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Programming WCF Services by Juval Lowy

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Service Concurrency Mode

Concurrent access to the service instance is governed by the ConcurrencyMode property of the ServiceBehavior attribute:

public enum ConcurrencyMode
{
   Single,
   Reentrant,
   Multiple
}

[AttributeUsage(AttributeTargets.Class)]
public sealed class ServiceBehaviorAttribute : ...
{
   public ConcurrencyMode ConcurrencyMode
   {get;set;}
   //More members
}

The value of the ConcurrencyMode enum controls if and when concurrent calls are allowed on the service instance.

ConcurrencyMode.Single

When the service is set with ConcurrencyMode.Single, WCF will provide automatic synchronization to the service instance and disallow concurrent calls by associating the service instance with a synchronization lock. Every call coming into the service must first try to acquire the lock. If the lock is unowned, the caller will lock the lock and be allowed in. Once the operation returns, WCF will unlock the lock and thus allow another caller in. The important thing is that only one caller at a time is ever allowed. If there are multiple concurrent callers while the lock is locked, the callers are all placed in a queue, and are served out of the queue in order. If the call times out while blocked, WCF will remove the caller from the queue and the client will get a TimeoutException. ConcurrencyMode.Single is the WCF default setting, so these definitions are equivalent:

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)] class MyService : IMyContract {...} [ServiceBehavior(InstanceContextMode ...

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