I have a program with an associated file type. However, when I double-click a file to launch the program, I can't figure out where that path was passed to the program.
Initially I thought the file path would be in Environment.CommandLine, but the command line only contained the program path.
I specified the file association using the Publish Options in visual studio like so: extension: .ext, Description: Some File, ProgId: ProjectName, Icon: SomeIcon.ico.
You are right, it should indeed be passed as a command line argument and be available through Environment.CommandLine.
My guess is that something is wrong with the file association itself. Here is a dated but still useful TechNet article on how these things work. In particular, the "%1" in the screenshot below is essential.
I'm not sure about the DDE thing, so if you're not using it, turn it off.
As a guess, your file type association may not be configured properly. If you look in the registry at your "Open" command for your file type, the command is probably configured as "c:\your-company\your-app.exe", instead of as "c:\your-company\your-app.exe" "%1", which is what it would need to be if you wanted the file path to show up on the command line.