I know the class gb.args and how command line arguments will be passed to a Gambas application. It's probably me but I have no idea how to proceed with this use case:
In a filemanager (i.e. KDE/Dolphin) I want to right-click an image. The context menue appears and I want to start my Gambas fileviewer-app to open and view the selected file. The filemanager opens my coded Gambas-fileviewer-app in the background passing just a single argument (path/filename as string) to the app.
Unfortunately, Gambas arguments require two dashes "–" upfront and that's why is null in my fileviewer-app. Any idea how to pass a single argument to the app without two dashes?

I don't really understand your question or why you are talking about gb.Args, I think you just need the standard Application class.

I would have thought you could just start your Gambas program with something like: myImageViewer filename

…and then read the filename from Application.Args[0] in your code.

 sorry for the misunderstanding. I want the filemanager Dolphin to start my app (see attachment). Application.Args[0] shows only the path and name of the app but not the filename I clicked in Dolphin. I would expect the filename L1007019.jpg in Application.Args[1] but it's empty.

(Click to enlarge)

There is no need for gb.args. The basic one works fine for what you are looking for. The 'Arguments' are passed to Gambas as an array. The 1st argument, [0], in the array is the name of your program so what you need is the 2nd argument, [1]. Your file manager will pass the full path to Gambas so, try this code: -

Yes sorry, its argument [1] as Charlie said, not [0] which is always the name of the program.

I modified some code to test it:-

I also modified "Open With..." properties in the file manager so it always loads my program if I double click on a RAW file type …but you may want to chose each time.

Thanks. Seems to be a user problem. This is my original example:

I found out that arg[1] is empty because the function val() destroys obviously the string. When I replace
with
it works! Thanks for your time.