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WCF InstanceContextMode and ConcurrencyMode

I recently ran into a concurrency issue with WCF on a customer’s system. I’ll get into the details later (though not too much detail… NDA ;) ), but it involved some testing of the various combinations of InstanceContextMode and ConcurrencyMode settings for a WCF service. The table below is an attempt to clarify the results…

InstanceContextMode
ConcurrencyMode
Resulting Behaviour
PerCall
Single
  • New InstanceContext created per call
  • Calls processed concurrently, because each call gets it's own instance

Multiple
Same as with ConcurrencyMode.Single
PerSession
Single
  • New InstanceContext is created for each client proxy connection
  • Each call is pooled and processed serially in a FIFO manner

Multiple
  • New InstanceContext is created for each client proxy connection
  • Each call is processed as soon as it arrives
  • A single instance of the proxy is required for concurrent processing from a single client
Singlton
Single
  • Single instance of an InstanceContext
  • Each call is pooled and processed serially in a FIFO manner

Multiple
  • Single instance of an InstanceContext
  • Each call is processed as soon as it arrives
  • The developer must manage thread-safety

It’s also worth noting that if you are using ServiceHost in an executable assembly, that the Main() method not be marked with the single threaded appartment attribute [System.STAThreadAttribute()]. This causes the application to be single threaded, and the additional threads will not be created regardless of the ConcurrencyMode.Multiple setting. Rather, it should be decorated with the multi-threaded appartment attribute [System.MTAThreadAttribute()].

Questions?

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