WCF Interview questions and answers (reference only, uuber level)

The intent to draft this list of questions is for learning purpose only and at very high level understanding of WCF services. This is not a dump :)

Question 1: What is the difference WCF and Web services?
Answer: -

Web services can only be invoked by HTTP. While Service or a WCF component can be invoked by any protocol and any transport type. Second web services are not flexible. However, Services are flexible. If you make a new version of the service then you need to just expose a new end. Therefore, services are agile and which is a very practical approach looking at the current business trends.

Question 2: What are different bindings supported by WCF?
Answer: -

WCF includes predefined bindings. They cover most of bindings widely needed in day-to-day application. However, just incase you find that you need to define something custom WCF does not stop you. So let us try to understand what each binding provides.

BasicHttpBinding: - This binding is used when we need to use SOAP over HTTP. This binding can also be configured to be used as HTTPS. It can be also configured to send data in plain text or in optimized form like MTOM.
WsHttpBinding: - It is same like BasicHttpBinding. In short, it uses SOAP over HTTP. But with it also supports reliable message transfer, security and transaction. WS-Reliable Messaging, security with WS-Security, and transactions with WS-Atomic Transaction supports reliable message.
NetTcpBinding: - This binding sends binary-encoded SOAP, including support for reliable message transfer, security, and transactions, directly over TCP. The biggest disadvantage of NetTcpBinding is that both server and client should be also made in .NET language.
NetMsmqBinding: - This binding sends binary-encoded SOAP over MSMQ. This binding can only be used for WCF-to-WCF communication.
NetNamedPipesBinding:-Ths binding Sends binary-encoded SOAP over named pipes. This binding is only usable for WCF-to-WCF communication between processes on the same Windows-based machine.

Note: - An interprocess control (IPC) protocol is used for exchanging information between two applications, possibly running on different computers in a network. The difference between Named pipes and TCP is that named pipes have good performance in terms of communication with in processes. But when it comes to communicate across network TCP holds the best choice. So if you are using WCF to communicate with process it’s the best choice to use in terms for performance. Named pipes do not perform when the traffic is heavy as compared to TCPIP.

Question 3: What are the main components of WCF?
Answer: -

We need to define three main components in WCF:-

• Service class. • Hosting environment • End point

Question 4: What are the various ways of hosting a WCF service?
Answer: -

There are three major ways to host a WCF service:-

• Self-hosting the service in his own application domain. This we have already covered in the first section. The service comes in to existence when you create the object of Service Host class and the service closes when you call the Close of the Service Host class.
• Host in application domain or process provided by IIS Server.
• Host in Application domain and process provided by WAS (Windows Activation Service) Server.

Question 5: What is a service level message and transport level message?
Answer: -
You can log WCF message at two levels one is service level and the other is transport level. Service level:-In this the messages are logged as they enter the user code or leave the user code. Transport level: - In this the messages are logged as they are ready to be encoded / decoded. All transport level, infrastructure messages and also reliable messaging is logged. You specify the message levels in the diagnostics node as shown in the below code snippet.

Question 6: For which bindings are transport, message and mixed mode supported?
Answer: -
Below is a table which shows for which binding which mode is supported. We did not discuss the mixed mode. It’s nothing but combination of transport and mixed mode. For instance data encrypted and passed over WsHttp using HTTPS is a mixed mode of security. Encryption is nothing but message security and HTTPS is a transport mode. In a combination they form mixed mode.

Question 7: Which bindings in WCF support the message streaming?
Answer: -
Following bindings supports the streaming in WCF:
1. basicHttpBinding 2. netTcpBinding 3. netNamedPipeBinding

Question 8: What are the various ways of hosting a WCF Service?
Answer: - 1.IIS 2.Self Hosting 3.WAS (Windows Activation Service)

Question 9: What is the use of ServiceBehavior attribute in WCF ?
Answer: -
ServiceBehaviour attribute is used to specify the InstanceContextMode for the WCF Service class (This can be used to maintained a state of the service or a client too).

There are three instance Context Mode in the WFC PerSession : This is used to create a new instance for a service and the same instance is used for all method for a particular client. (eg: State can be maintained per session by declaring a variable) PerCall : This is used to create a new instance for every call from the client whether same client or different. (eg: No state can be maintained as every time a new instance of the service is created) Single : This is used to create only one instance of the service and the same instance is used for all the client request. (eg: Global state can be maintained but this will be applicable for all clients)

Question 10: How the concurrency mode is specified in WCF service?
Answer: - The concurrency mode is specified using the ServiceBehavior attribute on the class that implements the service. Ex. [ServiceBehavior(ConcurrencyMode=ConcurrencyMode.Single)] Public class ServiceClass : IServiceInterface{ //Implementation Code } There are 3 possible values of ConcurrencyMode enumeration Single Reentrant Multiple.

Question 11: hat the different transaction options ?
Answer:-

We can specify transaction in 3 ways in WCF:-
TransactionFlowOption.NotAllowed This is a default option. Using this option no transaction will be propagated across the binding. If any client attempts to call the WCF service in a transaction it will be ignored for this option. This option specifies that client can call this WCF service in a transaction. It’s not compulsory that the service needs to be called in a transaction. You can call without the transaction also. TransactionFlowOption.Mandatory This option specifies that client must call the WCF service in a transaction mode. If the WCF service is called without transaction, ‘FaultException’ will be raised.

Question 12: Wat are the advantages of hosting WCF Services in IIS as compared to self-hosting?
Answer: - There are two main advantages of using IIS over self-hosting:-

Automatic activation IIS provides automatic activation that means the service is not necessary to be running in advance. When any message is received by the service it then launches and fulfills the request. But in case of self hosting the service should always be running.
Process recycling If IIS finds that a service is not healthy that means if it has memory leaks etc, IIS recycles the process. Ok let us try to understand what is recycling in IIS process. For every browser instance, a worker process is spawned and the request is serviced. When the browser disconnects the worker, process stops and you loose all information. IIS also restarts the worker process. By default, the worker process is recycled at around 120 minutes. So why does IIS recycle. By restarting the worker process it ensures any bad code or memory leak do not cause issue to the whole system. In case of self-hosting both the above features, you will need to code yourself.

Question 13: What is trace listener?

Answer - ‘Tracelistener’ are objects that get tracing information from the trace class and they output the data to some medium. For instance you can see from the figure ‘TraceListener’ how it listens to the trace object and outputs the same to UI, File or a windows event log. There are three different types of ‘tracelistener’ first is the ‘defaulttracelistener’ (this outputs the data to UI), second is ‘textwritertracelistener’ (this outputs to a file) and the final one is ‘Eventlogtracelistener’ which outputs the same to a windows event log.

Question 14: What is service host factory in WCF?
Answer-

1. Service host factory is the mechanism by which we can create the instances of service host dynamically as the request comes in.
2. This is useful when we need to implement the event handlers for opening and closing the service.
3. WCF provides ServiceFactory class for this purpose.

Question 15: What are the advantages of hosting WCF service in WAS?

Answer- WAS (Windows Activation Service) is a component of IIS 7.0. Following are few advantages : 1. We are not only limited to HTTP protocol. We can also use supported protocols like TCP, named pipes and MSMQ 2. No need to completely install IIS. We can only install WAS component and keep away the WebServer.

Question 16: In WCF which bindings supports the reliable session?

Answer- In WCF, following bindings supports the reliable session - 1. wsHttpBinding 2. wsDualHttpBinding 3. wsFederationHttpBinding 4. netTcpBinding

Reference - Prasham on dotnetfunda.com and Pro WCF: Practical Microsoft SOA Implementation -- Chris and Denis Mulder – Apress 2007.

- DJ