namespace System.Net.Sockets
{
	public static class SocketExtensions
	{
		private const int BytesPerLong = 4; // 32 / 8
		private const int BitsPerByte = 8;

		/// <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.Tcp, SocketOptionName.KeepAlive, true);
				socket.IOControl(IOControlCode.KeepAliveValues, inValue, outValue);
			}
			catch (SocketException e)
			{
				Console.WriteLine("Failed to set keep-alive: {0} {1}", e.ErrorCode, e);
				return false;
			}

			return true;
		}
	}
}

Sets the keep-alive interval for the socket.

.Net Socket keep-alive extension

c#

C#