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

      #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.
         OleInitialize(NULL);
         {
            // Get CLSID for Excel.Application from registry.
            hr = CLSIDFromProgID(L"Excel.Application", &clsid);
            if(FAILED(hr)) {
               MessageBox(NULL, "Excel not registered.", "Error",
                          MB_SETFOREGROUND);
               return;
            }
            // 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",
                          MB_SETFOREGROUND);
               return;
            }

            // 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",
                    dispID);
            MessageBox(NULL, buf, "Debug Notice", MB_SETFOREGROUND);

            // Initiate parameters to set visible property to true.
            VariantInit(&parm1);
            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,
               DISPATCH_PROPERTYPUT | DISPATCH_METHOD,
               &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.
         OleUninitialize();

      }