1. Sub Procedures And Functions

2. Introduction
In Visual Basic .NET There Are Two Broad Categories of Procedures:
Sub Procedures and Function Procedures

3. 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

4. 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

5. Sub Procedure Uses As Event handling code
As General code that is executed by statements in other procedures

6. 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

7. 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("Hello from btnGo_Click procedure.")
lstOutput.Items.Add("Calling the DisplayMessage " & _
"procedure.")
DisplayMessage()
lstOutput.Items.Add("Now I am back in the btnGo_Click
procedure.")
End Sub
Sub DisplayMessage()
'A Sub procedure that displays a message.
lstOutput.Items.Add("")
lstOutput.Items.Add("Hello from DisplayMessage.")
End Sub

8. 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

9. 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)

10. 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

11. 6.3 Passing Values to a Sub Procedure
When a Procedure Is Called, Values May Be Passed to It

12. 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)
End Sub
The value, 5, is copied into the storage location of 'number' in DisplayValue
Then DisplayValue executes

13. 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.

14. 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)

15. Function Procedures
A Function Procedure Returns a Value to the Part of the Program That Called the Function Procedure

16. Declaring a Function Procedure
[AccessSpecifier] Function FunctionName ([ParameterList]) _
As DataType
[Statements]
End 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

17. 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

18. 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 <= 100 Then
status = True
Else
status = False
End If
Return status
End Function