Simple Network Sniffer - Open Source - C# (C-sharp)

// Author: Toomas Kaljus
// http://www.digigrupp.com

namespace DG
{
    public class UDPPacket // rfc768
    {
        public ushort SourcePort;
        public ushort DestinationPort;
        public ushort Length;
        public ushort Checksum;
        public byte[] PacketData;
        
        public UDPPacket() : base() { }

        public UDPPacket(ref byte[] Packet) : base()
        {
            try {
                SourcePort = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 0));
                DestinationPort = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 2));
                Length = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 4));
                Checksum = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 6));
                PacketData = new byte[Packet.Length - 8];
                System.Buffer.BlockCopy(Packet, 8, PacketData, 0, Packet.Length - 8);
            } catch { }
        }

        public byte[] GetBytes(ref System.Net.IPAddress SourceAddress, ref System.Net.IPAddress DestinationAddress)
        {
            if (PacketData == null) PacketData = new byte[0];
            byte[] Packet = new byte[8 + PacketData.Length];
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)SourcePort)), 0, Packet, 0, 2);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)DestinationPort)), 0, Packet, 2, 2);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)Length)), 0, Packet, 4, 2);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes((short)0), 0, Packet, 6, 2);
            System.Buffer.BlockCopy(PacketData, 0, Packet, 8, PacketData.Length);
            Checksum = GetChecksum(ref Packet, 0, 8 - 1, ref SourceAddress, ref DestinationAddress);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes((short)Checksum), 0, Packet, 6, 2);
            return PacketData;
        }

        public ushort GetChecksum(ref byte[] Packet, int start, int end, ref System.Net.IPAddress SourceAddress, ref System.Net.IPAddress DestinationAddress)
        {
            byte[] PseudoPacket;
            PseudoPacket = new byte[12 + Packet.Length];
            System.Buffer.BlockCopy(SourceAddress.GetAddressBytes(), 0, PseudoPacket, 0, 4);
            System.Buffer.BlockCopy(DestinationAddress.GetAddressBytes(), 0, PseudoPacket, 4, 4);
            PseudoPacket[8] = 0;
            PseudoPacket[9] = 17;
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)Packet.Length)), 0, PseudoPacket, 10, 2);
            System.Buffer.BlockCopy(Packet, 0, PseudoPacket, 12, Packet.Length);
            return IPPacket.GetChecksum(ref PseudoPacket, 0, PseudoPacket.Length - 1);
        }
    }
}