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.
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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.
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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.
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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 |
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