Passing info btn 2 exe's MDI child docs

[Google VB.NET]

Many years ago I developed a VB6 application that graphed custom data.
The data was kept in a text based file with a unique file extension.
The file extension was mapped to this application. When the user double
clicked on the data file, it launched the app and graphed the data.
Later I made the app a MDI app, therefore multiple graphs could be
loaded and viewed. To stop multiple instances of the app running, I
coded for this to. The newly launched app would check to see if another
instance was alreay running, and if so, pass the file details to that
instance and then terminate.

NOW, in vb.net, I can check for another instance, make it active and
then terminate the new instance. The problem is I do not know how to
pass the file details to the original instance of the app, so that I
can graph the data.

I don't have the origianal VB6 code anymore, so I cannot even see what
I did.

Can anyone send me a code snippet to show me how to pass the
'secondary' file details to the 'primary' instance of the application.

Thanks
Stuman