From the registry, for a given file type, I get a string containing something like this:
\"C:\\Program Files\\AppName\\Executable.exe\" /arg1 /arg2 /arg3
I believe (it has been a while since I did this) that you can just use:
System.Diagnostics.Process.Start(/*File to open*/);
and it will open the file with the default application if there is one. You did not need to know the application it is going to use.
Am I understanding what you are looking? Or did I miss something?