Visual Basic 2012 Programmer's Reference

DELEGATES

A delegate is a type that refers to a subroutine, function, or other method. The method can be an instance method provided by an object, a class’s shared method, or a method defined in a code module. A delegate variable acts as a pointer to a subroutine or function. Delegate variables are sometimes called type-safe function pointers.

The Delegate keyword defines a delegate type and specifies the parameters and return type of the method to which the delegate will refer.

The following code demonstrates a delegate:

' Define a StringDisplayerType delegate to be a pointer to a subroutine
' that has a string parameter.
Private Delegate Sub StringDisplayerType(ByVal str As String)
 
' Declare a StringDisplayerType variable.
Dim DisplayStringRoutine As StringDisplayerType
 
' Assign the variable to a subroutine.
DisplayStringRoutine = AddressOf ShowStringInOutputWindow
 
' Invoke the delegate's subroutine.
DisplayStringRoutine("Hello world")

The code uses a Delegate statement to declare the StringDisplayerType to be a reference to a subroutine that takes a string as a parameter. Next, the code declares the variable DisplayStringRoutine to be of this type. This variable can hold a reference to a subroutine that takes a string parameter. The code then sets the variable equal to the ShowStringInOutputWindow subroutine. Finally, the code invokes the delegate’s subroutine, passing it a string.

The delegate in the preceding example holds a reference to a subroutine defined in a code module. ...