Top 10 WCF Interview Questions

By | June 15, 2012

ThisTop10WCF Tutorials post has most important andbasicquestionsthatareaskedduringaWCF interview on hot topics with detailed answers and explanation. You can find acomprehensivelistofWCF Interview Questions and Answers as:

1. What is the difference between WCF and ASMX Web Services?

Simple and basic difference is that ASMX or ASP.NET web service is designed to send and receive messages using SOAP over HTTP only. While WCF can exchange messages using any format (SOAP is default) over any transport protocol (HTTP, TCP/IP, MSMQ, NamedPipes etc).Another tutorial WCF Vs ASMX has detailed discussion on it.

2. What are WCF Service Endpoints? Explain.

For Windows Communication Foundation services to be consumed, it’s necessary that it must be exposed; Clients need information about service to communicate with it. This is where service endpoints play their role.A WCF service endpoint has three basic elements i.e. Address, Binding and Contract.Address: It defines “WHERE”. Address is the URL that identifies the location of the service.
Binding: It defines “HOW”. Binding defines how the service can be accessed.
Contract: It defines “WHAT”. Contract identifies what is exposed by the service.

3. What are the possible ways of hosting a WCF service? Explain.

For a Windows Communication Foundation service to host, we need at least a managed process, a ServiceHost instance and an Endpoint configured. Possible approaches for hosting a service are:

1.    Hosting in a Managed Application/ Self Hosting
a.    Console Application
b.    Windows Application
c.    Windows Service
2.    Hosting on Web Server
a.    IIS 6.0 (ASP.NET Application supports only HTTP)
b.    Windows Process Activation Service (WAS) i.e. IIS 7.0 supports HTTP, TCP,
NamedPipes, MSMQ.

4. How we can achieve Operation Overloading while exposing WCF Services?

By default, WSDL doesn’t support operation overloading. Overloading behavior can be achieved by using “Name” property of OperationContract attribute.

 [ServiceContract]
 interface IMyCalculator
 {
        [OperationContract(Name = "SumInt")]
        int Sum(int arg1,int arg2);        [OperationContract(Name = "SumDouble")]
        double Sum(double arg1,double arg2);
  }

When the proxy will be generated for these operations, it will have 2 methods with different names i.e. SumInt and SumDouble.


Important Note: Remember that during a technical Interview, interviewer always ask about the latest feature of that particular technology, so be prepare for it also. For latest features series on Windows Communication Foundation v4.5, Click here.


5. What Message Exchange Patterns (MEPs) supported by WCF? Explain each of them briefly.

  1. Request/Response
  2. One Way
  3. Duplex

Request/Response
It’s the default pattern. In this pattern, a response message will always be generated to consumer when the operation is called, even with the void return type. In this scenario, response will have empty SOAP body.
One Way
In some cases, we are interested to send a message to service in order to execute certain business functionality but not interested in receiving anything back. OneWay MEP will work in such scenarios.
If we want queued message delivery, OneWay is the only available option.
Duplex
The Duplex MEP is basically a two-way message channel. In some cases, we want to send a message to service to initiate some longer-running processing and require a notification back from service in order to confirm that the requested process has been completed.

Please follow for detailed implementation for Message Exchange Patterns in WCF here.

6. What is DataContractSerializer and How its different from XmlSerializer?

Serialization is the process of converting an object instance to a portable and transferable format. So, whenever we are talking about web services, serialization is very important.

Windows Communication Foundation has DataContractSerializer that is new in .NET 3.0 and uses opt-in approach as compared to XmlSerializer that uses opt-out. Opt-in means specify whatever we want to serialize while Opt-out means you don’t have to specify each and every property to serialize, specify only those you don’t want to serialize.
DataContractSerializer is about 10% faster than XmlSerializer but it has almost no control over how the object will be serialized. If we wanted to have more control over how object should be serialized that XmlSerializer is a better choice.

7. How we can use MessageContract partially with DataContract for a service operation in WCF?

MessageContract must be used all or none. If we are using MessageContract into an operation signature, then we must use MessageContract as the only parameter type and as the return type of the operation.

8. Which standard binding could be used for a service that was designed to replace an existing ASMX web service?

The basicHttpBinding standard binding is designed to expose a service as if it is an ASMX/ASP.NET web service. This will enable us to support existing clients as applications are upgrade to WCF.

9. Please explain briefly different Instance Modes in WCF?

WCF will bind an incoming message request to a particular service instance, so the available modes are:

Per Call: instance created for each call, most efficient in term of memory but need to maintain session.

Per Session: Instance created for a complete session of a user. Session is maintained.

Single: Only one instance created for all clients/users and shared among all.Least efficient in terms of memory.

Please follow “3 techniques for Instance Management in WCF“.

10. Please explain different modes of security in WCF? Or Explain the difference between Transport and Message Level Security.

In Windows Communication Foundation, we can configure to use security at different levels

a.    Transport Level security means providing security at the transport layer itself. When dealing with security at Transport level, we are concerned about integrity, privacy and authentication of message as it travels along the physical wire. It depends on the binding being used that how WCF makes it secure because most of the bindings have built-in security.

  <netTcpBinding>
         <binding name=”netTcpTransportBinding”>
                    <security mode=”Transport”>
                          <Transport clientCredentialType=”Windows” />
                    </security>
          </binding>
  </netTcpBinding>

b.    Message Level Security
For Tranport level security, we actually ensure the transport that is being used should be secured but in message level security, we actually secure the message. We encrypt the message before transporting it.

   <wsHttpBinding>
            <binding name=”wsHttpMessageBinding”>
                          <security mode=”Message”>
                                     <Message clientCredentialType=”UserName” />
                          </security>
             </binding>
     </wsHttpBinding>

It totally depends upon the requirements but we can use a mixed security mode also as follows:

      <basicHttpBinding>
             <binding name=”basicHttp”>
                          <security mode=”TransportWithMessageCredential”>
                               <Transport />
                               <Message clientCredentialType=”UserName” />
                          </security>
              </binding>
       </basicHttpBinding>

Note: In this WCF Tutorial, we discussed about the most important top 10 interview questions and answers only but you can go through a series of questions on WCF Interview Questions here.

Top 10 Interview Questions and Answers Series:

  • Karthik

    Imran..you done a very good explaination about WCF questions…Keep it up..Post more questions on this.

  • Anonymous

    Good work and Expect more from you Imran………

  • Anonymous

    Thank you

  • http://www.blogger.com/profile/00842963615291720456 suresh e

    Thank you.. Keep it up

  • Anonymous

    Good work.. keep it up..

  • http://www.blogger.com/profile/17654498887181200149 Ravindran

    good work

  • Anonymous

    Hey Imran, you are doing absolutely great job, It helps us a lot !!!!.

  • Anonymous

    Love this post

  • Anonymous

    Thank you! A good collection of questions.

  • Anonymous

    Thanks a lot Imran ..Really these are the top 10 question !!!!!!!!!!!!

  • Anonymous

    Thanks Imran nice basic understaning for WCF (Moez)

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    I would like to thank you for the efforts you have made in writing this article. I am hoping the same best work from you in the future as well.

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    Imran you doing Good work thanks for This :)

    Top 10 Interview Tips

  • http://www.blogger.com/profile/09763529050792902787 Syed Farhan Ashraf

    Great Work,,,

  • Anonymous

    Thanks Imran really helpful. More power to you !!!

  • Anonymous

    Thanks very nice top 10 questions

  • Anonymous

    I think “single ” instance mode should be most efficient in terms of memory. There is only one object being created

    • http://www.blogger.com/profile/10867674357364468276 imranghani

      Dear, single is least efficient in terms of memory. Although its a single instance but it lives in memory almost forever and disposed off only when host process gets down.

  • Anonymous

    thank you :)