SetSocketKeepAliveValues

/// <summary>
/// Using IOControl code to configue socket KeepAliveValues for line disconnection detection(because default is toooo slow) 
/// </summary>
/// <param name="tcpc">TcpClient</param>
/// <param name="KeepAliveTime">The keep alive time. (ms)</param>
/// <param name="KeepAliveInterval">The keep alive interval. (ms)</param>
public static void SetSocketKeepAliveValues(this TcpClient tcpc, int KeepAliveTime, int KeepAliveInterval)
{    
    //KeepAliveTime: default value is 2hr
    //KeepAliveInterval: default value is 1s and Detect 5 times
   
    uint dummy = 0; //lenth = 4
    byte[] inOptionValues = new byte[System.Runtime.InteropServices.Marshal.SizeOf(dummy) * 3]; //size = lenth * 3 = 12
    bool OnOff = true;

    BitConverter.GetBytes((uint)(OnOff ? 1 : 0)).CopyTo(inOptionValues, 0);
    BitConverter.GetBytes((uint)KeepAliveTime).CopyTo(inOptionValues, Marshal.SizeOf(dummy));
    BitConverter.GetBytes((uint)KeepAliveInterval).CopyTo(inOptionValues, Marshal.SizeOf(dummy) * 2);
    // of course there are other ways to marshal up this byte array, this is just one way
    // call WSAIoctl via IOControl

    // .net 1.1 type
    //int SIO_KEEPALIVE_VALS = -1744830460; //(or 0x98000004)
    //socket.IOControl(SIO_KEEPALIVE_VALS, inOptionValues, null); 

    // .net 3.5 type
    tcpc.Client.IOControl(IOControlCode.KeepAliveValues, inOptionValues, null);
}

TcpClient t = new TcpClient();
t.SetSocketKeepAliveValues(300000, 1000);