Q181444: HOWTO: Pass an Automation Object from VB to a Visual C/C++ DLL

Article: Q181444
Product(s): Microsoft C Compiler
Version(s): 4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbole kbAutomation kbMFC kbOSWin2000 kbVC400 kbVC500 kbVC600
Last Modified: 12-JUN-2002

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Editions, versions 4.0, 5.0, 6.0 
- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual C++.NET (2002) 
- the operating system: Microsoft Windows 95 
- the operating system: Microsoft Windows NT 
- the operating system: Microsoft Windows 2000 
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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.

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

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


Additional query words:

======================================================================
Keywords          : kbcode kbole kbAutomation kbMFC kbOSWin2000 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbOSWin2000 kbOSWin95 kbVBSearch kbAudDeveloper kbOSWinSearch kbVB400Search kbVB400 kbVC500 kbVC600 kbVC32bitSearch kbOSWinNTSearch kbVCNET kbVC500Search
Version           : :4.0,5.0,6.0
Issue type        : kbhowto

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