One-Way | WCF Tutorial pdf

In One-Way operation mode, client will send a request to the server and does not care whether it is success or failure of service execution. There is no return from the server side, it is one-way communication.
Client will be blocked only for a moment till it dispatches its call to service. If any exception thrown by service will not reach the server.
Client can continue to execute its statement, after making one-way call to server. There is no need to wait, till server execute. Sometime when one-way calls reach the service, they may not be dispatched all at once but may instead be queued up on the service side to be dispatched one at a time, according to the service's configured concurrency mode behavior. If the number of queued messages has exceeded the queue's capacity, the client will be blocked even if it's issued a one-way call. However, once the call is queued, the client will be unblocked and can continue executing, while the service processes the operation in the background.

Definition :
One-way operation can be enabled by setting IsOneWay property to true in Operation contract attribute.
[ServiceContract]
public interface IMyService
{
   [OperationContract(IsOneWay=true)]
   void MyMethod(EmployeeDetails emp);
}
One-Way Operations and Sessionful Services:
Let us see the example, what will happen when you use the one-way communication with Sessionful service.
    [ServiceContract(SessionMode = SessionMode.Required)]
   interface IMyContract
   {
       [OperationContract(IsOneWay = true)]
       void MyMethod();
   }
As per above configuration, when client makes one-way call using MyMethod() operation and if it close the proxy. Client will be blocked until operation completes. It will be good practice, that one-way operation should be applied on per-call and singleton service.
Suppose If you want to make use of One-way operation in Sessionful service, use in the last operation of the service which will terminate the session. This operation should not return any value.
    [ServiceContract(SessionMode = SessionMode.Required)]
   interface IMyContract
   {
       [OperationContract]
       void MyMethod1();

       [OperationContract]
       string MyMethod2();

       [OperationContract(IsOneWay = true, IsInitiating = false,
                                          IsTerminating = true)]
       string CloseSessionService(int id);
      
   }
One-Way Operations and Exceptions:
Suppose when we are using BasicHttpBinding or WSHttpBinding, i.e. no transport session is used, if any exception throw by service will not affect the client. Client can make a call to the service using same proxy
[ServiceContract]
interface IMyContract
{
  [OperationContract(IsOneWay = true)]
  void MethodWithError( );

  [OperationContract]
  void MethodWithoutError( );
}
//Client side without transport session
MyContractClient proxy = new MyContractClient( );
proxy.MethodWithError( ); //No exception is thrown from serivce
proxy.MethodWithoutError( ); //Operation will execute properly
proxy.Close( ); In the presence of transport session, any exception thrown by service will fault the client channel. Client will not be able to make new call using same proxy instance.
//Client side transport session
MyContractClient proxy = new MyContractClient( );
proxy.MethodWithError( ); 
proxy.MethodWithoutError( ); //Can not executre because channel is faulted
proxy.Close( );