Programming WCF Services

Singleton Service

The singleton service is the ultimate sharable service. When a service is configured as a singleton, all clients independently get connected to the same single well-known instance, regardless of which endpoint of the service they connect to. The singleton service lives forever and is only disposed of once the host shuts down. The singleton is created exactly once, when the host is created.

Using a singleton does not require the clients to maintain a session with the singleton instance, or to use a binding that supports a transport-level session. If the contract the client consumes has a session, then during the call the singleton will have the same session ID as the client (binding permitting), but closing the client proxy will only terminate the session, not the singleton instance. In addition, the session will never expire. If the singleton service supports contracts without a session, those contracts will not be per-call: they too will be connected to the same instance. By its very nature, the singleton is shared, and each client should simply create its own proxies to it.

You configure a singleton service by setting the InstanceContextMode property to InstanceContextMode.Single:

[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single)]
class MySingleton : ...
{...}

Example 4-6 demonstrates a singleton service with two contracts, one that requires a session and one that does not. As you can see from the client call, the calls on the two endpoints ...