How to pass an automation object from Visual Basic to a C/C++ DLL

MORE INFORMATION

The basic concept is to pass your automation object "ByVal As Object", expect an IUnknown pointer in the DLL and then call IUnknown::QueryInterface() for the IDispatch interface.

Steps to create DLL

The following steps illustrate how to create a Microsoft Visual C++ DLL project and then add and export the talkToObject() routine.

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.

         // Helper message function...
         void ShowMsg(char *msg, HRESULT hr) {
               char buf[1024];
               if((long)hr) {
                  sprintf(buf, "%s, HRESULT = %08lx", msg, (long)hr);
               }
               else {
                  sprintf(buf, "%s", msg);
               }
               ::MessageBox(NULL, buf, "C/C++ DLL message",
                            MB_SETFOREGROUND | MB_OK);
         }
   
         // The exported function, called from Microsoft Visual Basic...
         void _stdcall talkToObject(IUnknown *pUnk) {
               // QueryInterface for a IDispatch pointer...
               IDispatch *pDisp;
               HRESULT hr = pUnk->QueryInterface(IID_IDispatch,
                                                (void **)&pDisp);
               if(FAILED(hr)) {
                  ShowMsg("QueryInterface() failed", hr);
               }
               else {
                  ShowMsg("We got the dispatch pointer!!!", hr);
   
                  // Attach dispatch to a COleDispatchDriver class.
                  COleDispatchDriver disp(pDisp); // Uses constructor
   
                  // Set visible to FALSE...
                  static BYTE parms[] = VTS_BOOL;
                  disp.InvokeHelper(0x17, DISPATCH_PROPERTYPUT, VT_EMPTY,
                                    NULL, parms, FALSE);
   
   
                  ShowMsg("Microsoft Word 97 shouldn't be visible now.", 0);
   
                  // Set visible to TRUE...
                  disp.InvokeHelper(0x17, DISPATCH_PROPERTYPUT, VT_EMPTY,
                                    NULL, parms, TRUE);
                  ShowMsg("Microsoft Word 97 should now be visible again!",
                          0);
            }
         }
   					

3. Add the following line tothe end of the vcvbdll.def file so that the talkToObject function is exported:

         talkToObject
   					

4. Compile and then copy the .dll to the \Windows\System directory for testing.

Steps to create Visual Basic project

Next, build a Microsoft Visual Basic 5.0 project that automates Microsoft Word 97 and passes an Application object into the talkToObject function in vcvbdll.dll.

1. Create a new Microsoft Visual Basic 5.0 project.
2. Add a Command button to Form1.
3. Add the following code to Form1.

         Private Declare Sub talkToObject Lib "vcvbdll.dll" ( _
            ByVal pUnk As Object)
   
         Private Sub Command1_Click()
            Dim obj As Object
   
            ' Start automation to Microsoft Word 97.
            Set obj = CreateObject("Word.Application")
   
            ' Make Microsoft Word 97 visible.
            obj.Visible = True
            MsgBox "Preparing to call into C/C++ dll..."
   
            ' Pass automation interface to C/C++ dll.
            talkToObject obj
   
            ' Close Microsoft Word 97.
            obj.Quit
   
         End Sub