HOWTO: Use an OLE Control as an Automation Server

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2, 4.2b, 5.0

SUMMARY

In situations where an OLE container doesn't support control containment, you may want to use an OLE control as an automation server to gain access to its properties and methods. This article explains the necessary modifications you need to make in order for an OLE control to function as a normal automation server.

MORE INFORMATION

Prior to Visual C++ 4.0, an OLE Control could be used as an automation server without any modification. However, in MFC 4.0, the framework's implementation of IDispatch::Invoke calls the virtual function IsInvokeAllowed to determine if an automation server is in the appropriate state to handle automation calls. The default implementation in CCmdTarget::IsInvokeAllowed returns TRUE, implying that a server can handle automation calls.

In the case of an OLE control, COleControl::IsInvokeAllowed checks to see if the control has been either initialized or loaded properly through the persistent storage interfaces. If the control has the appropriate state information, then this function returns TRUE. When an OLE control is created as a normal automation server, it is not created as an embedding in the client. Hence, none of the persistent state initialization will take place, which thereby causes IsInvokeAllowed to return FALSE. The effect of this is that a call to an automation object method generates a run-time error of (8000ffff) : "Catastrophic failure".

In order to use an OLE control only as an automation server, you need to override COleControl::IsInvokeAllowed() and return TRUE. If any of the control's properties and methods should not be accessed when invoked as a normal automation server, then that automation function could be bypassed and/or an error code can be returned when COleControl::m_bInitialized is FALSE.

For example, the default OLE control behavior may pose a problem when an automation client calls CreateDispatch to create the OLE control as an automation server. Prior to making the above modification, CreateDispatch may succeed; however, any automation calls on the OLE control would fail.

Sample Code

   BOOL CMyOleControl::IsInvokeAllowed (DISPID)
   {
      // You can check to see if COleControl::m_bInitialized is FALSE
     // in your automation functions to limit access.
     return TRUE;
   }