PRB: COleControl::Serialize Not Called with VB as Container

The information in this article applies to:

• Standard and Professional Editions of Microsoft Visual Basic programming system for Windows, version 4.0
• The Microsoft Foundation Classes (MFC) included with:

      - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0
      - Microsoft OLE Control Developer's Kit (CDK)
  

SYMPTOMS

Although OLE controls are used with certain control containers (such as Microsoft Visual Basic version 4.0), the Serialize function for the control- derived class is not called. These containers allow the control to store its persistent information either by using the property sets or by using the IPersistPropertyBag interface. Both these methods call COleControl::DoPropExchange directly without calling the control's Serialize function.

The resolution section of this article discusses a technique that you can use in the DoPropExchange method to store CObject-derived objects.

CAUSE

COleControl::Serialize is called by the framework when an OLE control container uses one of the following persistent storage interfaces for loading and saving the control: IPersistStorage, IPersistStreamInit, or IPersistMemory.

If a control container uses any other method to store the control's persistent information, then Serialize for the control-derived class will not be called. Microsoft Visual Basic, for example, uses either IPersistPropertyBag or property sets to store the persistent information for an OLE control; therefore, the Serialize function for a control is not called when Visual Basic is used as the control container.

RESOLUTION

Although there is no direct support for serializing CObject-derived objects in COleControl::DoPropExchange, you might want to use the following technique to store objects in an OLE control:

1. Allocate a block of memory with GlobalAlloc.

      // cbGuess is a guess of how much memory will be needed.
      // If more is needed, CSharedFile will reallocate.
      HGLOBAL hMem = GlobalAlloc(GPTR, cbGuess);
      BYTE *pbMem = (BYTE *)hMem;
   
   

2. Construct an instance of CSharedFile and attach it to the memory block, starting four bytes in. Because the CSharedFile class is not yet documented, include afxpriv.h:

   CSharedFile file; file.Attach(pMem + sizeof(DWORD), cbGuess - sizeof(DWORD));

3. Construct an instance of CArchive on the file:

   CArchive ar(&file, CArchive::store);

4. Write the CObject-derived objects into the archive:

      // store data in the archive
      // for example, if m_myObject is a CObject-derived object, then
      m_myObject.Serialize(ar);
   
   

5. Get the length of the file and write it into the first DWORD of the memory block:

   *(DWORD*)pbMem = file.GetLength();

6. Pass the memory block to PX_Blob:

   PX_Blob(pPX, _T("MyObjects"), hMem);

   ARTICLE-ID: Q137333
   TITLE     : DOCERR: How to Use the PX_Blob Function

STATUS

This behavior is by design.

MORE INFORMATION

An OLE control generated using ControlWizard can read and write its persistent state using one of the following interfaces: IPersistMemory, IPersistStorage, IPersistStreamInit, IPersistPropertyBag (not implemented in versions before Visual C++ 4.x), and IDataObject through the property sets implementation. Each of these interfaces with the exception of IPersistPropertyBag and IDataObject call COleControl::Serialize passing in a CArchive. This archive could be used to store CObject-derived objects as part of the control's persistence.

Some OLE control containers (like Microsoft Visual Basic) use the "save as text" mechanism in order to allow as much of the OLE control's state to be represented in a human-readable format. For optimizing this mechanism, the interfaces IPropertyBag and IPersistPropertyBag are used and therefore are recommended for containers like Visual Basic. IPropertyBag is implemented by the container and is roughly analogous to IStream. IPersistPropertyBag is implemented by controls and is roughly analogous to IPersistStream(Init).

Visual Basic uses the control's IPersistPropertyBag interface, if one is implemented by the control, or it uses the property sets. Property sets are communicated from and to the control through IDataObject::GetData and IDataObject::SetData, implemented by the control. Note that OLE controls generated using earlier versions of Visual C++ don't provide an implementation for IPersistPropertyBag.

The implementation provided by the MFC framework for IPersistPropertyBag and property sets directly call COleControl::DoPropExchange passing in an instance of either CPropbagPropExchange or CPropsetPropExchange respectively.

REFERENCES

OLE Controls Inside Out - by Adam Denning