WCF Interview Questions

WCF Interview Questions on Concurrency

2012-01-14T06:06:00.000-08:00

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What are the different concurreny modes available in WCF?
1. Single
2. Reentrant
3. Multiple

Are WCF services protected from concurrent access by default?
Yes, WCF services are protected from concurrent access by default, as the concurrency mode of a WCF service is set to Single by default.

Explain Single Concurrency Mode?
In a Single concurrency mode only one request is processed by the same service instance. A lock is acquired while a request is processed by a service instance. Other threads, if any, are queued, until they timeout. Once the lock is released when the current request completes, next thread in the queue can then access the objects.

What effect does Single concurreny mode setting have on PerCall, PerSession and Singleton services?
PerCall services and Single Concurreny mode : A new service instance is allocated for each thread. Here, concurrency is not an issue and Single mode setting has no impact on the throughput and concurrent calls can be processed.

PerSession services and Single Concurreny mode : Service instances are protected against multithreaded clients. Single mode impacts throughput of single client but multiple clients can get through and concurrent calls can be processed.

Singleton services and Single Concurreny mode : Service instances are protected against any concurrent calls. Single concurrency mode impacts throughput of singleton. Multiple threads and clients cannot get through and no concurrent calls can be processed.

Explain Reentrant Concurrency Mode?
Reentrant concurrency mode is useful when services issue callbacks to clients, If callback operations are not one-way. Services release the acquired lock upon exit to make the callback and another thread is able to acquire the lock. In this case, return from callback will queue.

What effect does Reentrant concurreny mode setting have on PerCall, PerSession and Singleton services?
PerCall services and Reentrant Concurreny mode : In a case where PerCall services may need reentrancy, if we set the concurrency mode to Single, deadlock is guaranteed, where as if it is In Reentrant mode, we will have no problem.

PerSession services and Reentrant Concurreny mode : PerSession services allow a multithreaded client to access the service instance

Singleton services and Reentrant Concurreny mode : Singleton services allow any threads to access the service instance

What is the effect of Multiple concurrency mode on PerSession services with multithreaded clients?
PerSession services with multithreaded clients can have increased throughput, with Multiple concurrency mode, as no locks are acquired when requests are being processed by a service instance. However, care should be taken to protect shared resources.

What are different .NET multithreading techniques available to protect a shared resource?
1. Monitor
2. Mutex
3. Semaphore
4. ReadWriterLock
5. Interlocked

What are the factors that can influence overall throughput for a service, when multiple concurrency mode is enabled?
1. Instancing mode
2. Concurrency mode
3. Throttling behavior

What is ServiceThrottleBehavior?
ServiceThrottleBehavior provides several settings for throughput control, as shown below.

MaxConcurrentCalls: Maximum concurrent requests allowed. The default is 16 .

MaxConcurrentInstances: Maximum concurrent service instances allowed . The default is int.MaxValue.

MaxConcurrentSessions: Maximum concurrent active sessions. This includes transport, reliable, secure, and application sessions. The default is 10.

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WCF Interview Questions on Instancing Modes

2011-12-14T10:33:00.000-08:00

WCF Interview Questions on Messaging Patterns

2011-11-14T12:43:00.000-08:00

What is MTOM?
MTOM stands for Message Transmission Optimization Mechanism and is an Interoperable standard that reduces the overhead of large binary data transfera. Removes bloat and processing overhead of base64 encoded data. Improves overall message transfer performance.

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Method overloading in WCF

2011-11-11T10:15:00.000-08:00

To correct this we can use the Name property of OperationContractAttribute as shown below. After this change, the service works fine both at compile and runtime.

WCF Interview Questions on Bindings

2011-09-09T13:08:00.000-07:00

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What are bindings in WCF?
Bindings in WCF define the configuration of communication channel between clients and services. Binding specifies
1. Transport Protocol
2. Message encoding
3. Security Mechanism
4. Reliable Messaging
5. Transactions Support

What are different Transport protocols available in WCF?
1. TCP
2. HTTP
3. Named Pipes
4. MSMQ

What message encoding formats are available in WCF?
1. Text - For interoperability
2. MTOM (Transmission Optimization Mechanism) - For transfering large objects
3. Binary - For speed

What are the 2 ways available for configuring bindings in WCF?
1. Declaratively using a configuration file
2. Programatically in the code.

Using configuration file has several advantages.

List a few standard bindings available in WCF?
1. BasicHttpBinding
2. NetPeerTcpBinding
3. WSFederationHttpBinding
4. NetNamedPipeBinding
5. WSHttpBinding
6. WSDualHttpBinding
7. NetTcpBinding
8. NetMsmqBinding
9. MsmqIntegrationBinding

Can you create a customized binding in WCF?
Yes, bindings in WCF are completely extensible and we can create a customized one.

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WCF Interview Questions by topic

2011-05-22T13:19:00.000-07:00

On this page you can see list of all WCF Interview Questions by topic. In most of the interviews, these questions are very frequently asked. If you have an asp.net question that is asked in a previous interview, please post it here.

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Interview Questions - Exception Handling in WCF

2011-03-15T15:52:00.000-07:00

What is IExtensibleDataObject

2011-02-25T15:28:00.000-08:00

Disabling support for IExtensibleDataObject in Code using attributes

Disabling support for IExtensibleDataObject in configuration

Basic WCF Interview Questions

2011-02-21T14:51:00.000-08:00

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What is WCF?
WCF stands for Windows Communication Foundation (WCF) and is considered as the Microsoft Service-Oriented Architecture (SOA) platform for building distributed and interoperable applications. WCF unifies ASMX, Remoting, and Enterprise Services stacks and provides a single programming model. WCF services are interoperable and supports all the core Web services standards. WCF services also provide extension points to quickly adapt to new protocols and updates and integrates very easily with the earlier microsoft technologies like Enterprise Services, COM and MSMQ.

What is the version of the .NET framework in which WCF is released?
WCF - Windows Communication Foundation is released as part of .NET Framework 3.0. WPF (Windows Presentation Foundation), WF (Workflow Foundation) and Card Space are also part of .NET Framework 3.0.

What is the advantage of using WCF over other distributed programming models like Web Services(ASMX), .NET Remoting, Enterprise Services stack etc.?
To understand the advantage of using WCF over other distributed programming models like Web Services(ASMX), .NET Remoting, Enterprise Services stack etc, let's consider the following scenario. We have developed an application using web services. As we know web services use HTTP protocl and XML SOAP formatted messages, they are good for developing interoperable applications in a heterogeniuos environment. We have a new client. Our new client is using .NET and he wants binary formmatted messages over TCP protocol, because interoperability is not a concern and binary formmatted messages over TCP protocol are much faster than XML SOAP formmatted messages over HTTP. To satisfy the requirement of this client, now we cannot use our existing web service. So, we have to develop a brand new remoting application from the scratch. The business functionality is the same in web services and remoting application. Since our different clients have different requirements, we ended up creating the same business application using web services and remoting technologies. This approach has several disadvantages as listed below.
1. Developers have to be familiar with two different technologies (Web Services and Remoting).
2. We end up creating duplicate business applications with different technologies which also leads to maintainance overhead.

On the other hand WCF unifies Web Services, .NET Remoting, and Enterprise Services stacks under one roof. For the same requirement that we have seen untill now, we just create one application and expose multiple end points to satisfy the requirements of multiple clients. In WCF configuration drives protocol choices, messaging formats, process allocation, etc. WCF services are loosely coupled, meaning that a WCF service is not bound to a particular protocol, encoding format, or hosting environment. Everything is configurable.

Why are WCF Services are considered as loosely coupled?
WCF Services are considered as loosely coupled because WCF services are not tightly bound to a particular protocol, encoding format, or hosting environment. All of these are configurable. At the time of designing WCF services, we donot have to worry about what protocol, encoding format, or hosting environment to use to expose the service. We can worry about all these at the time of deployment.

What are the 3 things that a WCF Services end point must have?
OR
What are the ABC of a WCF service?
Address - The address where the WCF Service is hosted.
Binding - The binding that decides the protocol, message encoding and security to use. Binding also decides whether to use reliable messaging and transaction support.
Contract - The service contract defines what service operations are available to the client for consumption.

So the Address(A), Binding(B) and Contract(C) are called as the ABC of the service end point.

What is the role of WSDL in WCF?
OR
What is WSDL?
WSDL stands for Web Service Description Language. The WCF service exposes the WSDL document for the clients, to generate proxies and the configuration file. The WSDL file provides the following information for the consumers of the WCF service.
1. Provides the information about the service contract and operations available.
2. Provides the information about all the end points exposed by the WCF service.
3. Provides the information about the messages and types that can be exchanged between the client and the WCF service.
4. WSDL also provides any information about the policies used.

What is the tool that a client application can use to generate the proxy for a WCF service?
Service Utility (svcutil.exe) can be used by the clients to generate the proxy and configuration file. For the client to be able to generate proxies, the service should enable metadata exchange.

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WCF Interview Questions on Contract Versioning

2011-02-21T14:43:00.000-08:00

WCF Interview Questions

2011-02-19T12:11:00.000-08:00

WCF Interview Questions on Contracts - Part 4

2011-02-19T11:27:00.000-08:00

MessageContract attribute is supported by MessageHeader attribute and MessageBodyMember attribute. You can apply MessageHeader attribute to fields or properties of message contract. This is a simple technique for creating custom headers.You can provide Name, Namespace and ProtectionLevel. You may also set SOAP protocol settings like Relay, Actor, MustUnderstand.

MessageBodyMember attribute can also be Applied to fields or properties of message contract.Can have several body elements. This is equivalent to multiple parameters in operation and this is the only way to return multiple complex types. It is suggested as a good practice to always supply Order. You can also set Name, Namespace, ProtectionLevel. The example below shows how to use MessageHeader and MessageBodyMember attributes.

WCF Interview Questions on Contracts - Part 3

2011-02-19T11:16:00.000-08:00

In Example 2, we are associating known type, CorporateCustomer on SaveCorporateCustomer(Customer customer) and GetCorporateCustomer(int CustomerId) operations using ServiceKnownType attribute.

In Example 3, we are associating known types, CorporateCustomer and PremiumCustomer to the service contract ICustomerService as a whole.

It is also possible to specify known types in a configuration file rather than in code. Example 4 shows how to specify known types in configuration.file.

WCF Interview Questions on Contracts - Part 2

2011-02-19T11:02:00.000-08:00

WCF Interview Questions on Contracts - Part 1

2011-02-19T10:31:00.000-08:00

2. Operation Contract - All methods in a service contract should have OperationContract attribute. You can also provide explicit Name, Action and ReplyAction as shown in the example below.

Can you apply, ServiceContract attribute to a class rather than an interface in WCF?
Yes, a ServiceContract attribute can be applied either to a class or an interface, but defining service contracts using interfaces rather classes has the following benifits.

1. Defining service contracts using interfaces, removes coupling to service implementation. Later the implementation can be changed at will without affecting the clients.
2. Defining service contracts using interfaces, also allows a service to implement more than 1 contract.

What is the purpose of MessageParameter attribute in WCF?
MessageParameter attribute is used to control the parameter and returned object names from a service operation. Consider the example below. On the service side, the method parameter name in SaveCustomer([MessageParameter(Name = "Customer")] Customer cust) is cust. If we donot use MessageParameter attribute, then "cust" is what is exposed as parameter name to the client, which is not very proffesional. So we are using MessageParameter attribute to expose the method parameter name as Cutomer.