By and large, the service instance mode is strictly a service-side implementation detail that should not manifest itself on the client side in any way. To support that and a few other local service-side aspects, WCF defines the notion of behaviors. A behavior is a local attribute of the service or the client that does not affect its communication patterns. Clients should be unaware of service behaviors, and behaviors do not manifest themselves in the service’s binding or published metadata.
WCF defines two types of declarative service-side behaviors, governed by two corresponding attributes. The ServiceBehaviorAttribute is used to configure service behaviors; that is, behaviors that affect all endpoints (all contracts and operations) of the service. Apply the ServiceBehavior attribute directly on the service implementation class.
Use the OperationBehaviorAttribute to configure operation behaviors; that is, behaviors that affect only the implementation of a particular operation. The OperationBehavior attribute can be applied only on a method that implements a contract operation, never on the operation definition in the contract itself.
The ServiceBehavior attribute is used to configure the service instance mode. As shown in Example 1, the attribute defines the InstanceContextMode property of the enum type InstanceContextMode. The value of the InstanceContextMode enum controls which instance mode is used for the service.
Example 1. ServiceBehaviorAttribute used to configure the instance context mode
public enum InstanceContextMode{ PerCall, PerSession, Single}[AttributeUsage(AttributeTargets.Class)]public sealed class ServiceBehaviorAttribute : Attribute,...{ public InstanceContextMode InstanceContextMode {get;set;} //More members}The enum is correctly called InstanceContextMode rather than InstanceMode because it actually controls the instantiation mode of the context hosting the instance, rather than that of the instance itself. By default, however, the instance and its context are treated as a single unit, so the enum does control the life of the instance as well.
