Programming WCF Services, 2nd Edition

Callbacks and Client Safety

There are quite a few cases when a client might receive concurrent callbacks. For instance, if the client has provided a callback reference to multiple services, those services could call back to the client concurrently. Even if it has only provided a single callback reference, the service might launch multiple threads and use all of them to call on that single reference. Duplex callbacks enter the client on worker threads, and if they are processed concurrently without synchronization they might corrupt the client's state. The client must therefore synchronize access to its own in-memory state, as well as to any resources the callback thread might access. Similar to a service, a callback client can use either manual or declarative synchronization. The CallbackBehavior attribute introduced in Chapter 6 offers the ConcurrencyMode and the UseSynchronizationContext properties:

[AttributeUsage(AttributeTargets.Class)]
public sealed class CallbackBehaviorAttribute : Attribute,...
{
   public ConcurrencyMode ConcurrencyMode
   {get;set;}
   public bool UseSynchronizationContext
   {get;set;}
}

Both of these properties default to the same values as with the ServiceBehavior attribute and behave in a similar manner. For example, the default of the ConcurrencyMode property is ConcurrencyMode.Single, so these two definitions are equivalent:

class MyClient : IMyContractCallback
{...}

[CallbackBehavior(ConcurrencyMode = ConcurrencyMode.Single)] class MyClient : IMyContractCallback ...