Sub procedures are procedures that do not return any value. We have been using the Sub procedure Main in all our examples. We have been writing console applications so far, in these tutorials. When these applications start, the control goes to the Main Sub procedure, and it in turn, runs any other statements constituting the body of the program.
Defining Sub Procedures
The Sub statement is used to declare the name, parameter and the body of a sub procedure. The syntax for the Sub statement is:
[Modifiers] Sub SubName [(ParameterList)]
[Statements]
End Sub
Where,
Modifiers: specifiy the access level of the procedure; possible values are: Public, Private, Protected, Friend, Protected Friend and information regarding overloading, overriding, sharing, and shadowing.
SubName: indicates the name of the Sub
ParameterList: specifies the list of the parameters
Example
The following example demonstrates a Sub procedure CalculatePay that takes two parameters hours and wages and displays the total pay of an employee:
Module mysub
Sub CalculatePay(ByVal hours As Double, ByVal wage As Decimal)
'local variable declaration
Dim pay As Double
pay = hours * wage
Console.WriteLine("Total Pay: {0:C}", pay)
End Sub
Sub Main()
'calling the CalculatePay Sub Procedure
CalculatePay(25, 10)
CalculatePay(40, 20)
CalculatePay(30, 27.5)
Console.ReadLine()
End Sub
End Module
When the above code is compiled and executed, it produces following result:
Total Pay: $250.00
Total Pay: $800.00
Total Pay: $825.00
Passing Parameters by Value
This is the default mechanism for passing parameters to a method. In this mechanism, when a method is called, a new storage location is created for each value parameter. The values of the actual parameters are copied into them. So, the changes made to the parameter inside the method have no effect on the argument.
In VB.Net, you declare the reference parameters using the ByVal keyword. The following example demonstrates the concept:
Module paramByval
Sub swap(ByVal x As Integer, ByVal y As Integer)
Dim temp As Integer
temp = x ' save the value of x
x = y ' put y into x
y = temp 'put temp into y
End Sub
Sub Main()
' local variable definition
Dim a As Integer = 100
Dim b As Integer = 200
Console.WriteLine("Before swap, value of a : {0}", a)
Console.WriteLine("Before swap, value of b : {0}", b)
' calling a function to swap the values '
swap(a, b)
Console.WriteLine("After swap, value of a : {0}", a)
Console.WriteLine("After swap, value of b : {0}", b)
Console.ReadLine()
End Sub
End Module
When the above code is compiled and executed, it produces following result:
Before swap, value of a :100
Before swap, value of b :200
After swap, value of a :100
After swap, value of b :200
It shows that there is no change in the values though they had been changed inside the function.
Passing Parameters by Reference
A reference parameter is a reference to a memory location of a variable. When you pass parameters by reference, unlike value parameters, a new storage location is not created for these parameters. The reference parameters represent the same memory location as the actual parameters that are supplied to the method.
In VB.Net, you declare the reference parameters using the ByRef keyword. The following example demonstrates this:
Module paramByref
Sub swap(ByRef x As Integer, ByRef y As Integer)
Dim temp As Integer
temp = x ' save the value of x
x = y ' put y into x
y = temp 'put temp into y
End Sub
Sub Main()
' local variable definition
Dim a As Integer = 100
Dim b As Integer = 200
Console.WriteLine("Before swap, value of a : {0}", a)
Console.WriteLine("Before swap, value of b : {0}", b)
' calling a function to swap the values '
swap(a, b)
Console.WriteLine("After swap, value of a : {0}", a)
Console.WriteLine("After swap, value of b : {0}", b)
Console.ReadLine()
End Sub
End Module
When the above code is compiled and executed, it produces following result:
Before swap, value of a : 100
Before swap, value of b : 200
After swap, value of a : 200
After swap, value of b : 100
