.NET中的Socket类提供了网络通信常用的方法，分别提供了同步和异步两个版本，其中异步的实现是基于APM异步模式实现，即BeginXXX/EndXXX的方式。异步方法由于其非阻塞的特性，在需考虑程序性能和伸缩性的情况下，一般会选择使用异步方法。但使用过Socket提供的异步方法的同学，应该都会注意到了Socket的异步方法是无法设置Timeout的。以Receive操作为例，Socket提供了一个ReceiveTimeout属性，但该属性设置的是同步版本的Socket.Receive()方法的Timeout值，该设置对异步的Socket.BeginReceive()无效：如果对方没有返回任何消息，则BeginReceive操作将无法完成，其中提供的回调函数也将不会调用。如下示例代码所示：
private static void TestSocketBeginReceive()
{
Socket socket = new Socket(AddressFamily.InterNetwork,
SocketType.Dgram, ProtocolType.Udp);
byte[] content = Encoding.ASCII.GetBytes("Hello world");
IPAddress ip = Dns.Resolve("www.google.com").AddressList[0];
IPEndPoint receiver = new IPEndPoint(ip, 80);
socket.BeginSendTo(content, 0, content.Length, SocketFlags.None,
receiver, SendToCb, socket);
Console.WriteLine("Sent bytes: " + content.Length);
}
private static void SendToCb(IAsyncResult ar)
{
var socket = ar.AsyncState as Socket;
socket.EndSendTo(ar);
byte[] buffer = new byte[1024];
IAsyncResult receiveAr = socket.BeginReceive(buffer, 0, buffer.Length,
SocketFlags.None, null, null);
int received = socket.EndReceive(receiveAr);
Console.WriteLine("Received bytes: " + received);
}
由于接收方不会返回任何消息，Socket.BeginReceive将永远不会完成，SentToCb方法中的socket.EndReceive()调用将永远阻塞，应用程序也无法得知操作的状态。
支持Timeout
在个别的应用场景下，我们希望既能使用Socket的异步通信方法，保证程序的性能，同时又希望能指定Timeout值，当操作没有在指定的时间内完成时，应用程序能得到通知，以进行下一步的操作，如retry等。以下介绍的就是一种支持Timeout的Socket异步Receive操作的实现，方式如下：
1.基于APM异步模式封装Socket.BeginReceive/EndReceive方法。
2.使用ThreadPool提供的RegisterWaitForSingleObject()方法注册一个WaitOrTimerCallback，如果指定时间内操作未完成，则结束操作，并设置状态为Timeout。
3.将上述封装实现为Socket的扩展方法方便调用。
以下代码简化了所有的参数检查和异常处理，实际使用中需添加相关逻辑。
AsyncResultWithTimeout
首先看一下IAsyncResult接口的实现：
public class AsyncResultWithTimeout : IAsyncResult
{
private ManualResetEvent m_waitHandle = new ManualResetEvent(false);
public AsyncResultWithTimeout(AsyncCallback cb, object state)
{
this.AsyncState = state;
this.Callback = cb;
}
#region IAsyncResult
public object AsyncState { get; private set; }
public WaitHandle AsyncWaitHandle { get { return m_waitHandle; } }
public bool CompletedSynchronously { get { return false; } }
public bool IsCompleted { get; private set; }
#endregion
public AsyncCallback Callback { get; private set; }
public int ReceivedCount { get; private set; }
public bool TimedOut { get; private set; }
public void SetResult(int count)
{
this.IsCompleted = true;
this.ReceivedCount = count;
this.m_waitHandle.Set();
if (Callback != null) Callback(this);
}
public void SetTimeout()
{
this.TimedOut = true;
this.IsCompleted = true;
this.m_waitHandle.Set();
}
}
AsyncResultWithTimeOut类中包含了IAsyncResult接口中4个属性的实现、用户传入的AsyncCallback委托、接收到的字节数ReceivedCount以及两个额外的方法：
1.SetResult(): 用于正常接收到消息时设置结果，标记操作完成以及执行回调。
2.SetTimeout()：当超时时，标记操作完成以及设置超时状态。
StateInfo
StateInfo类用于保存相关的状态信息，该对象会作为Socket.BeginReceive()的最后一个参数传入。当接收到消息时，接收到的字节数会保存到AsyncResult属性中，并设置操作完成。当超时时，WatchTimeOut方法会将AsyncResult设置为TimeOut状态，并通过RegisteredWaitHandle属性取消注册的WaitOrTimerCallback.
public class StateInfo
{
public StateInfo(AsyncResultWithTimeout result, Socket socket)
{
this.AsycResult = result;
this.Socket = socket;
}
public Socket Socket { get; private set; }
public AsyncResultWithTimeout AsycResult { get; private set; }
public RegisteredWaitHandle RegisteredWaitHandle { get; set; }
}
封装Socket.BeginReceive
与Socket.BeginReceive方法相比，BeginReceive2添加了一个参数timeout，可以设置该操作的超时时间，单位为毫秒。BeginReceive2中调用Socket.BeginReceive()方法，其中指定的ReceiveCb回调将在正常接收到消息后将结果保存在stateInfo对象的AsyncResult属性中，该属性中的值就是BeginReceive2()方法返回的IAsyncResult。BeginReceive2调用Socket.BeginReceive后，在ThreadPool中注册了一个WaitOrTimerCallback委托。ThreadPool将在Receive操作完成或者Timeout时调用该委托。
public static class SocketExtension
{
public static int EndReceive2(IAsyncResult ar)
{
var result = ar as AsyncResultWithTimeout;
result.AsyncWaitHandle.WaitOne();
return result.ReceivedCount;
}
public static AsyncResultWithTimeout BeginReceive2
(
this Socket socket,
int timeout,
byte[] buffer,
int offset,
int size,
SocketFlags flags,
AsyncCallback callback,
object state
)
{
var result = new AsyncResultWithTimeout(callback, state);
var stateInfo = new StateInfo(result, socket);
socket.BeginReceive(buffer, offset, size, flags, ReceiveCb, state);
var registeredWaitHandle =
ThreadPool.RegisterWaitForSingleObject(
result.AsyncWaitHandle,
WatchTimeOut,
stateInfo, // 作为state传递给WatchTimeOut
timeout,
true);
// stateInfo中保存RegisteredWaitHandle，以方便在úWatchTimeOut
// 中unregister.
stateInfo.RegisteredWaitHandle = registeredWaitHandle;
return result;
}
private static void WatchTimeOut(object state, bool timeout)
{
var stateInfo = state as StateInfo;
// 设置的timeout前，操作未完成，则设置为操作Timeout
if (timeout)
{
stateInfo.AsycResult.SetTimeout();
}
// 取消之前注册的WaitOrTimerCallback
stateInfo.RegisteredWaitHandle.Unregister(
stateInfo.AsycResult.AsyncWaitHandle);
}
private static void ReceiveCb(IAsyncResult result)
{
var state = result.AsyncState as StateInfo;
var asyncResultWithTimeOut = state.AsycResult;
var count = state.Socket.EndReceive(result);
state.AsycResult.SetResult(count);
}
}
试一下
以下代码演示了如何使用BeginReceive2：
private static void TestSocketBeginReceive2()
{
Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
byte[] content = Encoding.ASCII.GetBytes("Hello world");
IPAddress ip = Dns.Resolve("www.google.com").AddressList[0];
IPEndPoint receiver = new IPEndPoint(ip, 80);
socket.BeginSendTo(content, 0, content.Length, SocketFlags.None, receiver, SendToCb2, socket);
Console.WriteLine("Sent bytes: " + content.Length);
}
private static void SendToCb2(IAsyncResult ar)
{
var socket = ar.AsyncState as Socket;
socket.EndSendTo(ar);
byte[] buffer = new byte[1024];
AsyncResultWithTimeout receiveAr = socket.BeginReceive2(2000, buffer, 0, buffer.Length, SocketFlags.None, null, null);
receiveAr.AsyncWaitHandle.WaitOne();
if (receiveAr.TimedOut)
{
Console.WriteLine("Operation timed out.");
}
else
{
int received = socket.EndReceive(ar);
Console.WriteLine("Received bytes: " + received);
}
}
输出结果如下：
上述实现是针对BeginReceive的封装，还可以以相同的方式将Send/Receive封装以支持Timeout， 或者更进一步支持retry操作。
附示例代码：files.cnblogs.com/dytes/SocketAsyncOpWithTimeOut.zip
本文转自：http://www.csharpwin.com/csharpspace/13263r8436.shtml