BUG: Blob Persistent Data Incorrect for Ported OLE Control

An OLE control using PX_Blob(), created using the Control Developer's Kit (CDK) that shipped with Visual C++ 2.2 or earlier and saved in a Visual Basic 4.0 form, may give undefined behavior if the control is ported to Visual C++ 4.0 or 4.1 and the Visual Basic form containing the control is reopened. To read in the blob data correctly, you must force the container to use an interface other than IPropertyBag to serialize the data. In this case, a 20-byte header will not be expected and the blob data should be read in properly. The technique demonstrated in the "Sample Code" section of this article unexposes the control's IPersistPropertyBag interface thereby forcing the container to use an interface other than IPropertyBag to read in the blob data.

OLE Controls created using the CDK that shipped with Visual C++ 2.2 or earlier did not implement the IPersistPropertyBag interface. Controls created with Visual C++ 4.0 or 4.1 do implement IPersistPropertyBag.

The undefined behavior is due to Visual Basic's implementation of IPropertyBag. Visual Basic is currently adding a 20-byte header to a blob property's persistent data, and expecting that header to be in any blob persistent data previously saved. In the case of controls created previously to Visual C++ 4.0 or 4.1, IPersistPropertyBag, which corresponds to the container's IPropertyBag was not implemented. Therefore, the container used a different means to serialize the blob data which did not attach a 20 byte header. The undefined behavior may behave a number of ways including the blob data being reinitialized, blob data being initialized incorrectly, or the control not being created altogether.

Microsoft has confirmed this to be a bug in Microsoft Visual Basic 4.0.

Sample Code to Demonstrate Workaround

The following is a step by step process to unexpose the IPersistPropertyBag interface in an OLE control generated with Visual C++ 4.0 or 4.1. This is most easily done by implementing a new interface map, which does not expose the IPersistPropertyBag interface.

1. In the declaration of your COleControl derived class (in the header file), add a DECLARE_INTERFACE_MAP() macro if there isn't one already there.

      class CBlobTestCtrl : public COleControl
      {
      ...
          DECLARE_INTERFACE_MAP()
      };
   						

2. In the .cpp file for the control, implement a new interface map specifying all but IID_IPersistPropertyBag.

      BEGIN_INTERFACE_MAP(CBlobTestCtrl, CWnd)  // use CWnd not COleControl
          INTERFACE_PART(CBlobTestCtrl, IID_IOleObject, OleObject)
          INTERFACE_PART(CBlobTestCtrl, IID_IConnectionPointContainer,
            ConnPtContainer)
          INTERFACE_PART(CBlobTestCtrl, IID_IOleControl, OleControl)
          INTERFACE_PART(CBlobTestCtrl, IID_IPersist, PersistStorage)
          INTERFACE_PART(CBlobTestCtrl, IID_IPersistMemory, PersistMemory)
          INTERFACE_PART(CBlobTestCtrl, IID_IPersistStreamInit,
            PersistStreamInit)
          INTERFACE_PART(CBlobTestCtrl, IID_IOleInPlaceObject,
            OleInPlaceObject)
          INTERFACE_PART(CBlobTestCtrl, IID_IOleInPlaceActiveObject,
            OleInPlaceActiveObject)
          INTERFACE_PART(CBlobTestCtrl, IID_IDispatch, Dispatch)
          INTERFACE_PART(CBlobTestCtrl, IID_IOleCache, OleCache)
          INTERFACE_PART(CBlobTestCtrl, IID_IViewObject, ViewObject)
          INTERFACE_PART(CBlobTestCtrl, IID_IViewObject2, ViewObject)
          INTERFACE_PART(CBlobTestCtrl, IID_IDataObject, DataObject)
          INTERFACE_PART(CBlobTestCtrl, IID_ISpecifyPropertyPages,
            SpecifyPropertyPages)
          INTERFACE_PART(CBlobTestCtrl, IID_IPerPropertyBrowsing,
            PerPropertyBrowsing)
          INTERFACE_PART(CBlobTestCtrl, IID_IProvideClassInfo,
            ProvideClassInfo)
          INTERFACE_PART(CBlobTestCtrl, IID_IProvideClassInfo2,
            ProvideClassInfo)
          INTERFACE_PART(CBlobTestCtrl, IID_IPersistStorage, PersistStorage)
      END_INTERFACE_MAP()
   						

MFC Technical Note #38 about "MFC/OLE IUnknown implementation" Inside Ole Second Edition by Kraig Brockschmidt