I am trying to access a TCP service on an AS/400 via C# with no success. The service takes requests to cancel shipments and returns if the shipment was successfully cancelled or has already been shipped.

I have tried coding the connection using both the TCPClient class and a synchronous Socket, but neither worked. When I used the socket, I could verify that I was actually connected via the socket's Connected property. I sent my request message, but I never got a response. Once, a shipment was actually cancelled, but I didn't get a response from the server. I found out by viewing the server's log.

I checked that my request syntax matched the command protocol many times. I played around with variations on the exposed properties of sockets and TCPClients without success.

I didn't use an asynchronous socket because the method making this request of the server cannot proceed until it finds out if the shipment could be cancelled.

I am wondering how to proceed to troubleshoot this. I am not an AS/400 programmer, but I have colleagues here who are. Not knowing much about network programming, I am hoping that someone has suggestions.

I thought about having the AS/400 log every request the service got, so I could verify that my requests were being received correctly. It does log successful cancellation requests currently.

Any troubleshooting tips for my client app? I am starting to suspect the server, only I know that other applications are accessing it and it is working at least sometimes, because a half-dozen or so requests are successfully fulfilled every day.