OperationContract

WCF uses an opt-in model to define what belongs to one of its contracts. In a service contract interface, only methods decorated with [OperationContract] are exposed to the client. That means that, in the interface below, if used within a WCF service, a client could call both Add and Subtract operations, but not Multiply.
[ServiceContract]
public interface ICalculator
{
    [OperationContract]
    int Add(int x, int y);

    [OperationContract]
    int Subtract(int x, int y);

    // Not decorated
    int Multiply(int x, int y);
}


There are two overload of OperationContract one is without parameter and other is with parameter.

Parameters are

Action → Gets or sets the WS-Addressing action of the request message.
AsyncPattern  → Indicates that an operation is implemented asynchronously using a Begin and End method pair in a service contract.HasProtectionLevel → Gets a value that indicates whether the messages for this operation must be encrypted, signed, or both.
IsInitiating → Gets or sets a value that indicates whether the method implements an operation that can initiate a session on the server (if such a session exists).
IsOneWay → Gets or sets a value that indicates whether an operation returns a reply message.
IsTerminating → Gets or sets a value that indicates whether the service operation causes the server to close the session after the reply message, if any, is sent.
Name → Gets or sets the name of the operation.
ProtectionLevel → Gets or sets a value that specifies whether the messages of an operation must be encrypted, signed, or both.
ReplyAction →  Gets or sets the value of the SOAP action for the reply message of the operation.
TypeId → When implemented in a derived class, gets a unique identifier for this
Attribute. → (Inherited from Attribute.)