In the case of a connection failure when a System.Net.DnsEndPoint was specified for the SocketAsyncEventArgs.RemoteEndPoint property, the SocketAsyncEventArgs.ConnectByNameError property will contain the exception that indicates the detailed cause of the socket error.

When an System.Net.IPEndPoint was specified for the SocketAsyncEventArgs.RemoteEndPoint property and a connection failure occurs, the SocketAsyncEventArgs.ConnectByNameError property will be a null reference.

The SocketAsyncEventArgs.SocketError property is always set in the case of a connection failure. The SocketAsyncEventArgs.ConnectByNameError property contains more information about the failure if it was a failure connecting to a System.Net.DnsEndPoint. If an application is only interested in whether the connect operation succeeded or failed, then the application only needs to check the SocketAsyncEventArgs.SocketError property.