We have a very old windows forms application that communicates with the server using .net remoting.
Can anyone recommend a method or a tool to performance test this.
Last time I looked there were no good tools for performance testing a windows application with many users.
You can use a profiler to see what is going on if the single user case is too slow. If you only are about a hand full of users, most UI test tools could be used to record a script that you played back on a few machines.
Otherwise you need to write a command line application that talks to the server in the same way as the windows forms app, and then run many copies of that command line app.
Or get the server to log all calls in such a way as the calls can be played back.
In the past I have thought about getting each client to log the calls to the sever in the form of C# statements that could then be compiled to create a performance test program.
There are some systems that claims to record the communication between the client and the sever at the network level, and then play it back many times. This may work if it is sensible for lot of clients to send the some requests with the same params to the server, otherwise there will be lots of scripting within the tool to create custom requests for each client.