Concurrency issues arise when multiple threads attempt to access the same resources at runtime. When requests arrive to a service, the service model dispatches the message on a thread from the thread pool. Certainly, if multiple clients call the same service, multiple concurrent requests can arrive at the service. The particular service object handling each request is based on the instancing mode for the service. For PerCall services, a new service object is granted for each request. For PerSession services, the same service object receives requests from the same client. For Single instancing mode, all client requests are sent to the same singleton service object. Based on this alone, PerSession services are at risk of concurrent access when the client is multithreaded, and Single services are perpetually at risk.

By default, only one request thread is granted access to any service object, regardless of the instancing mode. The concurrency setting for a service is controlled by the ConcurrencyMode property of the ServiceBehaviorAttribute. The default setting is Single, as shown here:

public class MessagingService : IMessagingService

This property can be set to any of the following ConcurrencyMode enumeration values:


A single request thread has access to the service object at a given time.


A single request thread has access to the service object, but the thread can exit the service and reenter without ...

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