How Do I Access A Class Froma Another Class Without Using New

When you use new you create a new instance of the class, which has all it's fields and properties independent to any existing instances: it's like cars. It you put your wallet in the glove box of your car, then buy a new car - would you expect to find your wallet in the new car's glove box?

So you are right not to use new if you are trying to access the existing main window - you need to get the actual instance of the window that the user is looking at.

If you are doing this from within the MainWindow class, then that;s pretty easy - you don't have to do anything because your code will automatically reference the current instance when you use a property or field name (unless you are inside a static method, where there is no instance).

If you are outside the class, then it's a lot, lot harder - and will depend on how your code works - because you wouldn't normally want code to refer to external windows as that breaks the OOPs principles. What are you actually trying to achieve that you need this?