If I select multiple files in explorer and hit enter it opens each file in a separate instance of my application. What do I need to do to get all the files to be passed on the command line in only one instance of my application?

Anyone have an idea?

You'll need to fiddle around with synchronization objects and inter-process communication to achieve what you are after.

http://msdn.microsoft.com/en-us/library/windows/desktop/ms686364
http://msdn.microsoft.com/en-us/library/windows/desktop/aa365574

http://stackoverflow.com/questions/8799646/preventing-multiple-instances-of-my-application