Inconsistent behavior of Socket.SendToAsync(SocketAsyncEventArgs)

[sakno]

Socket.SendToAsync(SocketAsyncEventArgs) method has inconsistent behavior on different platforms if used for UDP socket for sending datagram to loopback address. On Linux (Ubuntu 18.04.03 LTS x64) it returns synchronously and SocketAsyncEventArgs.SocketError is SocketError.ConnectionRefused. On Windows 10 it returns synchronously but SocketError is SocketError.Success.

[scalablecory]

It looks like Windows might have similar behavior if you set the SIO_UDP_CONNRESET option.

[karelz]

Triage:

• We should not hide additional information we got on specific OS (Linux) and give it to the user if we can.
• We may consider improving Windows to behave as Linux -- see Inconsistent behavior of Socket.SendToAsync(SocketAsyncEventArgs)  #34474 (comment) above
• We should verify if the socket is still usable when we receive the error (if not we may want to fix it)
• We may want to mention it in documentation that the calls might fail

[wfurt]

triage: there was never guarantee about finishing synchronously or not. This is not part of the contract and it should be relayed on. We can investigate SIO_UDP_CONNRESET to make it more consistent but that may break others.
Low priority for now. We should document it better to avoid surprises.