature.
When you overload a procedure:
Each overloaded version uses the same procedure
name.
Each overloaded version differs from all the other
overloaded versions in one of the following ways:
®The number of arguments
®The order of the arguments
®The data types of the arguments


Procedure Overloading (Contd.)
You cannot overload a procedure by varying only one or
more of the following items:
The procedure modifiers, such as Public, Shared,
and Static
The argument names
The argument modifiers, such as ByRef and Optional
The data type of the return value


Procedure Overriding
Means redefining a base class procedure in a derived class
without changing the name of the procedure. Rules:
You can override procedures that are declared with the
Overridable keyword in the base class.
You need to explicitly declare a procedure in the base
class with the Overridable keyword in order to
override it in the derived class.
Overridden procedures need to have the same
arguments as the inherited members from the base
class.
The redefined implementation of a procedure in the
derived class can call the implementation defined in the
parent class by specifying MyBase keyword before the
procedure name.

Built-in Functions in Visual Basic .NET
Functions Usage

Len (string expression) To find the length of the string
expression that is passed as the
argument
Mid (string expression, starting position, To extract a particular number of
number of characters) characters, starting at a given character
position, from the string expression that
is passed as the argument
CDate (string expression) To convert the string expression that is
passed as the argument to the Date
type
CTime (string expression) To convert the string expression that is
passed as the argument to the Time
type
Val (string/object/ char expression) To convert the numbers contained in a
string/char/object to a numeric value


Built-in Functions in Visual Basic .NET(Contd.)
Functions Usage

CBool (expression) To convert a string or numeric expression
to Boolean values

CByte (string/numeric expression) To convert a string or a numeric
expression to a Byte data type

CInt (string/numeric expression) To convert a string or a decimal
expression to an integer
CObj (string/numeric expression) To convert a string or numeric value to an
object
CChar (string/numeric expression) To convert a string or numeric value to a
Char
CStr (string/numeric expression) To convert a string or numeric value to a
String
CDec (string/numeric expression) To convert a string or numeric value to a
Decimal
CType(expression.datatype/object/class/ To convert an expression from one type
structure) to another


Problem Statement 4.D.1
The customer data entry forms at the call centers of Diaz
Telecommunications need to include options to check for a
valid customer ID and telephone number. The customer ID
should start with the letter ‘C’ and have three digits after it.
The telephone number should consist of eight digits. The form
should also have the facility to display error messages when
incorrect data is entered. Additionally, the form should have
the provision to clear the last customer details entered.


Task List
Identify the checks that need to be applied on the form.
Identify the changes needed in the design of the form.
Identify the mechanism to display error messages.
Add the additional controls to the form.
Check the data and display an error message.
Save the application.
Run the application to validate the checks applied on the
form.


Task 1: Identify the checks that need to be applied on the
form.
Result:
As per the problem statement, the customer data entry form
needs to have a provision to check for a valid customer id
and telephone number.
You can add the provision for checking the validity of the
customer id and the telephone number by including
procedures in the customer data entry application.
Since Visual Basic .NET allows the use of procedure
overloading, you can create two versions of a procedure in
the customer data entry application to check the validity of
the customer id and the telephone number.


Task 2: Identify the changes needed in the design of
the form.
You will create an overloaded procedure for checking the
validity of the customer id and the telephone number.
You can name the procedure Check_data ().
Result:
The following table recommends suitable prefixes that you
can use for the two buttons:
Object Prefix Example

Button cmd cmdCheckdata

Button cmd cmdReset


Task 3: Identify the mechanism to display error
messages.
Messages can be displayed by using either
Msgbox ()Function
MessageBox Class


Task 3: Identify the mechanism to display error
messages. (Contd.)
Result:
To display an error message to users when an invalid
customer id or telephone number is entered in the customers
data entry form, you can use either the MessageBox
class or the built-in MsgBox()function.
Since MessageBox class offers greater control over the
interface of the message box, such as displaying
appropriate icons, you should use the MessageBox class to
display error message when invalid customer id or telephone
number is entered.


Task 4: Add the additional controls to the form.
Task 5: Check the data and display an error
message.
Task 6: Save the application.
Task 7: Run the application to validate the checks
applied on the form.


Problem Statement 4.P.1
The Employee data entry form in the call centers at Diaz
Telecommunications requires options to check for valid
employee ID and age entries. The employee ID should start
with the letter ‘E’ and have three digits after it. The age should
be more than 20 years and less than 61 years. The form
should also have the facility to display error messages when
incorrect data is entered. Additionally, the form should have a
provision to clear the last employee details entered. The
details of an employee essentially include the employee id,
the employee’s first name, the employee’s last name, the
address, the age, the date of joining, the department, and the
salary.


Just a Minute…
2. There are two labels named Label1 and Label2 and one
button named Button1 on the form, Form1. Predict the
output of the code, when Button1 is clicked at run time.
3. There are two labels named Label1 and Label2 and one
button named Button1 on the form, Form1. You have
created a user-defined Sub procedure called MySub to
process information when the user clicks Button1. Predict
the output of the code, when the program is executed.
4. There is one label named Label1 and one button named
Button1 on the form, Form1. What will be the output of the
code when Button1 is clicked at run time?


Common Dialog Classes
Are used to access the default Font dialog box to change
the font of the text or used to open a file by using the Open
dialog box and display the contents.
Are of the following types:
ColorDialog
FontDialog
FileDialog
PrintDialog
PageSetupDialog


ColorDialog Class
Is used to change the background and the foreground color
of text.
FontDialog Class
Is used to change the font, the font style, and the size of
text.
FileDialog Class
Is an abstract class that is inherited from the
CommonDialog class.
You cannot instantiate it directly. However, you can use the
OpenFileDialog or SaveFileDialog class to open a file
or save an existing file.


PrintDialog Class
Is used to print text or graphics.

PageSetupDialog Class
Used to set the page details for printing in Windows
applications.


Just a Minute…
2. What will be the output of the code when Button1 is clicked
at run time?
3. The code snippet is written in the Code Editor window of
Form1. While building the project, there was a build error.
What would you do to resolve the error?


Summary
In this lesson, you learned that:
A procedure is a set of one or more program statements
that can be executed by referring to the procedure name.
Procedures are of three types:
Sub
Function
Property
Sub procedures can be of two types:
General
Event-handling

Summary (Contd.)
Property procedure is of two types:
Get procedures are used to retrieve values from a
property.
Set procedures are used to assign values to a property.
Property procedures are used to access properties declared
within a module, a class, or a structure.
Arguments can be passed to a procedure either by value or
by reference.
Parameter arrays enable you to pass an array of values as
an argument to a procedure.
Procedure overloading is defining multiple procedures
having the same name but different argument lists.


Summary (Contd.)
Procedure overriding enables you to redefine a base class
procedure in a derived class without changing the name of
the procedure.
The CommonDialog class is the base class for the most
commonly used dialog boxes, such as, Font, File, Print, and
Page Setup.


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