22 Essential WCF Interview Questions Your Team Lead Can't Answer

Windows Communication Foundation (WCF) is a framework for building service-oriented applications. Regardless of it’s age, WCF it is still a framework that can allow you to build really complex solutions, control all aspects of your data flow and stays an essential skill of any experienced .NET Developer.

Originally published on Fullstack.Cafe - Never Fail Tech Interview Again

Q1: What is WCF?

Topic: WCF
Difficulty: ⭐

Windows Communication Foundation (WCF) is a framework for building service-oriented applications. Using WCF, you can send data as asynchronous messages from one service endpoint to another. A service endpoint can be part of a continuously available service hosted by IIS, or it can be a service hosted in an application. An endpoint can be a client of a service that requests data from a service endpoint. The messages can be as simple as a single character or word sent as XML, or as complex as a stream of binary data.

🔗 Source: docs.microsoft.com

Q2: Could you name basic WCF service components?

Topic: WCF
Difficulty: ⭐⭐

Have a look at this mindmap to navigate yourself through WCF service components:

🔗 Source: codeproject.com

Q3: What is a service contract in WCF?

Topic: WCF
Difficulty: ⭐⭐

A service contract defines the operations which are exposed by the service to the outside world. A service contract is the interface of the WCF service and it tells the outside world what the service can do. It may have service-level settings, such as the name of the service and namespace for the service.

[ServiceContract]
interface IMyContract { 
    [OperationContract]
    string MyMethod();
}

class MyService: IMyContract {
    public string MyMethod() {
        return "Hello World";
    }
}

🔗 Source: dotnettricks.com

Q4: Explain what is SOA?

Topic: WCF
Difficulty: ⭐⭐

SOA stands for **service-oriented architecture. Service Oriented Architecture is an architectural approach in software development where the application is organized as "Services". Services are a group of methods that contain the business logic to connect a DB or other services. For instance you go to a hotel and order food. Your order first goes to the counter and then it goes to the kitchen where the food is prepared and finally the waiter serves the food.

Some important characteristics of Service Oriented Architecture

• SOA services should be independent of other services. Altering a service should not affect the client calling the service.
• Services should be self-contained. ** Services should be able to **define themselves (in the Web Service Description Language (WSDL)). It should be able to tell the client what all of the operations it does, what are all the data types it uses and what kind of value it will return.

🔗 Source: c-sharpcorner.com

Q5: What are the features and advantage of WCF?

Topic: WCF
Difficulty: ⭐⭐

Features of WCF**

Windows Communication Foundation (WCF) is a secure, reliable, and scalable messaging platform for the .NET Framework 3.0,

• Service Orientation
• Interoperability
• Multiple Message Patterns
• Service Metadata
• Data Contracts
• Security
• Multiple Transports and Encodings
• Reliable and Queued Messages
• Durable Messages
• Transactions
• AJAX and REST Support
• Extensibility

Advantages of WCF:

1. Service Oriented
2. Location Independent
3. Language Independent
4. Platform Independent
5. Support Multiple operation
6. WCF can maintain transaction like COM+ Does
7. It can maintain state
8. It can control concurrency
9. It can be hosted on IIS, WAS, Self hosting, Windows services.

🔗 Source: docs.microsoft.com

Q6: What is transport in WCF?

Topic: WCF
Difficulty: ⭐⭐⭐

The WCF programming model separates endpoint operations (as expressed in a service contract) from the transport mechanism that connects two endpoints. This gives you the flexibility to decide how to expose your services to the network. The transport layer is at the lowest level of the channel stack. A transport sends or receives the serialized form of a message to or from another application. The main transports used in Windows Communication Foundation (WCF) are:

• HTTP,
• HTTPS,
• TCP
• named pipes.

🔗 Source: weblogs.asp.net

Q7: What is WCF Binding and how many of them do you know?

Topic: WCF
Difficulty: ⭐⭐⭐

Bindings specify how a Windows Communication Foundation (WCF) service endpoint communicates with other endpoints. At its most basic, a binding must specify the transport (for example, HTTP or TCP) to use. You can also set other characteristics, such as security and transaction support, through bindings.

WCF provides nine built-in bindings:

• BasicHttpBinding: Basic web service communication. Exposes WCF services as legacy ASMX web services. Used for interoperability. No security by default.
• WSHttpBinding: Web services with WS-* support. Supports transactions and reliable messaging.
• WSDualHttpBinding: Web services with duplex contract and transaction support.
• WSFederationHttpBinding: Web services with federated security. Supports transactions.
• MsmqIntegrationBinding: Communication directly with MSMQ applications. Supports transactions.
• NetMsmqBinding: Communication between WCF applications by using queuing. Supports transactions.
• NetNamedPipeBinding: Communication between WCF applications on same computer. Supports duplex contracts and transactions.
• NetPeerTcpBinding: Communication between computers across peer-to-peer services. Supports duplex contracts.
• NetTcpBinding: Communication between WCF applications across computers. Supports duplex contracts and transactions.

🔗 Source: weblogs.asp.net

Q8: What are the Possible Ways of Hosting a WCF Service?

Topic: WCF
Difficulty: ⭐⭐⭐

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
   1. Console Application
   2. Windows Application
   3. Windows Service
2. Hosting on Web Server
   1. IIS 6.0 (ASP.NET Application supports only HTTP)
   2. Windows Process Activation Service (WAS) i.e. IIS 7.0 supports HTTP, TCP, NamedPipes, MSMQ.

🔗 Source: codeproject.com

Q9: What is Message Contract in WCF?

Topic: WCF
Difficulty: ⭐⭐⭐

Sometimes complete control over the structure of a SOAP message is just as important as control over its contents (that defined by Data Contracts). This is especially true when interoperability is important or to specifically control security issues at the level of the message or message part. In these cases, you can create a message contract that enables you to specify the structure of the precise SOAP message required.

🔗 Source: c-sharpcorner.com

Q10: What is an Operation Contract in WCF?

Topic: WCF
Difficulty: ⭐⭐⭐

An operation contract is defined within a service contract. It defines the parameters and return type of an operation. An operation contract can also defines operation-level settings, like as the transaction flow of the operation, the directions of the operation (one-way, two-way, or both ways), and fault contract of the operation.

[ServiceContract]
interface IMyContract { 
    [FaultContract(typeof(MyFaultContract))]
    [OperationContract]
    string MyMethod();
}

🔗 Source: dotnettricks.com

Q11: Name some different types of contracts in WCF

Topic: WCF
Difficulty: ⭐⭐⭐

WCF contract specifies the service and its operations. WCF has five types of contracts:

• service contract,
• operation contract,
• data contract,
• message contract
• fault contract.

🔗 Source: dotnettricks.com

Q12: Explain the difference between WCF vs ASP.NET Web API?

Topic: WCF
Difficulty: ⭐⭐⭐

In the scenarios listed below you should go for WCF:

• If you need to send data on protocols like TCP, MSMQ or MIME
• If the consuming client just knows how to consume SOAP messages WEB API is a framework for developing RESTful/HTTP services.

Consider:

🔗 Source: stackoverflow.com

Q13: Why we need Streaming?

Topic: WCF
Difficulty: ⭐⭐⭐

When you have a large amount of data to transfer, the streaming transfer mode in WCF is a feasible alternative to the default behavior of buffering and processing messages in memory in their entirety.

In WCF any receiving message is delivered only once the entire message has been received. What I mean here is that first message is buffered at the receiving side and once it is fully received it gets delivered to the receiving end. The main problem with this approach is that the receiver end is unresponsive while the message is getting buffered. So default way of message handling in WCF is ok for small size messages but for the large size messages, this approach is not good. So to overcome this problem Streaming in WCF come into action.

🔗 Source: c-sharpcorner.com

Q14: What is interoperability and how is it achieved with WCF services?

Topic: WCF
Difficulty: ⭐⭐⭐⭐

Interoperability is the ability to communicate with other platforms through standard protocols (such as SOAP). One of the major benefits of SOAP is that it is platform independent.

WCF is designed and built to interoperate with web services that support the Web Services Specification,
known as the WS-* specifications. As such, WCF comes with three system-provided bindings that facilitate
interoperability. These bindings are shown in the following list:

• BasicHttpBinding: WCF services can use this binding to configure and expose endpoints. These endpoints in turn can communicate with *.asmx-based web services as well as clients and services that comply with the WS-I Basic Profile 1.1.
• WsHttpBinding: Supports distributed transactions as well as secure and reliable sessions.
• WsDualHttpBinding: Support for duplex communication and communication via SOAP intermediaries.

🔗 Source: codeproject.com

Q15: When would you use Duplex WCF service?

Topic: WCF
Difficulty: ⭐⭐⭐⭐

You need duplex if you want to implement callback pattern. Callback means that client does not know when some event happens in server. If you do not know when event happens you have two options to implement:

• Polling - send requests every X minutes to check if event happened. Server should either return event details (if it happened) or return flag saying that you need to continue calling. Server also can return recommended timeout in advanced scenarios.
• Callback - client sends some form of description what server should do if event happened. This may be pointer to function in C, delegate in .NET or endpoint schema in WCF. Server remembers that info and makes call from their side when time came. As you can see duplex/callback means that at some point server works as client (initiates communication) and this is a big game change.

WCF duplex communications may require special network configuration because in many cases network allows you to call external services (you work as client) but forbids external resources to call you (external service works as client). This is implemented for security purposes.

🔗 Source: medium.com

Q16: Will it make any difference if I change the operation contract of methods having no return value by [OperationContract(IsOneWay=true)]?

Topic: WCF
Difficulty: ⭐⭐⭐⭐

Yes it will - and more importantly boost scalability too. Client is released as soon as call is taken up at the server and client does not have to wait until processing finishes. This helps the resources at both client and server to be released as soon as possible.

🔗 Source: stackoverflow.com

Q17: Explain Binary vs MTOM vs Streaming in WCF?

Topic: WCF
Difficulty: ⭐⭐⭐⭐⭐

• MTOM is a standard that uses multi-part mime-encoded messages to send portions of the message that are large and would be too expensive to base64 encode as pure binary. The SOAP message itself is sent as the initial part of the message and contains references to the binary parts which a web service software stack like WCF can then pull back together to create a single representation of the message.
• Binary encoding is entirely proprietary to WCF and really doesn't just have to do with large messages. It presents a binary representation of the XML Infoset which is far more compact across the wire and faster to parse than text based formats. If you happen to be sending large binary chunks of data then it just fits right in with the other bytes that are being sent.
• Streaming can be done used with any message format. That's more about when the data is written across the network vs. being buffered entirely in memoery before being presented to the network transport. Smaller messages make more sense to buffer up before sending and larger messages, especially those containing large binary chunks or streams, necessitate being streamed or will exhaust memory resources.

🔗 Source: stackoverflow.com

Q18: What is the difference between hosting WCF service on IIS, Windows Service and self-hosted app?

Topic: WCF
Difficulty: ⭐⭐⭐⭐⭐

There is no standard answer to this question. IIS provides you with a lot of out-of-the-box features, like app domain reloading, monitoring and so on. This is why you should answer this questions first: do you need all these features or not? If not - windows service can be considered.

Other thing to consider:

• IIS gives you on-demand loading - this can be a plus or a minus. When a request comes in, the ServiceHost is constructed, then the service class being hosted is instantiated, and the request is handled. Nothing needs to be running around the clock. But at the same time, this setup requires more time and effort every time a message comes in, and you as a programmer don't have much control over your service host, really.
• Self-hosting is often times faster, since your ServiceHost is already up and running - but it's up to you to make sure it really is up and running, there's no "on-demand" loading whenever a message comes in - either it's up and can service the request, or not. But you have a lot more control over the service host - when and how it's constructed etc., and you get to pick and define your service addresses as you see fit.

And some information from Microsoft:

🔗 Source: codeproject.com

Q19: Could we use WSHttpBinding with Request-CallBack (also called Duplex) exchange pattern?

Topic: WCF
Difficulty: ⭐⭐⭐⭐⭐

HTTP protocol does not support duplex communication — responses are valid only for each request, so the answer is no. This prevents HTTP-based bindings such as BasicHttpBinding and WSHttpBinding from supporting duplex. To address this limitation, WCF provides WSDualHttpBinding. This binding uses composite duplex communication, which means two communication channels are created to support calls in both directions.

🔗 Source: medium.com

Q20: What is the main difference between Request-Response and Duplex in WCF Message Exchange Pattern?

Topic: WCF
Difficulty: ⭐⭐⭐⭐⭐

The main difference is that, after the client establishes a channel to the service, the service can call the client independently at any time. In Request-Response the service only communicates back after receiving a Request from the client. So by using duplex you receive an event-like behavior from the client perspective. Obviously, such enhancement requires a Session to be maintained (instance mode PerSession on the service).

🔗 Source: stackoverflow.com

Q21: Should I use WCF or raw sockets?

Topic: WCF
Difficulty: ⭐⭐⭐⭐⭐

Sockets are very low level. Today I would never go down to such a low level as sockets unless you really have to. Working with high level abstractions is way more productive and creative. The big advantage of Remoting or WCF is that you can pass objects between server and client and call methods on them on each side. Also be aware that the performance of Net.tcp communication over WCF is much worse than plain socket communication. It is because WCF simplifies a lot of things but those simplification and generalizations slows processing. So it depends.

🔗 Source: stackoverflow.com

Q22: What replaces WCF in .Net Core?

Topic: WCF
Difficulty: ⭐⭐⭐⭐⭐

WCF client-side is already supported on.NET Core. WCF server-side is not supported in .NET Core since it's a Windows specific technology while .NET Core is supposed to be cross-platform. WCF won't likely be ported to .NET Core, because most of the code-base depends on Windows internal libraries. There are some alternatives that you could use for the server-side thought:

1. IpcServiceFramework - A .NET Core lightweight inter-process communication framework allowing invoking a service via named pipeline and/or TCP (in a similar way as WCF, which is currently unavailable for .NET Core). Secure communication over SSL is also supported.
2. gRPC - a high performance, open source RPC framework initially developed by Google. The framework is based on a client-server model of remote procedure calls. A client application can directly call methods on a server application as if it was a local object.
3. Core WCF - Core WCF is a port of Windows Communication Framework (WCF) to .NET Core. The goal of this project is to enable existing WCF projects to move to .NET Core.

🔗 Source: stackoverflow.com

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