Tuesday, 11 March 2014

Basic WCF Interview Questions and Answers for .NET Developers

Basic WCF Interview Questions and Answers for .NET Developers

If you are preparing of .NET interview, must brush up WCF concepts also. Mostly in every .NET project, WCF is used. So, every .NET developer must know at least basics of WCF. By keeping that in mind, I have tried to list down some basic WCF interview questions and answers which every .NET developer should know before going to the interview room. These WCF interview questions and answers cover basic concepts of WCF, SOA, difference between WCF and web services, need of WCF, endpoints in WCF like Address, Contracts and Bindings, types of Bindings in WCF, types of contracts in WCF, components of WCF, transport schemas in WCF, transactions in WCF, isolation levels in WCF, how to host WCF services, generating proxies for WCF services etc. So, lets have a look upon thes basic WCF interview questions and answers.

1. What is WCF (Windows Communication Foundation)?

Windows Communication Foundation (WCF) is an SDK for developing and deploying services on Windows. WCF provides a runtime environment for services, enabling you to expose CLR types as services, and to consume other services as CLR types.

WCF is part of .NET 3.0 and requires .NET 2.0, so it can only run on systems that support it. WCF is Microsoft’s unified programming model for building service-oriented applications with managed code. It extends the .NET Framework to enable developers to build secure and reliable transacted Web services that integrate across platforms and interoperate with existing investments.

Windows Communication Foundation combines and extends the capabilities of existing Microsoft distributed systems technologies, including Enterprise Services, System.Messaging, Microsoft .NET Remoting, ASMX, and WSE to deliver a unified development experience across multiple axes, including distance (cross-process, cross-machine, cross-subnet, cross-intranet, cross-Internet), topologies (farms, fire-walled, content-routed, dynamic), hosts (ASP.NET, EXE, Windows Presentation Foundation, Windows Forms, NT Service, COM+), protocols (TCP, HTTP, cross-process, custom), and security models (SAML, Kerberos, X509, username/password, custom).

2. What is SOA (Service Oriented Architecture)?

Service-oriented architecture (SOA) is an evolution of distributed computing based on the request/reply design paradigm for synchronous and asynchronous applications. An application's business logic or individual functions are modularized and presented as services for consumer/client applications.

3. What is the difference between WCF and Web Services?

1. Web services can only be invoked by HTTP. While Service or a WCF component can be invoked by any protocol and any transport type.

2. Second web services are not flexible. But Services are flexible. If you make a new version of the service then you need to just expose a new end point. So services are agile and which is a very practical approach looking at the current business trends.

4. What was the code name for WCF?

The code name of WCF was Indigo . 

WCF is a unification of .NET framework communication technologies which unites the following technologies:- 

NET remoting 
MSMQ 
Web services 
COM+

5. How does WCF work?

Follows the ‘software as a service’ model, where all units of functionality are defined as services.

A WCF Service is a program that exposes a collection of Endpoints. Each Endpoint is a portal (connection) for communication with either clients (applications) or other services.

Enables greater design flexibility and extensibility of distributed systems architectures.

A WCF application is represented as a collection of services with multiple entry points for communications.

6. What are the main components of WCF?

1. Service: The working logic or offering, implemented using any .Net Language©.

2. Host: The environment where the service is parked. E.g. exe, process, windows service

3. Endpoints: The way a service is exposed to outside world.

7. What is the endpoint in WCF? 

Every service must have Address that defines where the service resides, Contract that defines what the service does and a Binding that defines how to communicate with the service.

In WCF the relationship between Address, Contract and Binding is called Endpoint. The Endpoint is the fusion of Address, Contract and Binding.

1. Address: Specifies the location of the service which will be like http://Myserver/MyService.Clients will use this location to communicate with our service.

2. Contract: Specifies the interface between client and the server.It’s a simple interface with some attribute.

3. Binding: Specifies how the two paries will communicate in term of transport and encoding and protocols.

8. What is the binding in WCF and how many types of bindings are there in WCF?

A binding defines how an endpoint communicates to the world. A binding defines the transport (such as HTTP or TCP) and the encoding being used (such as text or binary).

A binding can contain binding elements that specify details like the security mechanisms used to secure messages, or the message pattern used by an endpoint.

WCF supports nine types of bindings.

1. Basic binding:

Offered by the BasicHttpBinding class, this is designed to expose a WCF service as a legacy ASMX web service, so that old clients can work with new services. When used by the client, this binding enables new WCF clients to work with old ASMX services.

2. TCP binding:

Offered by the NetTcpBinding class, this uses TCP for cross-machine communication on the intranet. It supports a variety of features, including reliability, transactions, and security, and is optimized for WCF-to-WCF communication. As a result, it requires both the client and the service to use WCF.

3. Peer network binding:

Offered by the NetPeerTcpBinding class, this uses peer networking as a transport. The peer network-enabled client and services all subscribe to the same grid and broadcast messages to it.

4. IPC binding:

Offered by the NetNamedPipeBinding class, this uses named pipes as a transport for same-machine communication. It is the most secure binding since it cannot accept calls from outside the machine and it supports a variety of features similar to the TCP binding.

5. Web Service (WS) binding:

Offered by the WSHttpBinding class, this uses HTTP or HTTPS for transport, and is designed to offer a variety of features such as reliability, transactions, and security over the Internet.

6. Federated WS binding:

Offered by the WSFederationHttpBinding class, this is a specialization of the WS binding, offering support for federated security.

7. Duplex WS binding:

Offered by the WSDualHttpBinding class, this is similar to the WS binding except it also supports bidirectional communication from the service to the client.

8. MSMQ binding:

Offered by the NetMsmqBinding class, this uses MSMQ for transport and is designed to offer support for disconnected queued calls.

9. MSMQ integration binding:

Offered by the MsmqIntegrationBinding class, this converts WCF messages to and from MSMQ messages, and is designed to interoperate with legacy MSMQ clients.

9. What are the contracts in WCF?

In WCF, all services expose contracts. The contract is a platform-neutral and standard way of describing what the service does.

WCF defines four types of contracts.

1. Service contracts: Describe which operations the client can perform on the service.

2. Data contracts: Define which data types are passed to and from the service. WCF defines implicit contracts for built-in types such as int and string, but we can easily define explicit opt-in data contracts for custom types.

3. Fault contracts: Define which errors are raised by the service, and how the service handles and propagates errors to its clients.

4. Message contracts: Allow the service to interact directly with messages. Message contracts can be typed or untyped, and are useful in interoperability cases and when there is an existing message format we have to comply with.

10. What is address in WCF and how many types of transport schemas are there in WCF?

Address is a way of letting client know that where a service is located. In WCF, every service is associated with a unique address. This contains the location of the service and transport schemas. 

WCF supports following transport schemas:

1. HTTP
2. TCP
3. Peer network
4. IPC (Inter-Process Communication over named pipes)
5. MSMQ

The sample address for above transport schema may look like

http://localhost:81
http://localhost:81/MyService
net.tcp://localhost:82/MyService
net.pipe://localhost/MyPipeService
net.msmq://localhost/private/MyMsMqService
net.msmq://localhost/MyMsMqService

11. Explain transactions in WCF.

Transactions in WCF allow several components to concurrently participate in an operation. Transactions are a group of operations that are atomic, consistent, isolated and durable. WCF has features that allow distributed transactions. Application config file can be used for setting transaction timeouts.

12. What are different isolation levels provided in WCF?

The different isolation levels:

1. READ UNCOMMITTED: An uncommitted transaction can be read. This transaction can be rolled back later.

2. READ COMMITTED: Will not read data of a transaction that has not been committed yet

3. REPEATABLE READ: Locks placed on all data and another transaction cannot read.

4. SERIALIZABLE: Does not allow other transactions to insert or update data until the transaction is complete.

13. How do I serialize entities using WCF?

LINQ to SQL supports serialization as XML via WCF by generating WCF serialization attributes and special serialization specific logic during code-generation. You can turn on this feature in the designer by setting serialization mode to ‘Unidirectional’. Note this is not a general solution for serialization as unidirectional mode may be insufficient for many use cases.

14. What are various ways of hosting WCF Services?

There are three major ways of hosting a WCF services 

1. 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. 

2. Host in application domain or process provided by IIS Server

3. Host in Application domain and process provided by WAS (Windows Activation Service) Server. 

15. What is Transport and Message Reliability?

Transport reliability (such as the one offered by TCP) offers point-to-point guaranteed delivery at the network packet level, as well as guarantees the order of the packets. Transport reliability is not resilient to dropping network connections and a variety of other communication problems. 

Message reliability deals with reliability at the message level independent of how many packets are required to deliver the message. Message reliability provides for end-to-end guaranteed delivery and order of messages, regardless of how many intermediaries are involved, and how many network hops are required to deliver the message from the client to the service.

16. What is service and client in perspective of data communication?

A service is a unit of functionality exposed to the world. The client of a service is merely the party consuming the service.

17. What is Proxy and how to generate proxy for WCF Services?

The proxy is a CLR class that exposes a single CLR interface representing the service contract. The proxy provides the same operations as service's contract, but also has additional methods for managing the proxy life cycle and the connection to the service. The proxy completely encapsulates every aspect of the service: its location, its implementation technology and runtime platform, and the communication transport. 

The proxy can be generated using Visual Studio by right clicking Reference and clicking on Add Service Reference. This brings up the Add Service Reference dialog box, where you need to supply the base address of the service (or a base address and a MEX URI) and the namespace to contain the proxy. 

Proxy can also be generated by using SvcUtil.exe command-line utility. We need to provide SvcUtil with the HTTP-GET address or the metadata exchange endpoint address and, optionally, with a proxy filename. The default proxy filename is output.cs but you can also use the /out switch to indicate a different name. 

SvcUtil http://localhost/MyService/MyService.svc /out:Proxy.cs 

When we are hosting in IIS and selecting a port other than port 80 (such as port 88), we must provide that port number as part of the base address: 

SvcUtil http://localhost:88/MyService/MyService.svc /out:Proxy.cs

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