Contracts : An overview

Posted Date: 03 Jul 2009	Resource Type: Articles	Category: WCF/Webservices
Author: Gaurav Arora	Member Level: Diamond
Rating:	Points: 35

In simple words contract defines the functionality provides/offers by a service and functionaly uses by the client.

Main point to note is : Contract can be completely independent of the implementation of service.

In WCF[Windows Comunication Foundation] can be grouped in following three different contract types:

Data Contract
Describes a data structure.Maps CLR types to XSD. In other words we can say that it defines the Data received by and returned from Service.

Here CLR types are mapped to XML schemas. Data Contract requires explicit marking of the fields that should be serialized with the [DataMember] attribute. [DataMember] attribute can be used regardless wheteher the field is private or public.

Data contract is different from .NET Serialization like :

runtime serialization: all fields are serialized including private fields.This mechanism is used by Remoting.
XML serialization: only public fields and properties are serialized. This mechanism is used by Web Services.

Following is the piece of example:


[DataContract (Namespace="http://www.msdotnetheaven.com/SampleServices/MsDnH/2009")]
public class MsDnHService
{
 [DataMember]
 public string ServiceId {get; set;}
}

Service Contract
Describes the operations a service can perform. Maps CLR types to WSDL. Also defines as,Service contract is used to define the WSDL that describes the service. This contract is defined with interfaces or classes with [ServiceContract] attributes and [OperationContract] attributes for method offered by the service. Here is an example:


[ServiceContract]
public interface IMsDnHService
{
 [OperationContract]
 bool StartServices(MsDnHService msdnhService);
}

Message Contract
Defines the structure of the message on the wire. Maps CLR types to SOAP messages.
It is used when complete control over the SOAP message is needed. With this one can specify that what part of the message should go into the SOAP header and what belongs in the SOAP body.[MessageContract] attribute is used to specify the Message Contract'. The header and the Body of the SOAP message are specified with the attributes [MessageHeader] and [MessageBodyMember].
Example:


[MessageContract]
public class ProcessMsDnHRequestMessage
{
 [MessageHeader]
 public int requiestId;

 [MessageBodyMember(Position=0) ]
 public MsDnHRequest msdnhRequest;
}

Now, above is used with the service contract as shown bellow:


[ServiceContract]
public interface IMsDnHRequest
{
 [OperationContract]
 public MsDnHResponseMessage msdnhRequest (ProcessMsDnHRequestMessage message);
}

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