[pywin32-bugs] [ pywin32-Bugs-986673 ] Python exceptions through COM custom interface

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Bugs item #986673, was opened at 2004-07-07 15:54
Message generated for change (Comment added) made by prittenh
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Category: com
Group: None
Status: Open
Resolution: None
Priority: 6
Submitted By: Phil Rittenhouse (prittenh)
Assigned to: Mark Hammond (mhammond)
Summary: Python exceptions through COM custom interface 

Initial Comment:
The problem occurs with Python COM servers raising 
COM exceptions across a custom interface. 

Using the dispatch interface works fine, but in the 
custom interface the exception's error information is 
lost.  The problem appears to be in 
PyGatewayBase::InterfaceSupportsErrorInfo() in 
PyGatewayBase.cpp. It is called to determine whether 
the GetErrorInfo API should be used to extract an error 
after an error HRESULT is returned from an interface.

The function is passed an IID of an interface, the code 
compares this to the return value of GetIID() which is 
always IID_IUnknown.  The comparison fails, so the 
function returns S_FALSE and no error info is extracted.

The comment above the definition of GetIID() says:
// Currently this is used only for ISupportErrorInfo,
// so hopefully this will never be called in this base class.
// (however, this is not a rule, so we wont assert or 
anything!)

But it appears that it is called in the base class (or at 
least not overloaded by anyone inheriting from it).

Everything seems to work fine if you just change 
InterfaceSupportsErrorInfo() to always return S_OK.





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>Comment By: Phil Rittenhouse (prittenh)
Date: 2004-10-22 18:48

Message:
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My colleague David has created a simple COM server and 
client to demonstrate the issue (see attached).

The Python COM server is fairly generic with one method
that simply raises an exception.   The C# client connects to 
the COM server via the custom interface and displays the 
results returned from the COM exception.

Our workaround of always returning S_OK from 
InterfaceSupportsErrorInfo() may not be the best solution.  
Perhaps the more relevant question for me to ask is why 
does GetIID() always returns IID_IUnknown in this situation?

Thanks for your help,
Phil

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Comment By: Mark Hammond (mhammond)
Date: 2004-10-08 23:45

Message:
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user_id=14198

What do you mean by "custom interface" in this context?  A
custom interface should be implemented such that GetIID()
returns other than IUnknown.  The ISupportErrorInfo
semantics indicate we do need to check the IID.  I'll need
more specific details.

Thanks

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