Rory Primrose

Learn from my mistakes, you don't have time to make them yourself

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ThreadStatic gotcha with ASP.Net

I had a requirement recently in a service implementation that business rules at any point throughout the execution of a service request could determine a message to return in the header of the response. Because these circumstances don’t affect the execution of the request (no exception is thrown), I needed a loosely coupled way of recording all the required messages and handling them higher up the stack.

Storing data against the executing thread was the obvious solution. I considered manually storing information in the context data of the thread, but then thought this would be a great opportunity to use the ThreadStaticAttribute. I coded this up and it all seemed to work well.

Yesterday a bug was raised against this code because messages unrelated to the service request were being returned. I looked at the code and rechecked the ThreadStatic documentation. It shouldn’t be possible, but it was definitely happening.

I quickly realised that the thread must be being reused across multiple service requests. This reminded me that ASP.Net uses a ThreadPool and it would not be doing any clean up of ThreadStatic data as it reused an existing thread for a new service request.

The simple fix was to reset the ThreadStatic reference when the response is generated.

Written on September 23, 2009