How to: Pass Procedures to Another Procedure in Visual Basic

This example shows how to use delegates to pass a procedure to another procedure.

A delegate is a type that you can use like any other type in Visual Basic. The AddressOf operator returns a delegate object when applied to a procedure name.

This example has a procedure with a delegate parameter that can take a reference to another procedure, obtained with the AddressOf operator.

Create the delegate and matching procedures

1. Create a delegate named MathOperator.

   Delegate Function MathOperator( 
       ByVal x As Double, 
       ByVal y As Double 
   ) As Double
   

2. Create a procedure named AddNumbers with parameters and return value that match those of MathOperator, so that the signatures match.

   Function AddNumbers( 
       ByVal x As Double, 
       ByVal y As Double 
   ) As Double 
       Return x + y
   End Function
   

3. Create a procedure named SubtractNumbers with a signature that matches MathOperator.

   Function SubtractNumbers( 
       ByVal x As Double, 
       ByVal y As Double
   ) As Double 
       Return x - y
   End Function
   

4. Create a procedure named DelegateTest that takes a delegate as a parameter.

   This procedure can accept a reference to AddNumbers or SubtractNumbers, because their signatures match the MathOperator signature.

   Sub DelegateTest( 
       ByVal x As Double, 
       ByVal op As MathOperator, 
       ByVal y As Double 
   )
       Dim ret As Double
       ret = op.Invoke(x, y) ' Call the method.
       MsgBox(ret)
   End Sub
   

5. Create a procedure named Test that calls DelegateTest once with the delegate for AddNumbers as a parameter, and again with the delegate for SubtractNumbers as a parameter.

   Protected Sub Test()
       DelegateTest(5, AddressOf AddNumbers, 3)
       DelegateTest(9, AddressOf SubtractNumbers, 3)
   End Sub
   

   When Test is called, it first displays the result of AddNumbers acting on 5 and 3, which is 8. Then the result of SubtractNumbers acting on 9 and 3 is displayed, which is 6.

See Also

Tasks

How to: Invoke a Delegate Method (Visual Basic)

Reference

AddressOf Operator (Visual Basic)

Delegate Statement

Other Resources

Delegates (Visual Basic)