SetKeepAlive - activates keep-alive mechanism for TcpClient.

TcpClientExtensions.cs

public static class TcpClientExtensions
{
    /// <summary>
    /// Activate Keep-Alive for TcpClient. Support only for Windows 2k and later
    /// </summary>
    /// <param name="client"></param>
    /// <param name="keepAliveTime">Time (in milliseconds) with no activity on a socket</param>
    /// <param name="keepAliveInterval">Interval (in milliseconds) between keep-alive packets. The number of packets depends on OS version.</param>
    public static void SetKeepAlive(this TcpClient client, uint keepAliveTime, uint keepAliveInterval)
    {
        List<byte> bytes = new List<byte>();
        bytes.AddRange(BitConverter.GetBytes((uint)(keepAliveTime != 0 ? 1 : 0)));
        bytes.AddRange(BitConverter.GetBytes(keepAliveTime));
        bytes.AddRange(BitConverter.GetBytes(keepAliveInterval));

        client.Client.IOControl(IOControlCode.KeepAliveValues, bytes.ToArray(), null);
    }
}