What is the difference between PerCall, PerSession, and Single instance modes?

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In WCF, PerCall, PerSession, and Single are instance context modes that define how and when WCF service instances are created and reused during client communication. 1. PerCall Description: A new service instance is created for every method call from any client.State: Not maintained between calls.Concurrency: No synchronization needed.Use case: Stateless operations (e.g., utility services, logging). Feature Value Instance Scope Per method call State Retention No Scalability High 2. PerSession Description: A single instance is created for the lifetime of a client session.State: Maintained throughout the session.Concurrency: Needed if multiple threads are involved.Use case: Services that require state tracking per client (e.g., shopping cart, workflow). Feature Value Instance Scope Per client session State Retention Yes (per client) Scalability Medium Requires Sessions Yes (NetTcp, etc.) 3. Single (Singleton) Description: A single service instance is created and shared across all clients and all calls.State: Shared across all clients.Concurrency: Critical — must handle simultaneous access.Use case: Centralized resource sharing, caching, or coordination services. Feature Value Instance Scope Entire application State Retention Yes (shared) Scalability Low Concurrency Must be handled Summary Table Mode Instance Created When State Retained Suitable For Needs Concurrency Handling PerCall On every operation call No Stateless services No PerSession Once per client session Yes (per client) Stateful workflows Yes Single Once per application Yes (shared) Shared-state services Yes

Feature	Value
Instance Scope	Per method call
State Retention	No
Scalability	High

Feature	Value
Instance Scope	Per client session
State Retention	Yes (per client)
Scalability	Medium
Requires Sessions	Yes (`NetTcp`, etc.)

Feature	Value
Instance Scope	Entire application
State Retention	Yes (shared)
Scalability	Low
Concurrency	Must be handled

Mode	Instance Created When	State Retained	Suitable For	Needs Concurrency Handling
PerCall	On every operation call	No	Stateless services	No
PerSession	Once per client session	Yes (per client)	Stateful workflows	Yes
Single	Once per application	Yes (shared)	Shared-state services	Yes

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What is the difference between PerCall, PerSession, and Single instance modes?

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1 Answers

1. PerCall

2. PerSession

3. Single (Singleton)

Summary Table