Monday, December 14, 2009

Fatal Execution Engine Error when calling a WCF service in IIS

If you like nasty errors, this is one! It means more or less that the .NET framework itself has crashed. I have not seen this kind of errors a lot(luckily). Everytime we called a specific service, we succeeded in crashing the .NET framework.  After some investigation, we started to suspect the usage of IEnumerable<T> in one of our services. And finding this MS Connect call confirmed this suspicion.

The fault could be found in the DataContractSerializer class. Microsoft indicated that the DataContractSerializer issue is fixed in .NET 4.0, but there is not a hotfix available for .NET 3.5sp1 at this time.
The workaround they proposed  was to place all assemblies that contain types "T" used in contracts that have IEnumerable<T> into the GAC.  (In other words, if your contract has IEnumerable<T> elements, then all types T have to be strong-named & in the GAC.)

Why does this work?  The bug with DataContractSerializer apparently does not manifest itself when assemblies are loaded as "domain neutral" (shared across all appdomains.)  You can force strong-named/GAC'd assemblies to be loaded as "domain neutral" by using the LoaderOptimization attribute.  But if you're hosting in IIS, you are automatically getting LoaderOptimization(LoaderOptimization.MultiDomainHost) behavior for your application.  If you're not hosting in IIS, this bug doesn't seem to appear at all.

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