OpenFileDialog issue

Hey so basically i did a lot of research on this but could not find a solution.
I need a function to occur if one or more files are opened by an openfiledialog
So if 1 file was selected i need <function> to occur.
But if no file was selected i do not want this function to occur while showing a msgbox asking the user to select 1 or more files first.
So this is what i thought.

If openfiledialog.[1 or more files chosen] then
   <Do Function>
Else
   msgbox("Select a file")
Endif

Any ideas??