One of the problems that existed in the CTP3 edition of StreamInsight was an error that occurred if you tried to access the grouping key from within your projection expression. That was a real issue as you always need access to the key. It's a bit like using a GROUP BY in TSQL and then not including the columns you're grouping by in the SELECT clause. You'd see the results but not be able to know which results are which. Look at the following code: var laneSpeeds = from e in vehicleSpeeds group e...(read more)