What is IErrorInfo::GetIID good for?   

Hello newsgroup,

I'm intrigued what the method IErrorInfo::GetIID is good for. I have some 
thoughts about this method but I'm not sure whether they're right or wrong.

AFAIK, COM exceptions are handled like Win32 GetLastError, e.g. that only the 
last COM exception is stored by COM (if an older exception is never "caught" via 
GetErrorInfo, a newer exception will replace the old exception). That's why it 
can be quite difficult to determine whether an exception actually belongs to a 
specific method invocation.

Let's suppose I have an IExampleInterface pointer to object A. If I invoke a 
method on this interface pointer and get an HRESULT that indicates an exception, 
I cannot be sure whether it was object A which threw the exception: It may be 
the case that the implementation of A is not aware of exceptions, and the 
exception that is currently stored by COM stems from an earlier method 
invocation on a completely different object. As I understand, we can use the 
ISupportErrorInfo interface to determine whether object A supports exceptions 
over its IExampleInterface interface.

So far, so good.

If we have ruled out by above mentioned way that the exception is actually from 
object A, there would be no need to check whether the exception really belongs 
to the interface IExampleInterface. So what is IErrorInfo::GetIID good for?

Thanks in advance,
Stuart

date: Fri, 18 Jan 2008 14:23:07 +0100   author:   Stuart Redmann

Re: What is IErrorInfo::GetIID good for?   

Stuart Redmann  wrote:
> AFAIK, COM exceptions are handled like Win32 GetLastError, e.g. that
> only the last COM exception is stored by COM (if an older exception
> is never "caught" via GetErrorInfo, a newer exception will replace
> the old exception). That's why it can be quite difficult to determine
> whether an exception actually belongs to a specific method invocation.

This is what ISupportErrorInfo is for. If you receive an error code from 
a method call and want to get rich error information, you have to query 
the object for ISupportErrorInfo and call InterfaceSupportsErrorInfo, 
passing the IID of the interface whose method has just failed. If this 
succeeds, then you know IErrorInfo object is in fact associated with the 
most recent error. Otherwise, it's a stale object from some previous 
error.

For this to work, an interface that claims to support rich error 
information (via ISupportErrorInfo::InterfaceSupportsErrorInfo) has to 
explicitly set or explicitly clear IErrorInfo whenever one of its 
methods is about to return a failed HRESULT.

> So what
> is IErrorInfo::GetIID good for?

I don't see the point either.
-- 
With best wishes,
    Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not 
necessarily a good idea. It is hard to be sure where they are going to 
land, and it could be dangerous sitting under them as they fly 
overhead. -- RFC 1925

date: Fri, 18 Jan 2008 12:29:15 -0500   author:   Igor Tandetnik