BinaryWriter writer = new BinaryWriter(new MemoryStream());

writer.Write((byte)0xFF);
writer.Write((byte)packetId);

writer.Write((short)0);

writer.Writer(...); // your data

writer.Position = 2;

writer.BaseStream.Write((short)writer.BaseStream.Length);

yourSocket.Send(((MemoryStream)writer.BaseStream).ToArray[]);