FIX: Error in COleDispatchException Constructor

WINDOWS        | WINDOWS NT

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++ for Windows, versions 1.5, 1.51, 1.52
      - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0
  

SYMPTOMS

In an MFC application, AfxThrowOleDispatchException is used to throw an exception from within an OLE Automation function. When Microsoft Visual Basic is used as the automation client application, it will not be able to correctly interpret the error code passed to AfxThrowOleDispatchException.

CAUSE

AfxThrowOleDispatchException constructs and throws a COleDispatchException object, which is used to handle exceptions specific to the OLE IDispatch interface. The constructor of COleDispatchException incorrectly initializes the COleDispatchException object, resulting in the Visual Basic client being unable to interpret the error code of the exception.

RESOLUTION

To implement the proper behavior for throwing an OLE dispatch exception from an MFC server, do not use AfxThrowOleDispatchException. Instead, construct your own COleDispatchException, set m_scError = 0, and throw the exception yourself as illustrated in the sample code in this article.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 4.1.

MORE INFORMATION

COleDispatchException has two public members (m_wCode and m_scError) which are mapped by COleDispatchException::Process() to the wCode and scode members of an EXCEPINFO structure. An EXCEPTINFO structure is used by an automation object to describe an exception that occurred during IDispatch::Invoke. When an automation object raises an exception, it should store an error code inside either wCode or scode but not both. If an error code is passed to the constructor of the COleDispatchException object, the values of both wCode and scode in the EXCEPINFO structure will be set.

Sample Code

// Insert the following code instead of AfxThrowOleDispatchException

                 _T("Some Exception"), 200, 425);

THROW(pException);

REFERENCES

"Inside OLE" second edition by Kraig Brockschmidt, published by Microsoft Press, Chapter 14, pages 658 - 660.