registering a c# rpogram

Assalmu 3alaykum. Lets say i am making a software like the Microsoft Word and i want to my software to read and write file in c#. Reading and writing is th easy stuff but the problem is i want to make a new thing which is operating system related and that is when i open a file "of extension .mw lets say" i want to automatically run my program and let the program reads the specific file just like Microsoft word .doc file or the html browser. I knw how to do that in the command prompt but the problem is that when i open the .mw file my software opens but ofcours without reading the opened file. Is there a way to read the opened .mw file from my software coded in c# ofcourse.