WCF and delayed computations (C# yield keyword)

Never use delayed computations in WCF service contract implementation! Reason: IErrorHandler component is never invoked in this case. If there is any kind of exception within delayed computation, it would be increasingly hard for you to find out the reason even with WCF tracing.
Let's have a detailed look how WCF works (this is very simplified version):
int --> WCF serializer --> some other staff :) --> try { result = CallYourCustomCode() } catch{ CallToErrorHandler() ... } --> WCF serializer (result) --> out
So, if you have delayed computations created with help of yield C# keyword, CallYourCustomCode returns not the actual result, but kind of reference to your implementation. This reference will be resolved and executed during serialization (!). So, any exception during serialization will close wcf channel, get round of IErrorHandler, and produce sensless exception to the WCF client.