HOWTO: Pass an Automation Object from VB to a C/C++ DLL

ID: Q181444

The information in this article applies to:

Microsoft Visual C++, 32-bit Editions, versions 4.0, 5.0, 6.0
on the following platforms: NT, Win95
Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0
on the following platforms: NT, Win95

SUMMARY

This article demonstrates how to pass an automation object from Microsoft 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.

Create a new Microsoft Visual C++ 5.0 "MFC AppWizard (dll)" project named "vcvbdll" and accept all of the default options.

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 **)&amp;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);
         }
      }

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

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.

Create a new Microsoft Visual Basic 5.0 project.

Add a Command button to Form1.

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

REFERENCES

For more information about VC++ OLE Automation or COleDispatchDriver, search the VC++ online help for "OLE Automation" or "COleDispatchDriver."

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


Keywords          : kbcode kbole kbAutomation kbMFC kbVC400 kbVC500 kbVC600 
Version           : WIN95:4.0; WINNT:4.0,5.0,6.0
Platform          : Win95 winnt 
Issue type        : kbhowto

Last Reviewed: July 21, 1999