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Home Interviews What are the different instance management modes available in WCF? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 27 May 2025
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In WCF (Windows Communication Foundation), instance management defines how service objects are created and maintained for client requests. WCF provides three main instance context modes, configured using the [ServiceBehavior] attribute's InstanceContextMode property.

1. PerCall (default)

  • A new service instance is created for each client request.
  • No state is maintained between calls.
  • Ideal for stateless services.
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
public class MyService : IMyService
{
    // A new instance is created per request
}
  1. Pros:
    1. High scalability.
    2. No need for synchronization.
    3. Low memory usage.
  2. Cons:
    1. Cannot maintain service-level state.

2. PerSession

  • One service instance is created per client session (requires session-enabled binding like NetTcpBinding or WSHttpBinding with reliable session).
  • State is preserved throughout the session.
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
public class MyService : IMyService
{
    // Maintains state per client session
}
  1. Pros:
    1. Maintains state for a specific client.
    2. Useful for multi-call workflows.
  2. Cons:
    1. Increased memory per client.
    2. Requires bindings that support sessions.

3. Single (Singleton)

  • A single service instance serves all client requests.
  • Shared state across all clients.
[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single)]
public class MyService : IMyService
{
    // Shared instance for all clients
}
  1. Pros:
    1. Easy to maintain shared state.
    2. Suitable for centralized services.
  2. Cons:
    1. Must handle concurrency (ConcurrencyMode.Multiple or Reentrant).
    2. Potential bottleneck if not managed correctly.

Summary Table

Mode Instance per State Preservation Concurrency Consideration
PerCall Call No Not needed
PerSession Session Yes (per client) Needed if multiple calls overlap
Single Application Yes (shared) Must handle explicitly

You can configure it like this:

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
public class MyService : IMyService
{
    // Implementation
}
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