VB .net tutorial - 4

Introducing Procedures and CommonDialog Classes in Visual Basic .NET

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

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



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, number of characters)

To extract a particular number of 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.)

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/structure)

To convert an expression from one type to another

Functions Usage



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…

1. 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.

2. 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.

3. 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…

1. What will be the output of the code when Button1 is clicked at run time?

2. 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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VB .net tutorial - 4