WCF Interview Questions and Answers for 10 years experience

1. What is WCF?

   • Answer: Windows Communication Foundation (WCF) is a framework for building service-oriented applications. It allows you to create applications that communicate over a network using various protocols like HTTP, TCP, named pipes, and MSMQ. WCF provides a unified programming model for developing services that can be consumed by various clients, including .NET and non-.NET clients.

2. Explain the different message patterns supported by WCF.

   • Answer: WCF supports various message patterns including Request-Reply (synchronous), One-way (asynchronous), Duplex (two-way communication), and Datagram (unreliable, connectionless).

3. What are bindings in WCF and explain some common ones?

   • Answer: Bindings define how a WCF service communicates with its clients. They specify the transport protocol (HTTP, TCP, etc.), encoding (Text, Binary, MTOM), and security settings. Common bindings include BasicHttpBinding, WSHttpBinding, NetTcpBinding, NetNamedPipeBinding, NetMsmqBinding.

4. Describe the role of behaviors in WCF.

   • Answer: Behaviors modify the runtime behavior of a WCF service or client. They can be used to add functionalities like error handling, transaction management, security, and throttling.

5. Explain different ways to handle errors in WCF.

   • Answer: WCF provides mechanisms like FaultContracts, custom exception handling, and using try-catch blocks to handle errors gracefully. FaultContracts allow you to define specific fault messages to be returned to the client.

6. What is a service contract?

   • Answer: A service contract defines the operations a WCF service exposes to its clients. It specifies the methods, parameters, and return types that are available for interaction. It is defined using the [ServiceContract] and [OperationContract] attributes.

7. What is a data contract?

   • Answer: A data contract defines the data exchanged between a WCF service and its clients. It specifies the data types and their serialization format. It is defined using the [DataContract] and [DataMember] attributes.

8. Explain different types of WCF security modes.

   • Answer: WCF offers various security modes including Transport, Message, and TransportWithMessageCredential. Transport security secures the message in transit, Message security secures the message itself, and TransportWithMessageCredential combines both.

9. How to implement transactions in WCF?

   • Answer: Transactions in WCF are managed using the TransactionScope class. It ensures that operations within the scope are either all committed or all rolled back.

10. What are the advantages of using WCF over Web Services (ASMX)?

    • Answer: WCF offers improved features compared to ASMX, including a unified programming model, support for various protocols and encodings, better security features, and more flexible configuration options.

11. Explain the difference between Session and PerCall instance modes in WCF.

    • Answer: PerCall creates a new instance of the service for each call, while Session maintains a single instance for multiple calls from the same client, providing statefulness within the session.

12. How do you handle concurrency in WCF?

    • Answer: Concurrency is handled using the `ConcurrencyMode` property in the `ServiceBehaviorAttribute`. Options include Single, Multiple, and Reentrant, affecting how multiple requests are processed concurrently.

13. What are Metadata in WCF and how are they used?

    • Answer: Metadata describes the service's contracts, operations, and data types. It's used by tools to generate client proxies and for service discovery.

14. Explain the concept of Message Inspectors in WCF.

    • Answer: Message inspectors allow you to inspect and modify messages before they are sent or received by the service. Useful for logging, security, or message transformation.

15. How to implement custom message handlers in WCF?

    • Answer: Custom message handlers provide more advanced control over message processing than inspectors. They allow for asynchronous processing and more complex modifications.

16. Describe different ways to host a WCF service.

    • Answer: WCF services can be hosted in various ways, including IIS, WAS (Windows Activation Service), Self-hosting (a console application or Windows Service), and Windows Services.

17. Explain how to implement a WCF service with RESTful behavior.

    • Answer: Use `WebHttpBinding` and decorate methods with `[WebGet]` and `[WebInvoke]` attributes to map HTTP verbs to service operations.

18. What are the benefits of using message queues (MSMQ) with WCF?

    • Answer: MSMQ provides reliable asynchronous messaging, ideal for scenarios requiring guaranteed delivery even if the client is offline.

19. How do you configure throttling in WCF?

    • Answer: Throttling limits the number of concurrent calls and the maximum session time to prevent service overload. Configured using the `ServiceThrottlingBehavior`.

20. Explain the role of the app.config file in a WCF application.

    • Answer: The `app.config` file contains configuration settings for the WCF service, including endpoints, bindings, behaviors, and security settings.

21. How do you debug a WCF service?

    • Answer: Use the standard debugging tools in Visual Studio, attaching the debugger to the hosting process (IIS, WAS, or self-host).

22. How do you monitor a WCF service?

    • Answer: Use performance counters and logging to monitor the service's health and performance. Tools like PerfMon can be used for monitoring.

23. Explain the concept of callbacks in WCF duplex communication.

    • Answer: In duplex communication, the service can initiate communication with the client using callbacks. Requires a callback contract and a binding that supports duplex communication (e.g., NetTcpBinding).

24. How do you handle large messages in WCF?

    • Answer: Use MTOM (Message Transmission Optimization Mechanism) for efficient transfer of large binary data or streaming techniques for large text data.

25. What are some common performance considerations when designing a WCF service?

    • Answer: Choosing the right binding, optimizing data contracts, efficient error handling, proper concurrency settings, and using caching where appropriate.

26. Explain the difference between a WCF service and a WCF client.

    • Answer: A WCF service exposes functionality, while a WCF client consumes that functionality. They both use the same contracts and bindings but have different roles in the communication.

27. How do you version your WCF services?

    • Answer: Use different namespaces or contracts for different versions of the service. Use a versioning strategy for clients to choose the appropriate service version.

28. Describe your experience with WCF in a production environment.

    • Answer: [This requires a personalized answer based on your own experience. Describe specific projects, challenges faced, and solutions implemented.]

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