How to: Pass Procedures to Another Procedure - Visual Basic (original) (raw)

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
  1. 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
  1. 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
  1. 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
  1. 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