c# how to open a file with your app

so i have a problem regarding opening files with my app.
so the problem is i can't open files when i select "open with > myapp" to open a file, it simply opens my application and doesnt display the contents of the file. i have no code regarding this so if someone could help then it would be great relief.

I will to try explain my problem with an example:

for example there is a text document when we double click it then notepad opens that file and display its contents.
i want my app to do the same.