Current implementation of QuicListener (backed by MsQuic) allows starting multiple listeners in the same process on the same machine port (each listener listening for connections with different ALPNs). When attempting to start a second listener with the same ALPN (e.g. second HTTP3 listener), we reported an exception with an internal error which caused some confusion among the users (see #73045).

This type of error does not fall in any of the existing QuicError members used to communicate the reason behind a QuicException. The proposed solution is to add a new member AlpnInUse to cover this error case.

    public enum QuicError
    {
        // existing members elided

+       /// <summary>
+       /// Another QUIC listener is already listening on one of the requested application protocols on the same port.
+       /// </summary>
+       AlpnInUse,
    }
}