Once you have created the file association it
should start your application. It is how your application treats the file passed to it that makes it operate like Word or Excel.
If your problem is that when you click on a file it opens a new instance of your application rather than using the already open one then you need to be searching for
c# single instance application.
here[
^] is one article describing how to do it. There are several ways to do it though so I recommend your searching for yourself and reading a couple of the links.
Good luck. :)
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