wcf callback exception after updating to .net 4.0

Question

I have a wcf service that uses callbacks with DualHttpBindings. The service pushes back a datatable of search results the client (for a long running search) as it finds them.

This worked fine in .Net 3.5. Since I updated to .Net 4.0, it bombs out with a System.Runtime.FatalException that actually kills the IIS worker process. I have no idea how to even go about starting to fix this. Any recommendations appreciated.

The info from the resulting event logs is pasted below:

---

An unhandled exception occurred and the process was terminated.

Application ID: /LM/W3SVC/2/ROOT/CP

Process ID: 5284

> Exception: System.Runtime.FatalException

> Message: Object reference not set to an instance of an object.

StackTrace: at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage4(MessageRpc& rpc) at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage31(MessageRpc& rpc) at System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean isOperationContextSet) at System.ServiceModel.Dispatcher.ChannelHandler.DispatchAndReleasePump(RequestContext request, Boolean cleanThread, OperationContext currentOperationContext) at System.ServiceModel.Dispatcher.ChannelHandler.HandleRequest(RequestContext request, OperationContext currentOperationContext) at System.ServiceModel.Dispatcher.ChannelHandler.AsyncMessagePump(IAsyncResult result) at System.Runtime.Fx.AsyncThunk.UnhandledExceptionFrame(IAsyncResult result) at System.Runtime.AsyncResult.Complete(Boolean completedSynchronously) at System.Runtime.InputQueue1.AsyncQueueReader.Set(Item item) at System.Runtime.InputQueue1.Dispatch() at System.ServiceModel.Channels.ReliableDuplexSessionChannel.ProcessDuplexMessage(WsrmMessageInfo info) at System.ServiceModel.Channels.ReliableDuplexSessionChannel.HandleReceiveComplete(IAsyncResult result) at System.ServiceModel.Channels.ReliableDuplexSessionChannel.OnReceiveCompletedStatic(IAsyncResult result) at System.Runtime.Fx.AsyncThunk.UnhandledExceptionFrame(IAsyncResult result) at System.Runtime.AsyncResult.Complete(Boolean completedSynchronously) at System.ServiceModel.Channels.ReliableChannelBinder1.InputAsyncResult1.OnInputComplete(IAsyncResult result) at System.Runtime.Fx.AsyncThunk.UnhandledExceptionFrame(IAsyncResult result) at System.Runtime.AsyncResult.Complete(Boolean completedSynchronously) at System.Runtime.InputQueue1.AsyncQueueReader.Set(Item item) at System.Runtime.InputQueue1.Dispatch() at System.Runtime.IOThreadScheduler.ScheduledOverlapped.IOCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* nativeOverlapped)
at System.Runtime.Fx.IOCompletionThunk.UnhandledExceptionFrame(UInt32 error, UInt32 bytesRead, NativeOverlapped* nativeOverlapped)
at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)

> InnerException: *> System.NullReferenceException*

Message: Object reference not set to an instance of an object.

StackTrace: at System.Web.HttpApplication.ThreadContext.Enter(Boolean setImpersonationContext) at System.Web.HttpApplication.OnThreadEnterPrivate(Boolean setImpersonationContext) at System.Web.AspNetSynchronizationContext.CallCallbackPossiblyUnderLock(SendOrPostCallback callback, Object state) at System.Web.AspNetSynchronizationContext.CallCallback(SendOrPostCallback callback, Object state) at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage4(MessageRpc& rpc)

Answer 1

OK - I found the answer. Very Strange, but simply put the following attribute on the WCF callback wrapper class:

[CallbackBehavior(UseSynchronizationContext=false)]

Thanks to Cauldwell.net for the answer: http://www.cauldwell.net/patrick/blog/CategoryView,category,CodeGen.aspx

From cauldwell.net:

The problem, it turned out, was that ASP.NET uses (by default) a little thing called the SynchronizationContext. As near as I can tell (I haven't researched this thoroughly, to be honest) one of it's jobs it to make sure that any callbacks get run on the UI thread, thereby obviating the need to call Control.Invoke like you do in WinForms. In my case, that additional lock was giving something fits, and it was trying to clean stuff up on a thread that wasn't around any more, hence to NullReferenceException.