Andrej Tozon's blog

While I was watching the Silverlight TV episode on common WCF service issues with Silverlight (Silverlight TV 46: What's Wrong with my WCF Service?) I remembered one more issue that bit my neck a while back and thought it was worth pointing it out again... It was one of those “Service returned ‘Not found’” issues, except nothing I would do seemed to help solving it.

My simple WCF service was returning an IEnumerable<T> and I was using a Linq query for filtering the results before returning them to the client. Can you guess where I’m going to?

Let’s step back and take a look at the symptoms I was facing: the original problem was that service call returned with the infamous “NotFound” exception.

I would try using Client Http Stack to get more info about the exception – nothing. Reverted to Browser Http Stack and implemented service stack to return Faults – still nothing. I wrapped my service method call into a try…catch statement inside service method – no errors would’ve been caught on the server side and the NotFound error would persist… Furthermore, stepping through the service code didn’t throw any exceptions and the error only happened when service call returned one or more records. When there was zero, it returned fine… Hmmm… Something happening on the wire? Fiddler wasn’t much of a help either.

What helped at last was a careful examination of the service method call. Facepalm! I was failing in materializing the Linq query by not forcing its evaluation with the .ToList() extension method (or similar) and data never made it to the other side.

Lesson learned – when your services return IEnumerable and you’re using Linq to query/transform data double check that you’re returning something that client would actually have a chance enumerating.