What are Contracts in WCF?

Contract provides Interoperability to communicate with the client. It is a type of contract between client and service. 

In WCF, we have the following types of contract.

• Service Contract
• Operation Contract
• Data Contract
• Message Contract
• Fault Contract

Service Contract: A service contract defines the operations which are exposed by the service to the outside world. Here's an example.

[ServiceContract]
interface IEmployee
{
    [OperationContract]
    EmployeeDetail GetEmployee(EmployeeInfo emp);
}

Operation Contract: We define Operation Contract inside Service Contract.

[ServiceContract]
interface IEmployee
{
    [OperationContract]
    EmployeeDetail GetEmployee(EmployeeInfo emp);
}

Data Contract: Data Contract describes the Data to be exchanged.

[DataContract]
public class EmployeeInfo
{
}

Message Contract: Message Contract gives more control over the actual SOAP message for a service operation request or reply. A message contract defines the elements of a message (like Message Header, Message Body, etc), as well as the message-related settings, such as the level of message security.

[ServiceContract]
public interface IEmployeeService
{
    [OperationContract]
    EmployeeDetail CalPrice(Employee request);
}
[MessageContract]
public class Employee
{
    [MessageHeader]
    public MyHeader SoapHeader
    {
        get;
        set;
    }
    [MessageBodyMember]
    public EmployeeInfo Employee
    {
        get;
        set;
    }
}
[DataContract]
public class MyHeader
{
    [DataMember]
    public string EmpID
    {
        get;
        set;
    }
}
[DataContract]
public class EmployeeInfo
{
    [DataMember]
    public string EMP_Name
    {
        get;
        set;
    }
    [DataMember]
    public sring Country
    {
        get;
        set;
    }
    [DataMember]
    public string City
    {
        get;
        set;
    }
    [DataMember]
    public string Mobile {
        get;
        set;
    }
}

Fault Contract: Fault Contract defines the errors raised by the service.

[ServiceContract]
interface IEmpContract
{
    [FaultContract(typeof(MyFaultContract1))]
    [FaultContract(typeof(MyFaultContract2))]
    [OperationContract]
    string GetEmployeeAddress();
    [OperationContract]
    string ShowEmployee();
}