Sub Procedures

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• Sub Procedures
• And Functions

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Introduction

• In Visual Basic .NET There Are Two Broad
  Categories of Procedures
• Sub Procedures and Function Procedures

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• A Sub Procedure is a collection of statements
  that performs a task
• The word Sub is an abbreviation for the older
  term subroutine
• Event procedures belong to this category
• A Function procedure is a collection of
  statements that performs a task and then returns
  a value to the part of the program that executed
  it
• Function procedures work like intrinsic
  functions, such as Val and IsNumeric

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Sub Procedures

• You Can Write Your Own General Purpose Sub
  Procedures That Perform Specific Tasks
• General Purpose Sub Procedures Are Not Triggered
  by Events but Called From Statements in Other
  Procedures

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Sub Procedure Uses

• As Event handling code
• As General code that is executed by statements in
  other procedures

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Uses for Sub Procedures

• To Modularize one's program
• Break it into small, manageable hunks
• To Simplify one's program
• Create individual sub procedures to perform tasks
  that are done repeatedly

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Sample Sub Procedure
Private Sub btnGo_Click(ByVal sender As
System.Object, _ ByVal e As System.EventArgs)
Handles btnGo.Click ' This procedure calls the
DisplayMessage procedure. lstOutput.Items.Add("He
llo from btnGo_Click procedure.") lstOutput.Items
.Add("Calling the DisplayMessage "
_ "procedure.") DisplayMessage() lstOutput.Ite
ms.Add("Now I am back in the btnGo_Click procedu
re.") End Sub
Sub DisplayMessage() 'A Sub procedure that
displays a message. lstOutput.Items.Add("") lstO
utput.Items.Add("Hello from DisplayMessage.") lst
Output.Items.Add("") End Sub
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Declaring a Sub Procedure
AccessSpecifier Sub ProcedureName
(ParameterList) Statement(s) End Sub

• AccessSpecifier gives accessibility to the
  program
• ParameterList is a list of variable values that
  are being passed to the sub procedure
• 'Sub' and 'End' are keywords

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Local Variables

• Within the Sub Procedure, declare the variable
  names that are needed
• These work as they do in other procedures
• They are only accessible from inside the sub
  procedure in which they are declared
• (Their values are not saved from one call to the
  sub procedure to the next)

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Static Local Variables

• Declared with
• These local variables do maintain their values
  from one call to the sub procedure to the next

Static VariableName As DataType
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6.3Passing Values to a Sub Procedure

• When a Procedure Is Called, Values May Be Passed
  to It

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Passing Parameters By Value, Example
DisplayValue(5) Sub DisplayValue(ByVal number
As Integer) ' This procedure displays a value in
a message box. MessageBox.Show(number.ToString) E
nd Sub

• The value, 5, is copied into the storage location
  of 'number' in DisplayValue
• Then DisplayValue executes

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Passing Multiple Parameters
ShowSum(5, 10) Sub ShowSum(ByVal num1 As
Integer, ByVal num2 As Integer) ' This procedure
accepts two arguments, and prints ' their sum on
the form. Dim sum As Integer sum num1
num2 MessageBox.Show("The sum is "
sum.ToString) End Sub

• Value of first argument is copied to first
• Second to second, etc.

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Passing Arguments by Reference

• Keyword 'ByVal' is used to indicate passing a
  parameter by value (value is copied)
• Keyword 'ByRef' indicates passing a parameter in
  another way
• Use of the corresponding variable in the sub
  procedure refers to the corresponding storage
  location in the calling program
• There is no corresponding storage location in the
  sub procedure (to receive a copy)

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Function Procedures

• A Function Procedure Returns a Value to the Part
  of the Program That Called the Function Procedure

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Declaring a Function Procedure
AccessSpecifier Function FunctionName
(ParameterList) _ As DataType Statements E
nd Function

• New keyword 'Function'
• Plus 'As DataType' qualifier to state the data
  type of the returned value
• Return value is specified in a Return expression

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Arguments Passed and Value Returned
total Sum(value1, value2) Function Sum(ByVal
num1 As Single, ByVal num2 As Single) _ As
Single Dim result As Single result num1
num2 Return result End Function
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Returning Nonnumeric Values
Function FullName(ByVal first As String, ByVal
last As String)_ As String Dim name As
String name last ", " first Return
name End Function
Function IsValid(num As Integer) As Boolean Dim
status As Boolean If num 0 And num Then status True Else status False End
If Return status End Function