Brain Spew - Neville Mehta's Blog

Saturday, December 02, 2006

What is the UseSynchronizationContext property in WCF

The UseSynchronizationContext from the ServiceBehavior attribute in WCF is used to determine which thread your service will execute on.

Basically, if you have something like this on your service class at the time of creating your ServiceHost:

[ServiceBehavior(UseSynchronizationContext = true)]
class MyServiceClass: IMyServiceContract
// Implementation

then the value from the System.Threading.SynchronizationContext.Current is read and cached so that when a request comes to the service, the host can marshal the request onto the thread that the host was created on using the cached SynchronizationContext.

If you are familiar with the concept behind methods like Control.Invoke, Control.BeginInvoke() and Control.InvokeRequired() then you will be familiar with what the System.Threading.SynchronizationContext class is trying to achieve. Basically the SynchronizationContext.Send() and SynchronizationContext.Post() methods are similar to the Control.Invoke() and Control.BeginInvoke() methods.

If you try to host a service in, for example, a WPF application and also call that service from the same thread in the WPF application, you will notice that you get a deadlock when the client tries to call the service. The reason for this is that the default value of the UseSynchronizationContext is true and so when you create the ServiceHost on the UI thread of the WPF application, then the current synchronization context is a DispatcherSynchronizationContext which holds a reference to a System.Windows.Threading.Dispatcher object which then holds a reference to the current thread. The DispatcherSynchronizationContext will then be used when a request comes in to marshal requests onto the UI thread. But if you are calling the service from the UI thread then you have a deadlock when it tries to do this!!

Ive types this up pretty quick while its still in my head so hopefully it makes sense :-)

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  • Dude,

    That was really helpful- it solved me the deadlock issue iv'e been working on for the last 3 days!

    I don't know why this important attribute and its (very problematic) default value isn't published or discussed in an official platform.


    By Anonymous Anonymous, at 10:42 pm  

  • I'm not as hard up as Anonymous was, but MAN, you just saved me hours of smashing my head against this "why doesn't it ever farkin' DO ANYTHING?" wall.

    Thanks a lot!

    (And thanks to the service trace for suggesting the idea of looking at the property at the higher level. Why this isn't in the Step By Step WCF book, I can't imagine.)

    By Blogger Sigivald, at 12:44 pm  

  • Thank you!!! This was a life saver, have been torturing myself for the last 4 days.

    By Anonymous Anonymous, at 11:14 am  

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