c# - 如何只让一个线程运行临界区，同时丢弃其他线程而不挂起

Question

我正在使用 .NET Framework 4.0 和 C# 开发 Windows 服务。

此服务将打开一个套接字来接收命令。

我有这个套接字监听器类:

public class SocketListener
{
    private System.Net.Sockets.TcpListener m_server;

    public SQLServerSocketListener()
    {
        IPEndPoint ip = new IPEndPoint(IPAddress.Any, 5445);
        m_server = new System.Net.Sockets.TcpListener(ip);
    }

    public void Start()
    {
        m_server.Start();
        m_server.BeginAcceptTcpClient(new AsyncCallback(Callback), m_server);
    }

    public void Stop()
    {
        if (m_server != null)
            m_server.Stop();
    }

    private void Callback(IAsyncResult ar)
    {
        if (!(m_server.Server.IsBound) ||
            (m_server.Server == null))
            return;

        TcpClient client;
        try
        {
            client = m_server.EndAcceptTcpClient(ar);
        }
        catch (ObjectDisposedException)
        {
            //Listener canceled
            return;
        }
        DataHandler dataHandler = new DataHandler(client);
        ThreadPool.QueueUserWorkItem(dataHandler.HandleClient, client);

        m_server.BeginAcceptTcpClient(new AsyncCallback(Callback), m_server);
    }
}

这个类处理通过套接字接收到的命令:

class DataHandler
{
    private bool m_disposed = false;
    private TcpClient m_controlClient;

    private IPEndPoint m_remoteEndPoint;
    private string m_clientIP;

    private NetworkStream m_controlStream;
    private StreamReader m_controlReader;

    public DataHandler(TcpClient client)
    {
            m_controlClient = client;
    }

    public void HandleClient(object obj)
    {
            m_remoteEndPoint = (IPEndPoint)m_controlClient.Client.RemoteEndPoint;

            m_clientIP = m_remoteEndPoint.Address.ToString();

            m_controlStream = m_controlClient.GetStream();

            m_controlReader = new StreamReader(m_controlStream, true);

            string line;
            try
            {
                    while (((line = m_controlReader.ReadLine()) != null) ||
                            (m_controlClient == null) ||
                            (!m_controlClient.Connected))
                    {
                            CommandHandler.ProcessCommand(line);
                    }
            }
            catch (Exception ex)
            {
                    Console.WriteLine("CodeServerService.DataHandler error: {0}", ex.Message);
            }
            finally
            {
                    Dispose();
            }
    }
}

还有，CommandHandler:

class CommandHandler
{
    public static void ProcessCommand(string command, string connStringINICIC, string connStringTRZIC, byte codeLevel)
    {
        switch (command)
        {
            case "GetNewCodes<EOF>":
                CodesIncremental.GetNewCodes();
                break;
        }
    }
}

和CodesIncremental :

public class CodesIncremental
{
    public static bool GetNewCodes()
    {
        [ ... ]
    }
}

我的问题是我可以收到 GetNewCodes<EOF>在第一个完成之前发出命令。所以，我需要不让 GetNewCodes<EOF>如果有另一个 GetNewCodes<EOF> 则运行运行。

我怎么能不让跑CodesIncremental.GetNewCodes();如果此代码在另一个线程中运行？

我需要一些东西来丢弃 CodesIncremental.GetNewCodes(); 时收到的命令正在运行。

在伪代码中:

如果CodesIncremental.GetNewCodes();正在运行什么都不做。

Answer 1

此版本不阻塞。 CompareExchange确保原子性，因此只有一个线程会交换 _running 变量的值，其余线程会立即返回。

public class CodesIncremental
{
    static Int32 _running = 0;

    public static bool GetNewCodes()
    {
        if (Interlocked.CompareExchange(ref _running, 1, 0) == 1)
            return false;

        try
        {
            // Do stuff...
            return true;
        }
        finally
        {
            _running = 0;
        }
    }
}

与监视器或其他同步方法不同的是，此方法几乎没有争用，而且速度相当快。