Marshaling a Delegate as a Callback Method


This sample demonstrates how to pass delegates to an unmanaged function expecting function pointers. A delegate is a class that can hold a reference to a method and is equivalent to a type-safe function pointer or a callback function.


When you use a delegate inside a call, the common language runtime protects the delegate from being garbage collected for the duration of that call. However, if the unmanaged function stores the delegate to use after the call completes, you must manually prevent garbage collection until the unmanaged function finishes with the delegate. For more information, see the HandleRef Sample and GCHandle Sample.

The Callback sample uses the following unmanaged functions, shown with their original function declaration:

  • TestCallBack exported from PinvokeLib.dll.

    void TestCallBack(FPTR pf, int value);
  • TestCallBack2 exported from PinvokeLib.dll.

    void TestCallBack2(FPTR2 pf2, char* value);

PinvokeLib.dll is a custom unmanaged library that contains an implementation for the previously listed functions.

In this sample, the LibWrap class contains managed prototypes for the TestCallBack and TestCallBack2 methods. Both methods pass a delegate to a callback function as a parameter. The signature of the delegate must match the signature of the method it references. For example, the FPtr and FPtr2 delegates have signatures that are identical to the DoSomething and DoSomething2 methods.

Public Delegate Function FPtr( ByVal value As Integer ) As Boolean
Public Delegate Function FPtr2( ByVal value As String ) As Boolean

Public Class LibWrap
    ' Declares managed prototypes for unmanaged functions.
    Declare Sub TestCallBack Lib "..\LIB\PinvokeLib.dll" ( ByVal cb _
        As FPtr, ByVal value As Integer )

    Declare Sub TestCallBack2 Lib "..\LIB\PinvokeLib.dll" ( ByVal cb2 _
        As FPtr2, ByVal value As String )
End Class

Public Class App
   Public Shared Sub Main()
        Dim cb As FPtr
        cb = AddressOf App.DoSomething
        Dim cb2 As FPtr2
        cb2 = AddressOf App.DoSomething2
        LibWrap.TestCallBack( cb, 99 )
        LibWrap.TestCallBack2( cb2, "abc" )
    End Sub 'Main

    Public Shared Function DoSomething( ByVal value As Integer ) As Boolean
        Console.WriteLine( ControlChars.CrLf + "Callback called with param: {0}", value )
        ' ...
        Return True
    End Function

    Public Shared Function DoSomething2( ByVal value As String ) As Boolean
        Console.WriteLine( ControlChars.CrLf + "Callback called with param: {0}", value )
        ' ...
        Return True
    End Function
End Class