Creating Modules and Procedures

Modules are separate sections of program code.  The module is a separate file with a .VB extension.  It contains variable declarations and program statements just like in the main program, but it can be executed with a single statement from anywhere in the program.

The scope of a variable becomes an issue when you begin to work with modules.  Variables may be “seen” by the program only within a module, or throughout all the programs module.  The variables only seen within the module are called local variables.  This variation is the default.  The variables which can be seen, assigned values and used by any module within the program are called public variables.

• Pull down the Project Menu, choose Add Module. 

• The Module icon should be automatically selected.
• Type a name in the textbox at the bottom of the window.
• Click Add.

The module will open in the IDE in the code view.

Declaring public variables

Variables which retain a value throughout the program, and that can be used anywhere in the program are called public variables.  Their scope extends through the main code and all other modules.  The declaration statement uses the Public keyword rather than Dim, which is used for local variables.  This statement can be at the top of the form’s program code, or within the module where it first occurs in code. I think the top of the form’s code is best.  Using this scheme, all public variables are in the same location.

Syntax:

   Public variableName As Type

Example:

Public Class Form1

Public RunningTotal As Interger

 

A procedure is the set of statements that perform a unique task.  The tasks are divided into two general categories by the type of task– those that do calculations and those that receive or process input, display output or set properties.

Procedures which perform a calculations are called Functions. A function receives a variable or number(the argument) and return a modified value.

The other category of procedures are called Sub Procedures.  They also revieve arguments and return modified values in an argument list

Procedures are useful because the same task may be required at several points in a program.  Using a procedure allows you to create the code once which saves development time.

Syntax:

Function FunctionName([arguments]) As Type

    Function statements go here

    [return value]

End Function

FunctionName is the name you have the function.

As Type declares the type of output the function will generate.

Arguments is a list of arguments to be used in the function.  They are optional and are separated by commas. The type must be included here as well.

The last line is usually an assignment statement.

Return Value is a list of arguments to be passed back from the function.  This is optional. Separate these with commas.  Any code following this line is ignored.

The last line is usually an assignment statement.

End Function separates this function’s code from what follows.

Example:

Function TotalTax(ByVal Cost as Single) as Single

     Dim StateTax, CityTax as Single

     StateTax = Cost * 0.05

     CityTax = Cost * 0.15

     TotalTax = StateTax + CityTax

End Function

Notice the text ByVal in the arguments section. This was not mentioned the Syntax description.  It tells VB to pass the value of the variable Cost to the function.

The function call

Somewhere in the program this statement occurs and causes the procedure to execute using the number 500 as the value for the variable Cost.

lblTaxes.Text = TotalTax(500)

This causes the function TotalTax to run using the value 500 as the contents for the variable Cost. The result will be returned, which is the total tax on a purchase of $500.

Function calls usually use a variable, like this:

 lblTaxes.Text = TotalTax(SalesTotal)

Sub procedures usually return input from a user, display or print something or change properties when a condition occurs.

Syntax

Sub ProcedureName([arguments])

Procedure statements go here

End Sub

ProcedureName is what you name the procedure.

Arguments is a list of items, optional and separated by commas, which will be used by the sub procedure.

The procedure statements are program statements, just like any others within your program, which are required to get the task accomplished.

Example:

Sub Greeting(ByVal FirstName as String)

            Msg = “Happy Birthday “ & FirstName

            MsgBox(Msg,,”Have a good day!”)

End Sub

The example creates a message Happy Birthday concatenated with the FirstName variable contents.  That message is concatenated with the message “Have a great day!”. The entire message is displayed in a message box.

This sub procedure is called by a statement like this:

FriendlyGreeting(FirstName)

ByRef and ByVal

When arguments may be passed ByRef.  This means the location in memory is passed back (ref).  With this technique, any changes that happened to the variable, as a result of the sub procedure will be passed.  The sub procedure is allowed to change the value of the variable.

When arguments are passed by value, the sub procedure cannot make any changes to the variable’s value.