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HOWTO: Use OLE Automation from a C Application Rather Than C++

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If you need to program in C, rather than C++, OLE Automation can be difficult because the Component Object Model (COM), the building blocks of OLE, was designed around the binary layout of a C++ class. The header files included by WINDOWS.H, however, have built-in support for C compilations; therefore, you can mimic the behavior of a C++ object and make OLE Automation calls. This article describes how to build a Win32 console application, using C, that starts up Microsoft Excel and makes it visible via OLE Automation.

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More information

Use the following method:
  1. Create a new "Win32 Console Application" project in Microsoft Visual C++ 5.0.
  2. Add a Main.c (not Main.cpp) text file to your project.
  3. Add the following code to your Main.c file:
          #include <stdio.h>
          #include <windows.h>
          void main(void) {
             IDispatch *pDisp; // Main IDispatch pointer.
             unsigned short *ucPtr; // Temporary variable to hold names.
             DISPID dispID; // Temporary variable to hold DISPIDs.
             CLSID clsid; // Holds CLSID of server after CLSIDFromProgID.
             HRESULT hr; // General error/result holder.
             char buf[8192]; // Generic buffer for output.
             // IDispatch::Invoke() parameters...
             DISPPARAMS dispParams = { NULL, NULL, 0, 0 };
             VARIANT parm1;
             DISPID dispidNamed = DISPID_PROPERTYPUT;
             // Initialize OLE Libraries.
                // Get CLSID for Excel.Application from registry.
                hr = CLSIDFromProgID(L"Excel.Application", &clsid);
                if(FAILED(hr)) {
                   MessageBox(NULL, "Excel not registered.", "Error",
                // Start Excel97, Excel 2000, or Excel 2002 and get its IDispatch pointer.
                hr = CoCreateInstance(&clsid, NULL, CLSCTX_LOCAL_SERVER,
                                      &IID_IDispatch, (void **)&pDisp);
                if(FAILED(hr)) {
                   MessageBox(NULL, "Couldn't start Excel.", "Error",
                // Get the 'visible' property's DISPID.
                ucPtr = L"Visible";
                pDisp->lpVtbl->GetIDsOfNames(pDisp, &IID_NULL, &ucPtr, 1,
                                             LOCALE_USER_DEFAULT, &dispID);
                sprintf(buf, "DISPID for 'Visible' property = 0x%08lx",
                MessageBox(NULL, buf, "Debug Notice", MB_SETFOREGROUND);
                // Initiate parameters to set visible property to true.
                parm1.vt = VT_I4;
                parm1.lVal = 1; // true
                // One argument.
                dispParams.cArgs = 1;
                dispParams.rgvarg = &parm1;
                // Handle special-case for property-puts!
                dispParams.cNamedArgs = 1;
                dispParams.rgdispidNamedArgs = &dispidNamed;
                // Set 'visible' property to true.
                hr = pDisp->lpVtbl->Invoke(pDisp,
                   dispID, &IID_NULL, LOCALE_SYSTEM_DEFAULT,
                   &dispParams, NULL, NULL, NULL
                if(FAILED(hr)) {
                   sprintf(buf, "IDispatch::Invoke() failed with %08lx", hr);
                   MessageBox(NULL, buf, "Debug Notice", MB_SETFOREGROUND);
                // All done.
                MessageBox(NULL, "done.", "Notice", MB_SETFOREGROUND);
             // Uninitialize OLE Libraries.
  4. Compile and run.

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For more general information regarding OLE, COM, and Automation, consult the book "Inside OLE" by Kraig Brockschmidt (Microsoft Press).

For more information about IDispatch, consult the Microsoft Visual C++ online help.

For more information about COM, OLE, and automating Microsoft Excel using Microsoft Visual C++, see Chapters 23-27 of the book "Inside Visual C++" by David J. Kruglinski (Microsoft Press).

(c) Microsoft Corporation 1999, All Rights Reserved. Contributions by Joe Crump, Microsoft Corporation.

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Keywords: KB181473, kbhowto

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Article Info
Article ID : 181473
Revision : 7
Created on : 2/12/2007
Published on : 2/12/2007
Exists online : False
Views : 577