Calling Hidden Default Method of an OLE Automation Collection

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0, 4.1

SUMMARY

A technical article found on the Microsoft Developer Network CD-ROM titled "Implementing OLE Automation Collections," describes the functionality necessary to create an OLE automation collection. This includes creating a method called Item that returns the dispatch interface of the indicated item in the collection.

All OLE automation collection objects must implement the Item method to iterate through the objects of the collection. The implementation of the method might have been created hidden in the .ODL file for some applications. You cannot find these hidden methods by calling IDispatch::GetIdsOfNames(). However, in many situations, this hidden method is the default method of the collection, in which case you can invoke it with an dispatch ID of DISPID_VALUE.

Visual Basic 3.0 depends on this Item method being the default method of the collection object.

MORE INFORMATION

The following code sample illustrates using COleDispatchDriver::InvokeHelper() to call the Item method from the Resources collection class provided by Microsoft Project 4.1. The Item method is hidden and cannot be found in the type library of the object:

Sample Code

   void IterateCollection(void)
   {
      Application App; // wrapper class for the 'Application' object
      Project Proj;    // wrapper class for the 'Project' object
      Resources Res;   // wrapper class for the 'Resources' object
      CString lpszFileName ("c:\\temp\\project2.mpp");

      App.CreateDispatch("MSProject.Application");

      VARIANT vFileName;
      VariantInit(&vFileName);
      V_VT(&vFileName) = VT_BSTR;
      V_BSTR(&vFileName) = lpszFileName.AllocSysString();
      App.FileOpen(vFileName);

      Proj.AttachDispatch(App.GetActiveProject());
      Res.AttachDispatch(Proj.GetResources());

      VARIANT vCount;
      VariantInit(&vCount);
      V_VT(&vCount) = VT_I4;
      vCount = Res.Count();

      // the index of the collection starts with 1, not 0
      for (int i = 1 ; i <= vCount.lVal ; i++)
      {
         Resource r; // wrapper class for the 'Resource' object

         VARIANT vResult, vIndex, vStr;
         VariantInit(&vResult);
         VariantInit(&vIndex);
         V_VT(&vIndex) = VT_I2;
         V_I2(&vIndex) = i;
         static BYTE parms[] = VTS_VARIANT;

         Res.InvokeHelper(DISPID_VALUE, DISPATCH_METHOD, VT_VARIANT,
   (void*)&vResult, parms, &vIndex);
         r.AttachDispatch(vResult.pdispVal);

         vStr = r.GetName();
         // ... 'vStr' now contains the Name property
      }
   }