PRB: How to Use Automation Error Code & Help from Visual Basic

• Microsoft Visual Basic Standard and Professional Editions for Windows, version 3.0
• The Microsoft Foundation Classes (MFC), included with:
  • Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5
  • Microsoft Visual C++, 32-bit Editions, version 2.0

SYMPTOMS

Visual Basic version 3.0 does not report automation errors that result from OLE automation exceptions. Instead, it reports an err value of 440 and a description string of "OLE Automation exception." The actual wCode passed by the automation server is lost.

WORKAROUND

The More Information section in this article explains and illustrates a method you can use to use the error information and help context code returned by Visual Basic version 3.0 to start WinHelp from an MFC OLE automation server.

STATUS

This behavior is by design in Visual Basic version 3.0. Visual Basic for Applications, included in Excel version 5.0, Visual Basic 4.x and later, and other Microsoft products, returns the wCode correctly.

MORE INFORMATION

When an MFC automation server throws an OLE dispatch exception by calling AfxThrowOleDispatchException, an object of class COleDispatchException is built. Its member variables include an OLE SCODE, a help context, string error description, and wCode (an integer code used by Visual Basic).

The only information that the Visual Basic automation controller preserves is the string description. However, the MFC OLE automation server can preserve the information used in the call to AfxThrowOleDispatchException() and expose an additional "Help" method that uses this information to provide help to the user. From within the Visual Basic error handling code, Err is 440, Error(Err) is "OLE Dispatch Exception" and Error$ is the actual string passed by the server.

Sample Code Snippets

/* Compile options needed:
   standard MFC OLE project generated by AppWizard*/  

1. The method in the Automation server that throws the dispatch exception should retain the help context and wCode as member variable of the automation object as in this function:

   
      void CMyObject::Exception()
      {
         m_nIDContext = <some context>;
         m_nSomeCode = <some code>;
         AfxThrowOleDispatchException(m_nSomeCode,"String",m_nIDContext);
      } 




2. The Automation server should expose a method that uses those member variables to start WinHelp or another help engine with the actual context from the exception as in this code:

   
      short CMyObject::GetError()
      {
         // From here you can use the member variable code
         // and context to start WinHelp or do whatever
         // help code you need to do ...
         AfxGetApp()->WinHelp(m_nIDContext);
         return m_nSomeCode;
      } 




3. The Visual Basic error handling routine should call the exposed error method in the Automation server when a dispatch exception occurs. Here is example Visual Basic code:

   
      Sub Command1_Click ()
         On Error GoTo EHandle
         Dim a As object
         Set a = CreateObject("TestAuto")
         ' Next line causes exception:
         a.exception
      EHandle:
         Msg = "The error message for error number "
         Msg = Msg & Err & " is:" & NL & NL
         Msg = Msg & """" & Error(Err) & """"
         MsgBox Msg  ' Display message.
         ' Next line starts WinHelp engine by calling method in server.
         a.GetError
         Exit Sub
      End Sub 

Additional query words: 1.00 1.50 2.00 2.50 3.00

Keywords : kbinterop kbole kbtshoot kbMFC kbVBp300 kbVC100 kbVC150 kbVC200
Version : 1.00 1.50 3.00 | 2.00
Platform : NT WINDOWS
Issue type : kbprb