I am attempting to make use of the XamDataGrid's external summaries feature. I have registered a QuerySummaryResult event handler and in that handler I am using QuerySummaryResultEventArgs.SetSummaryValue to provide the result of my custom summary calculation. This works - I can indeed provide a summary value and observe it in the grid. But, here is my problem: how do I determine the set of data records that need to be summarized? My QuerySummaryResult handler is being called to handle the SummaryDefinitions which I have placed in the xaml (summarize all of the data). My QuerySummaryResult handler is also being called to provide the summaries needed when the users performs a group by operation. How do I determine which data items need to be summarized?