I am using inno setup and I made an associated file type so that my app starts when a user double clicks an associated file. But how can I know if the application was started by a file so I can show the file? For now, my app simply starts running.
The filename is the first argument passed to your Application. You can (and must) handle it, there are several ways to access this in .net, e.g. with Environment.GetCommandLineArgs.