Programming WCF Services, 2nd Edition

Singleton Service

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

Tip

A singleton hosted in the WAS is created when the host process is launched (typically only when the first request to any service in that process is made).

Using a singleton does not require clients to maintain a logical 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, during the call the singleton will have the same session ID as the client (binding permitting), but closing the client proxy will terminate only the transport session, not the singleton instance. 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 proxy or 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 ...