Send and receive text between Arduino Mega 2560 and C# via ENC28J60
You can use TCP or UDP protocols.

https://www.arduino.cc/en/Tutorial/UDPSendReceiveString

https://www.arduino.cc/en/Tutorial/DhcpChatServer

You can use free UDP, TCP port listener sofwares for Windows, Linux etc.. for testing your setup.

For C# development:

https://stackoverflow.com/questions/20038943/simple-udp-example-to-send-and-receive-data-from-same-socket 

and

https://stackoverflow.com/questions/10182751/server-client-send-receive-simple-text