How To Callback Visual Basic Functions From a C DLL

With the new AddressOf operator in Microsoft Visual Basic 5.0, you can pass a DLL the pointer to a Microsoft Visual Basic function and have the DLL call that function through the pointer. This article gives you some sample code to get started and also demonstrates how to pass strings from your DLL to Microsoft Visual Basic.

To use this article, follow the steps below to create a Microsoft Visual Basic 5.0 .exe project and a Microsoft Visual C++ 5.0 .dll project. Then run your Microsoft Visual Basic program to test it.

Steps to Create the Microsoft Visual Basic 5.0 Project

1. Create a new Standard EXE project in Microsoft Visual Basic 5.0.
2. Add a Command button to Form1.
3. Add the following declare statement to the General Declarations section of Form1:

         Private Declare Sub ExecuteCallback Lib "vcvbdll" ( _
            ByVal pFunc as Long)
   
   						

4. Add the following code to the Click event of the Command button:

         Call ExecuteCallback(AddressOf MyCallback)
   
   						

5. Add a new module to the project, and add the following code to the module.

         Sub MyCallback(ByVal parm as String)
            MsgBox "You are inside the VB callback function!"
            MsgBox "Parameter passed in was: " & parm
         End Sub
   
   						

Steps to Create the Microsoft Visual C++ 5.0 Project

1. Create a new Microsoft Visual C++ 5.0 MFC AppWizard (dll) project named "vcvbdll" and accept all of the default options.
2. Add the following code to the end of the vcvbdll.cpp file:

         void __stdcall ExecuteCallback(long cbAddress) {
            // Declare the function pointer, with one string argument.
            typedef void (__stdcall *FUNCPTR)(BSTR pbstr);
            FUNCPTR vbFunc;
   
            // Point the function pointer at the passed-in address.
            vbFunc = (FUNCPTR)cbAddress;
   
            // Call the function through the function pointer.
            vbFunc(SysAllocString(L"Hi! This message came from your DLL!"));
         }
   
   						

3. Add the following line to the end of the vcvbdll.def file to export the ExecuteCallback function:

         ExecuteCallback
   
   						

4. Compile the project, and copy the .dll to the \Windows\System directory to test it.

Microsoft Visual Basic 5.0 Books Online: AddressOf

For additional information about using the AddressOf operator for callbacks, see the following article in the Microsoft Knowledge Base:

(c) Microsoft Corporation 1998, All Rights Reserved. Contributions by Joe Crump, Microsoft Corporation