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 ...
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