KB949753: Problem: MFC Document Interface ON_UPDATE_COMMAND_UI exception handling

Question

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Link to Original Article from Microsoft Support

Problem: MFC Document Interface ON_UPDATE_COMMAND_UI exception handling

Action

An MFC document interface application is consuming an unusual amount of CPU during idle operation.  When attaching a debugger it shows that there are an extreme number of exceptions being thrown causing the high CPU usage.

 

Result

•	System operation degrades slight to moderately.
•	If using a debugger to log first chance exceptions, a single type exception will be logged to disk as quickly as it can be written. System operation will be hampered more significantly in this case.

Answer 1

Cause

MFC continually pumps the ON_UPDATE_COMMAND_UI message during Idle periods in an MFC documented interface application due to the menu.  The CControlBar forwards this message to the handler.  If your application is coded to throw exceptions within this event handler then it ends up in a loop of non-stop exceptions. 

Resolution

•	If possible alter code not to throw exceptions in this event processing as attached debuggers will be rendered less useful due to an overload.
•	If unable to remove the exception throwing code, add a Sleep() call to give up the remainder of the timeslice.  Given that the message only occurs during idle times this will not effect the performance of the application.

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