C# Socket.BeginReceive/EndReceive

Question

Socket.BeginReceive/EndReceive 函数按什么顺序调用？

例如，我调用 BeginReceive 两次，一次是为了获取消息长度，第二次是为了获取消息本身。现在的情况是这样的，对于我发送的每条消息，我开始等待它的完成（实际上是对发送的消息的确认，我也在收到确认后等待操作的完成），所以我调用 BeginReceive 对于每个 BeginSend，但在每个 BeginReceive 的回调中，我检查是否收到长度或消息。如果我正在接收消息并且已完全接收，那么我会调用另一个 BeginReceive 来接收操作的完成情况。现在事情就变得不同步了。因为我的接收回调之一正在接收字节，它会将其解释为消息的长度，而实际上它是消息本身。

现在我该如何解决呢？

编辑：这是一个 C#.NET 问题:)

这是代码，基本上它太大了，抱歉

public void Send(string message)
{
    try
    {
        bytesSent = 0;

        writeDataBuffer = System.Text.Encoding.ASCII.GetBytes(message);
        writeDataBuffer = WrapMessage(writeDataBuffer);
        messageSendSize = writeDataBuffer.Length;

        clientSocket.BeginSend(writeDataBuffer, bytesSent, messageSendSize, SocketFlags.None,
                            new AsyncCallback(SendComplete), clientSocket);
    }
    catch (SocketException socketException)
    {
        MessageBox.Show(socketException.Message);
    }
}

public void WaitForData()
{
    try
    {
        if (!messageLengthReceived)
        {
            clientSocket.BeginReceive(receiveDataBuffer, bytesReceived, MESSAGE_LENGTH_SIZE - bytesReceived,
                                    SocketFlags.None, new AsyncCallback(RecieveComplete), clientSocket);
        }
}

public void Send(string message)
{
    try
    {
        bytesSent = 0;

        writeDataBuffer = System.Text.Encoding.ASCII.GetBytes(message);
        writeDataBuffer = WrapMessage(writeDataBuffer);
        messageSendSize = writeDataBuffer.Length;

        clientSocket.BeginSend(writeDataBuffer, bytesSent, messageSendSize, SocketFlags.None,
                            new AsyncCallback(SendComplete), clientSocket);
    }
    catch (SocketException socketException)
    {
        MessageBox.Show(socketException.Message);
    }
}

public void WaitForData()
{
    try
    {
        if (! messageLengthReceived)
        {
            clientSocket.BeginReceive(receiveDataBuffer, bytesReceived, MESSAGE_LENGTH_SIZE - bytesReceived,
                                    SocketFlags.None, new AsyncCallback(RecieveComplete), clientSocket);
        }
        else 
        {
            clientSocket.BeginReceive(receiveDataBuffer, bytesReceived, messageLength - bytesReceived,
                                    SocketFlags.None, new AsyncCallback(RecieveComplete), clientSocket);
        }
    }
    catch (SocketException socketException)
    {
        MessageBox.Show(socketException.Message);
    }
}

public void RecieveComplete(IAsyncResult result)
{
    try
    {
        Socket socket = result.AsyncState as Socket;
        bytesReceived = socket.EndReceive(result);

        if (! messageLengthReceived)
        {
            if (bytesReceived != MESSAGE_LENGTH_SIZE)
            {
                WaitForData();
                return;
            }

            // unwrap message length
            int length = BitConverter.ToInt32(receiveDataBuffer, 0);
            length = IPAddress.NetworkToHostOrder(length);

            messageLength = length;
            messageLengthReceived = true;

            bytesReceived = 0;

            // now wait for getting the message itself
            WaitForData();
        }
        else
        {
            if (bytesReceived != messageLength)
            {
                WaitForData();
            }
            else
            {
                string message = Encoding.ASCII.GetString(receiveDataBuffer);

                MessageBox.Show(message);

                bytesReceived = 0;
                messageLengthReceived = false;

                // clear buffer
                receiveDataBuffer = new byte[AsyncClient.BUFFER_SIZE];

                WaitForData();
            }
        }
    }
    catch (SocketException socketException)
    {
        MessageBox.Show(socketException.Message);
    }

}

public void SendComplete(IAsyncResult result)
{
    try
    {
        Socket socket = result.AsyncState as Socket;
        bytesSent = socket.EndSend(result);

        if (bytesSent != messageSendSize)
        {
            messageSendSize -= bytesSent;

            socket.BeginSend(writeDataBuffer, bytesSent, messageSendSize, SocketFlags.None,
                            new AsyncCallback(SendComplete), clientSocket);
            return;
        }

        // wait for data
        messageLengthReceived = false;
        bytesReceived = 0;

        WaitForData();
    }
    catch (SocketException socketException)
    {
        MessageBox.Show(socketException.Message);
    }
}

Answer 1

时间顺序应为：

1. BeginReceive 表示消息长度
2. EndReceive 表示 #1 的完成
3. BeginReceive 表示消息正文
4. EndReceive code> 用于完成 #3

例如不使用你可以拥有的回调：

var sync = socket.BeginReceive(....);
sync.AsyncWaitHandle.WaitOne();
var res = socket.EndReceive(sync);
sync = socket.BeginReceive(....);
sync.AsyncWaitHandle.WaitOne();
var res2 = socket.EndReceive(sync);

但是，你最好只使用Receive！

我认为您可能会发现对两个不同的接收使用单独的处理程序会更容易：

... Start(....) {
    sync = socket.BeginReceive(.... MessageLengthReceived, null);
}

private void MessageLengthReceived(IAsyncResult sync) {
  var len = socket.EndReceive(sync);
  // ... set up buffer etc. for message receive

 sync = socket.BeginReceive(... MessageReceived, null);
}

private void MessageReceived(IAsyncResult sync) {
  var len = socket.EndReceive(sync);
  // ... process message
}

最终将所有关联的状态对象放入状态对象中并从 BeginReceive 传递它（在通过 IAsyncResult.AsyncState 的完成委托访问中）可以让事情变得更容易，但确实需要从命令式代码的线性思维转变为完全采用事件驱动的方法。

---

2012 附录：

.NET 4.5 版本

随着 C#5 中的异步支持，出现了一个新选项。这使用编译器从内联代码生成手动延续（单独的回调方法）和闭包（状态）。然而，有两件事需要解决：

1. 虽然 System.Net.Sockets.Socket 有各种 ...Async 方法，但这些方法适用于基于事件的异步模式，而不是基于事件的异步模式。 C#5 的 await 使用的基于 Task 的模式。解决方案：使用 TaskFactory.FromAsync 从 Begin... End... 对中获取单个 Task。

2. TaskFactory.FromAsync 仅支持将最多三个附加参数（除了回调和状态之外）传递给 Begin...。解决方案：采用零个附加参数的 lambda 具有正确的签名，并且 C# 将为我们提供正确的闭包来传递参数。

因此（通过 Message 更充分地实现，它是另一种处理从初始发送以固定数量的字节编码的长度，然后将内容字节转换为内容缓冲区的长度）：

private async Task<Message> ReceiveAMessage() {
  var prefix = new byte[Message.PrefixLength];

  var revcLen = await Task.Factory.FromAsync(
                         (cb, s) => clientSocket.BeginReceive(prefix, 0, prefix.Length, SocketFlags.None, cb, s),
                         ias => clientSocket.EndReceive(ias),
                         null);
  if (revcLen != prefix.Length) { throw new ApplicationException("Failed to receive prefix"); }
  
  int contentLength = Message.GetLengthFromPrefix(prefix);
  var content = new byte[contentLength];
  
  revcLen = await Task.Factory.FromAsync(
                         (cb, s) => clientSocket.BeginReceive(content, 0, content.Length, SocketFlags.None, cb, s),
                         ias => clientSocket.EndReceive(ias),
                         null);
  if (revcLen != content.Length) { throw new ApplicationException("Failed to receive content"); }
  
  return new Message(content);
}

Answer 2

也许您想要做的是链接您的回调：

伪代码：

// read the first 2 bytes as message length
BeginReceive(msg,0,2,-,-,new AsyncCallback(LengthReceived),-)

LengthReceived(ar) {
  StateObject so = (StateObject) ar.AsyncState;
  Socket s = so.workSocket;
  int read = s.EndReceive(ar);
  msg_length = GetLengthFromBytes(so.buffer);
  BeginReceive(so.buffer,0,msg_length,-,-,new AsyncCallback(DataReceived),-)
}

DataReceived(ar) {
  StateObject so = (StateObject) ar.AsyncState;
  Socket s = so.workSocket;
  int read = s.EndReceive(ar);
  ProcessMessage(so.buffer);
  BeginReceive(so.buffer,0,2,-,-,new AsyncCallback(LengthReceived),-)
}

请参阅： http://msdn.microsoft.com/en-us/library/system.asynccallback.aspx 获取正确示例

Answer 3

通常，BeginXXX 方法表示异步操作，而您似乎希望以同步方式执行此操作。

如果您确实想要同步客户端/服务器，也许这会有所帮助 http://sharpoverride.blogspot.com/2009/04/another-tcpip-server-client-well-it.html

Answer 4

如果您描述所发送消息的结构将会有所帮助。

只要您只有一个 BeginReceive() 未完成，它就会完成并为您提供线路上的下一个可用数据字节。如果您同时有多个未完成的任务，那么所有的赌注都会被取消，因为 .net 不保证按照任何给定的顺序完成。

Answer 5

正如其他人所说，不要在这里使用全局变量 - 使用套接字状态类。类似于：

public class StateObject
{
    public const int DEFAULT_SIZE = 1024;           //size of receive buffer

    public byte[] buffer = new byte[DEFAULT_SIZE];  //receive buffer
    public int dataSize = 0;                        //data size to be received
    public bool dataSizeReceived = false;           //received data size?
    public StringBuilder sb = new StringBuilder();  //received data String
    public int dataRecieved = 0;

    public Socket workSocket = null;                //client socket.
    public DateTime TimeStamp;                      //timestamp of data
} //end class StateObject

在尝试重新发送消息之前，您应该验证套接字...您可能会遇到套接字异常。

您可能应该得到回报；在 ReceiveComplete 的“if”块中调用 WaitForData 之后。

Timothy Pratley 上面说过，第二次通过时会在 bytesRecieved 中出现一个错误。每次您仅测量从该 EndReceived 接收的字节数，然后将其与 messageLength 进行比较。您需要保留所有已接收字节的总和。

您最大的错误是，在第一次调用 ReceiveComplete 时，您考虑到消息可能（很可能）包含比消息大小更多的数据 - 它可能也包含消息的一半。您需要去掉数据大小，然后将消息的其余部分存储在消息变量中。