associate a file extension with my executable, how to receive filename?

I install the application.

I make a file association extension because I want to do this:  when double-click on a file with the extension ".elp" then runs Edit_epl.exe.

   example doubleclick on:  "my_sample.epl"  ---> then runs Edit_eplp.exe.   so far,so good.  

My question is how to know its filename (my_sample.epl) as "parameter" inside "Edit_eplp.exe"

Can anyone help me?

Thanks