HOWTO: Use an OLE Control as an Automation Server

ID: Q146120

The information in this article applies to:

The Microsoft Foundation Classes (MFC), used with:
- Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2, 4.2b, 5.0, 6.0

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.

To override COleControl::IsInvokeAllowed(), you may follow these steps:

In ClassView, right-click the C<name>Ctrl class for your control, where <name> is the base name of your control.

Choose Add Member Function from the Context menu.

In the Function Type field, type: BOOL

In the Function Declaration field, type: IsInvokeAllowed( DISPID ).

Leave Access set at Public, and the check boxes cleared. Click OK.

Inside the new function (which should appear automatically under the cursor), between the curly braces, type: return TRUE;

Build the control.

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;
   }

Additional query words: ocx OLE Automation error

Keywords : kbcode kbole kbAutomation kbCtrl kbMFC kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 kbocx
Version : winnt:4.0,4.1,4.2,4.2b,5.0,6.0
Platform : winnt
Issue type : kbhowto