INFO: COM Method Call In WM_PAINT Handler Returns 0x80010005

• The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++, 32-bit Editions, versions 4.2, 4.2b, 5.0

SUMMARY

When calling methods on a COM server from within OnDraw() or a WM_PAINT message handler of an MFC client application, you may receive an HRESULT of 0x80010005 (RPC_E_CANTCALLOUT_INEXTERNALCALL - it is illegal to call out while inside message filter) as the return value of a COM call to a server.

MORE INFORMATION

MFC has a default implementation of IMessageFilter(COleMessageFilter). IMessageFilter is used by COM servers and clients to selectively handle incoming and outgoing COM messages while waiting for a response from synchronous calls. MFC's implementation allows processing of WM_PAINT messages (to keep the UI updated) on the COM client side, while waiting on a synchronous COM call to a server (for example, a call to an automation method). This is done through IMessageFilter::MessagePending(). Thus, when calling a COM server method inside of a WM_PAINT handler, you could already be in the middle of a call to the server, and therefore receive the error 0x80010005.

As with any Windows application, the code in WM_PAINT handlers should be minimal. If at all possible, do not have any calls to COM servers in WM_PAINT handlers.

To resolve this problem you could use any one of the following three workarounds:

• In the WM_PAINT handler, post yourself a user-defined message; in the handler for the user-defined message, make the call to the COM server.

  -or-

  Create a secondary UI thread that creates and makes all calls to the COM servers.

  -or-

  Modify MFC's default implementation of MessageFilter::MessagePending(), which currently processes WM_PAINT messages. To do this, derive a class from COleMessageFilter and override the virtual function OnMessagePending():

        CMyMessageFilter : public COleMessageFiler
        {
           virtual BOOL OnMessagePending(const MSG* pMsg);
        }
  
        BOOL CMyMessageFilter::OnMessagePending(const MSG* pMsg)
        {
           //The base class function dispatches WM_PAINT
           //by returning FALSE no messages are being processed; the
           //user can add code here to appropriately handle messages.
           //WARNING: Not processing WM_PAINT messages will cause the user
           //         interface to appear to hang and not update until the
           //         current COM method call returns.
           return FALSE;
        }
  
  

   BOOL CmyApp::InitInstance()
   {
      // Initialize OLE libraries and registers default message filter.
      if (!AfxOleInit())
      {
         AfxMessageBox(IDP_OLE_INIT_FAILED);
         return FALSE;
      }

      CWinThread* pThread = AfxGetThread();
      if (pThread != NULL)
      {
         // Destroy message filter, thereby unregistering it.
         delete pThread->m_pMessageFilter;
         pThread->m_pMessageFilter = NULL;

         // Create the new message filter object.
         pThread->m_pMessageFilter = new CMyMessageFilter;
         ASSERT(AfxOleGetMessageFilter() != NULL);

         // Register the new message filter object.
         AfxOleGetMessageFilter()->Register();
      }
      ...
      ...
   }

REFERENCES

"Inside OLE", second edition, by Kraig Brockschmidt, Chapter 6, "Local/Remote Transparency," published by Microsoft Press.

Visual C++ Books Online and the MFC source code for the functions mentioned in this article.