Asynchronous operation in WCF using IAsyncResult

The WCF service operations can be implemented asynchronously or synchronously. Clients can consume service operations synchronously or asynchronously.

There are three ways to implement asynchronous operations

• IAsyncResult asynchronous
• Task-based asynchronous
• Event-based asynchronous

Here sample code is given for IasyncResult asynchronous operation.

Service Implementation

Asynchronous operations require Begin and End methods.

using System;

using System.ServiceModel;

namespace WCF

{

[ServiceContract]

public interface IMessageService

{

[OperationContract(AsyncPattern = true)]

IAsyncResult BeginGetMessage(string message, AsyncCallback callback, object state);

string EndGetMessage(IAsyncResult asyncResult);

}

}

In the service contract, the BeginGetMessage method takes a parameter, a callback object, and a state object, and returns an IasyncResult, while EndGetMessage method takes an IasyncResult and returns the value. Since the service operation returns IasyncResult, hence needs to be implemented first.

using System;

using System.Threading;

namespace WCF

{

public class AsyncResult : IAsyncResult, IDisposable

{

private AsyncCallback _callback;

private object _state;

private ManualResetEvent _manualResetEvent;

public AsyncResult(AsyncCallback callback, object state)

{

_callback = callback;

_state = state;

_manualResetEvent = new ManualResetEvent(false);

}

public bool IsCompleted

{

get { return _manualResetEvent.WaitOne(0, false); }

}

public WaitHandle AsyncWaitHandle

{

get { return _manualResetEvent; }

}

public object AsyncState

{

get { return _state; }

}

public ManualResetEvent AsyncWait

{

get { return _manualResetEvent; }

}

public bool CompletedSynchronously

{

get { return false; }

}

public void Completed()

{

_manualResetEvent.Set();

if (_callback != null)

_callback(this);

}

public void Dispose()

{

_manualResetEvent.Close();

_manualResetEvent = null;

_state = null;

_callback = null;

}

}

}

AsyncResult implements IasyncResult that has some properties like AsyncCallback, state object and ManualResetEvent, which handle waiting for asynchronous operation. IsCompleted is used to specify that the operation is completed. In the Complete() method ManualResetEvent has been set, which means that the event is signaled now and any other awaiting thread can follow.

For MessageServiceOperation, MessageAsyncResult is there which just inherit from AsyncResult to add Message property.

public class MessageAsyncResult : AsyncResult

{

public string Message { get; private set; }

public MessageAsyncResult(string message, AsyncCallback callback, object state)

: base(callback, state)

{

Message = message;

}

}

Now implementing the service operations

public class MessageService : IMessageService

{

public IAsyncResult BeginGetMessage(string message, AsyncCallback callback, object state)

{

var replyMessage = string.Format("{0}{1}", "Server says :", message);

var messageAsyncResult = new MessageAsyncResult(replyMessage, callback, state);

ThreadPool.QueueUserWorkItem(CompleteProcess, messageAsyncResult);

return messageAsyncResult;

}

private void CompleteProcess(object state)

{

var messageAsyncResult = state as MessageAsyncResult;

messageAsyncResult.Completed();

}

public string EndGetMessage(IAsyncResult asyncResult)

{

var messageAsyncResult = asyncResult as MessageAsyncResult;

messageAsyncResult.AsyncWait.WaitOne();

return messageAsyncResult.Message;

}

}

In the BeginGetMessage () method an instance of MessageAsyncResult is created by passing appropriate parameters to it then call ThreadPool.QueueUserWorkItem() method to add method to waiting queue for execution. The method CompleteProcess() will run whenever a thread becomes available and it finally call the Completed() method to release the thread and allow the other operations to execute. In the EndGetMessage () method, In WaitOne() method, wait for ManualResetEvent until the current execution ends then return the Result property.

Service Hosting

Here console application will be used as a server, to host service.

class Program

{

static void Main(string[] args)

{

var svcHost = new ServiceHost(typeof(MessageService));

Console.WriteLine("Available Endpoints :\n");

svcHost.Description.Endpoints.ToList().ForEach(endpoints =>Console.WriteLine(endpoints.Address.ToString()));

svcHost.Open();

Console.ReadLine();

}

}

And the configuration for that,

<configuration>

<system.serviceModel>

<behaviors>

<serviceBehaviors>

<behavior name="">

<serviceMetadata httpGetEnabled="true" />

<serviceDebug includeExceptionDetailInFaults="false" />

</behavior>

</serviceBehaviors>

</behaviors>

<services>

<service name="WCF.Asynchronous.Services.MessageService">

<endpoint address="" binding="basicHttpBinding" contract="WCF.Asynchronous.Services.IMessageService">

<identity>

<dns value="localhost" />

</identity>

</endpoint>

<endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />

<host>

<baseAddresses>

<add baseAddress="http://localhost/WCF.Asynchronous.Services/" />

</baseAddresses>

</host>

</service>

</services>

</system.serviceModel>

</configuration>

Client

Here is given a simple console app to consume this service,

public class Program

{

static void Main(string[] args)

{

var channelFactory = new ChannelFactory<IMessageService>("MessageServiceEndpoint");

var proxy = channelFactory.CreateChannel();

var message = Console.ReadLine();

while (message.Equals("exit", StringComparison.CurrentCultureIgnoreCase) == false)

{

proxy.BeginGetMessage(message, ClientCallBack, proxy);

message = Console.ReadLine();

}

}

private static void ClientCallBack(IAsyncResult ar)

{

var res = ar.AsyncState as IMessageService;

if (res != null)

{

var message = res.EndGetMessage(ar);

Console.WriteLine(message);

}

}

}

And the configuration file for this is,

<configuration>

<system.serviceModel>
<bindings />
<client>
<endpoint address="http://localhost/WCF.Asynchronous.Services/"
binding="basicHttpBinding" contract="WCF.Asynchronous.Services.IMessageService"
name="MessageServiceEndpoint">
</endpoint>
</client>
</system.serviceModel>

</configuration>