如何在C#中检查套接字是否已连接/断开？

Question

如果其他主机在断开连接时没有向您发送数据包(例如因为它不正常地断开连接),如何检查网络套接字(System.Net.Sockets.Socket)是否仍然连接？

Answer 1

由于保罗特纳 回答Socket.Connected不能在这种情况下使用.您需要每次轮询连接以查看连接是否仍处于活动状态.这是我用过的代码:

bool SocketConnected(Socket s)
{
    bool part1 = s.Poll(1000, SelectMode.SelectRead);
    bool part2 = (s.Available == 0);
    if (part1 && part2)
        return false;
    else
        return true;
}

它的工作原理如下:

• s.Poll 如果返回true
  • 连接已关闭,重置,终止或挂起(表示没有活动连接)
  • 连接处于活动状态,并且有可供读取的数据
• s.Available 返回可用于读取的字节数
• 如果两者都是真的:
  • 没有可读数据,因此连接无效

Answer 2

作为禅达写道,这是不错的使用Socket.Poll和Socket.Available,但你需要考虑到插座可能没有在第一时间进行初始化.这是最后一条(我相信)信息,由Socket.Connected酒店提供.修改后的方法版本如下所示:

 static bool IsSocketConnected(Socket s)
    {
        return !((s.Poll(1000, SelectMode.SelectRead) && (s.Available == 0)) || !s.Connected);

/* The long, but simpler-to-understand version:

        bool part1 = s.Poll(1000, SelectMode.SelectRead);
        bool part2 = (s.Available == 0);
        if ((part1 && part2 ) || !s.Connected)
            return false;
        else
            return true;

*/
    }

Answer 3

该Socket.Connected属性将告诉您套接字是否认为它已连接.它实际上反映了在套接字上执行的最后一次发送/接收操作的状态.

如果套接字已被您自己的操作关闭(处理套接字,调用方法断开连接),Socket.Connected将返回false.如果套接字已通过其他方式断开连接,则该属性将返回,true直到您下次尝试发送或接收信息为止,此时将抛出SocketException或ObjectDisposedException将被抛出.

您可以在发生异常后检查属性,但之前不可靠.

Answer 4

如果您拔掉网络电缆，接受的答案似乎不起作用。或者服务器崩溃。或者你的路由器崩溃了。或者，如果您忘记支付互联网账单。设置 TCP 保持活动选项以获得更好的可靠性。

public static class SocketExtensions
{
    public static void SetSocketKeepAliveValues(this Socket instance, int KeepAliveTime, int KeepAliveInterval)
    {
        //KeepAliveTime: default value is 2hr
        //KeepAliveInterval: default value is 1s and Detect 5 times

        //the native structure
        //struct tcp_keepalive {
        //ULONG onoff;
        //ULONG keepalivetime;
        //ULONG keepaliveinterval;
        //};

        int size = Marshal.SizeOf(new uint());
        byte[] inOptionValues = new byte[size * 3]; // 4 * 3 = 12
        bool OnOff = true;

        BitConverter.GetBytes((uint)(OnOff ? 1 : 0)).CopyTo(inOptionValues, 0);
        BitConverter.GetBytes((uint)KeepAliveTime).CopyTo(inOptionValues, size);
        BitConverter.GetBytes((uint)KeepAliveInterval).CopyTo(inOptionValues, size * 2);

        instance.IOControl(IOControlCode.KeepAliveValues, inOptionValues, null);
    }
}



// ...
Socket sock;
sock.SetSocketKeepAliveValues(2000, 1000);

时间值设置自上次发送数据以来的超时时间。然后它尝试发送和接收一个保持活动的数据包。如果失败，它会在确定连接失效之前指定的时间间隔内重试 10 次（自 Vista AFAIK 以来的硬编码次数）。

因此，上述值将导致 2+10*1 = 12 秒的检测。之后，套接字上的任何读取/写入/轮询操作都应该失败。

Answer 5

我根据此 MSDN文章提出了一种扩展方法。这样可以确定套接字是否仍然连接。

public static bool IsConnected(this Socket client)
{
    bool blockingState = client.Blocking;

    try
    {
        byte[] tmp = new byte[1];

        client.Blocking = false;
        client.Send(tmp, 0, 0);
        return true;
    }
    catch (SocketException e)
    {
        // 10035 == WSAEWOULDBLOCK
        if (e.NativeErrorCode.Equals(10035))
        {
            return true;
        }
        else
        {
            return false;
        }
    }
    finally
    {
        client.Blocking = blockingState;
    }
}

Answer 6

public static class SocketExtensions
{
    private const int BytesPerLong = 4; // 32 / 8
    private const int BitsPerByte = 8;

    public static bool IsConnected(this Socket socket)
    {
        try
        {
            return !(socket.Poll(1000, SelectMode.SelectRead) && socket.Available == 0);
        }
        catch (SocketException)
        {
            return false;
        }
    }


    /// <summary>
    /// Sets the keep-alive interval for the socket.
    /// </summary>
    /// <param name="socket">The socket.</param>
    /// <param name="time">Time between two keep alive "pings".</param>
    /// <param name="interval">Time between two keep alive "pings" when first one fails.</param>
    /// <returns>If the keep alive infos were succefully modified.</returns>
    public static bool SetKeepAlive(this Socket socket, ulong time, ulong interval)
    {
        try
        {
            // Array to hold input values.
            var input = new[]
            {
                (time == 0 || interval == 0) ? 0UL : 1UL, // on or off
                time,
                interval
            };

            // Pack input into byte struct.
            byte[] inValue = new byte[3 * BytesPerLong];
            for (int i = 0; i < input.Length; i++)
            {
                inValue[i * BytesPerLong + 3] = (byte)(input[i] >> ((BytesPerLong - 1) * BitsPerByte) & 0xff);
                inValue[i * BytesPerLong + 2] = (byte)(input[i] >> ((BytesPerLong - 2) * BitsPerByte) & 0xff);
                inValue[i * BytesPerLong + 1] = (byte)(input[i] >> ((BytesPerLong - 3) * BitsPerByte) & 0xff);
                inValue[i * BytesPerLong + 0] = (byte)(input[i] >> ((BytesPerLong - 4) * BitsPerByte) & 0xff);
            }

            // Create bytestruct for result (bytes pending on server socket).
            byte[] outValue = BitConverter.GetBytes(0);

            // Write SIO_VALS to Socket IOControl.
            socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
            socket.IOControl(IOControlCode.KeepAliveValues, inValue, outValue);
        }
        catch (SocketException)
        {
            return false;
        }

        return true;
    }
}

1. 将 SocketExtensions 类复制到您的项目中
2. 在您的套接字上调用 SetKeepAlive - socket.SetKeepAlive(1000, 2);
3. 添加一个定时器来检查 IsConnected 函数