WCF Interview Questions & Answers

Basics of WCF:

1. What is WCF?

   • Answer: Windows Communication Foundation (WCF) is a framework for building service-oriented applications. It enables the creation and consumption of distributed services.

2. Explain the key components of WCF.

   • Answer: WCF consists of Service, Client, Address, Binding, and Contract (ABC).

3. What is a service contract in WCF?

   • Answer: A service contract defines the operations that a service exposes. It includes methods, parameters, and return types.

4. Explain the ABCs of WCF.

   • Answer: ABC stands for Address, Binding, and Contract. Address specifies where the service is hosted, Binding defines communication protocols, and Contract defines the operations the service exposes.

5. What is a Data Contract in WCF?

   • Answer: A Data Contract defines the data types that are exchanged between the client and the service. It explicitly defines the structure of the data being passed.

WCF Service:

6. How do you host a WCF service?

   • Answer: WCF services can be hosted in IIS, self-hosted in a managed application, or in Windows services.

7. Explain the difference between BasicHttpBinding and WsHttpBinding.

   • Answer: BasicHttpBinding uses HTTP for communication and is suitable for interoperability, while WsHttpBinding uses WS-* standards and provides more features but may not be as interoperable.

8. What is the purpose of the ServiceBehavior attribute in WCF?

   • Answer: The ServiceBehavior attribute is used to configure the runtime behavior of a WCF service, such as instance management, transaction settings, and error handling.

9. How do you enable metadata exchange in WCF?

   • Answer: Metadata exchange is enabled by adding the <serviceMetadata> behavior in the configuration file.

10. Explain the concept of instancing in WCF.

• Answer: Instancing refers to how service instances are created and managed. WCF supports Single, PerCall, and PerSession instancing.

WCF Contracts:

11. What is the role of the OperationContract attribute in WCF?

• Answer: The OperationContract attribute is used to define methods as service operations that can be invoked by clients.

12. Explain the FaultContract attribute in WCF.

• Answer: FaultContract is used to specify the types of exceptions that a service operation can throw.

13. What is the MessageContract attribute used for?

• Answer: MessageContract is used to define the structure and format of the messages exchanged between clients and services.

WCF Bindings:

14. What is a binding in WCF?

• Answer: A binding in WCF specifies how communication between the client and service should be done, including the transport protocol, encoding, and security requirements.

15. Differentiate between WSHttpBinding and NetTcpBinding.

• Answer: WSHttpBinding uses HTTP/HTTPS and supports WS-* standards, while NetTcpBinding uses TCP for communication, offering better performance but less interoperability.

16. Explain the purpose of the MEX endpoint.

• Answer: The MEX (Metadata Exchange) endpoint provides metadata about the service, allowing clients to understand the service's contract and configuration.

Security in WCF:

17. What are the different security modes in WCF?

• Answer: WCF supports three security modes: None, Transport, and Message.

18. Explain Transport-level security in WCF.

• Answer: Transport-level security secures communication at the transport layer, using protocols such as HTTPS. It ensures data confidentiality and integrity during transmission.

19. What is Message-level security in WCF?

• Answer: Message-level security secures individual messages, providing end-to-end security regardless of the transport protocol.

Error Handling and Debugging in WCF:

20. How do you handle errors in WCF?

• Answer: WCF provides a FaultContract attribute to handle errors. Additionally, exception details can be logged or returned to the client.

21. Explain the use of the ServiceDebug behavior.

• Answer: The ServiceDebug behavior is used for debugging purposes and provides settings for exposing metadata, including exception details.

22. How can you enable tracing in WCF?

• Answer: Tracing in WCF can be enabled by configuring the <system.diagnostics> section in the configuration file.

WCF Transactions:

23. What is a transaction in WCF?

• Answer: A transaction in WCF ensures that multiple operations either complete successfully or leave no lasting effect if an error occurs.

24. Explain the TransactionFlow attribute in WCF.

• Answer: TransactionFlow is used to enable or disable transaction propagation in a WCF service.

25. What is the role of the OperationBehavior attribute in transactions?

• Answer: OperationBehavior is used to configure transaction-related properties for an operation, such as TransactionScopeRequired.

WCF Throttling:

26. What is throttling in WCF?

• Answer: Throttling in WCF controls the maximum number of concurrent sessions, instances, and calls that a service can handle.

27. How do you configure throttling in WCF?

• Answer: Throttling settings can be configured in the service behavior section of the configuration file using attributes such as maxConcurrentCalls, maxConcurrentSessions, and maxConcurrentInstances.

RESTful Services in WCF:

28. What is REST in the context of WCF?

• Answer: REST (Representational State Transfer) is an architectural style for building web services. WCF supports RESTful services through the WebHttpBinding.

29. Explain the WebGet and WebInvoke attributes in WCF.

• Answer: WebGet and WebInvoke are used to define the URI template and HTTP method for operations in a RESTful service.

WCF Duplex Services:

30. What is a duplex contract in WCF?

• Answer: A duplex contract allows a service to send messages back to the client, creating a two-way communication channel.

31. How do you implement a duplex contract in WCF?

• Answer: Duplex contracts are implemented using the CallbackContract attribute, which defines the interface on the client to receive callback messages.

WCF Routing:

32. What is message routing in WCF?

• Answer: Message routing in WCF allows messages to be directed through a series of intermediary services before reaching the final destination.

33. Explain the use of the RoutingService in WCF.

• Answer: The RoutingService in WCF is a built-in service that facilitates message routing based on configured routing rules.

WCF Instance Management:

34. What is the purpose of instance management in WCF?

• Answer: Instance management in WCF determines how service instances are created, reused, and released.

35. **Explain the PerSession instance

context mode.**

• Answer: PerSession creates a new service instance for each client session and maintains that instance for the duration of the session.

36. What is the difference between PerCall and Singleton instance modes?

• Answer: PerCall creates a new instance for each method call, while Singleton creates a single instance for all clients.

WCF Discovery:

37. What is service discovery in WCF?

• Answer: Service discovery allows clients to dynamically find and connect to available services without prior knowledge of their locations.

38. Explain the use of the ServiceDiscoveryBehavior.

• Answer: The ServiceDiscoveryBehavior is added to the service's behaviors to enable service discovery. It publishes service metadata to allow clients to discover the service.

WCF Extensibility:

39. What is WCF extensibility?

• Answer: WCF extensibility allows developers to customize and extend the behavior of WCF by creating custom behaviors, bindings, and channels.

40. Explain the role of the IErrorHandler interface.

• Answer: IErrorHandler is used to handle errors globally in a WCF service. It allows developers to customize error handling and logging.

Testing WCF Services:

41. How do you test a WCF service?

• Answer: WCF services can be tested using tools like WCF Test Client or by creating client applications that consume the service.

42. Explain the purpose of unit testing in WCF.

• Answer: Unit testing in WCF involves testing individual components or methods of a service to ensure they work as expected.

WCF RESTful Authentication:

43. How can you implement authentication in WCF RESTful services?

• Answer: Authentication in WCF RESTful services can be implemented using standard HTTP authentication mechanisms such as Basic Authentication or OAuth.

44. Explain the use of the WebHttpBehavior in authentication.

• Answer: The WebHttpBehavior is used to configure the behavior of a RESTful service, including authentication settings.

WCF Scalability:

45. What strategies can be employed to improve the scalability of a WCF service?

• Answer: Strategies include load balancing, optimizing instance management, and using asynchronous operations.

46. Explain the use of the ServiceThrottlingBehavior in scalability.

• Answer: ServiceThrottlingBehavior is used to configure throttling settings to control the number of concurrent calls, sessions, and instances.

WCF Performance Optimization:

47. How can you optimize the performance of a WCF service?

• Answer: Performance optimization can be achieved by using efficient bindings, minimizing message size, and optimizing serialization.

48. Explain the impact of choosing the right binding on performance.

• Answer: The choice of binding can significantly impact performance. For example, the NetTcpBinding generally offers better performance than the BasicHttpBinding.

WCF Versioning:

49. How do you handle versioning in WCF?

• Answer: WCF supports versioning through techniques such as backward compatibility, versioning the service contract, and versioning data contracts.

50. Explain the role of the ServiceBehavior attribute in versioning.

• Answer: The ServiceBehavior attribute can be used to specify settings related to versioning, such as setting the ConfigurationName property to maintain compatibility with older versions of the service.