Accessing XAML from non-main class

Hello,

I am making a video game. Some time ago, one person recommended me to use separate classes, and not have everything in one file. I did manage to separate some of functions and variables, but unfortunatelly, only those that don't affect xaml elements.

 

Mostly, xaml elements that change during game: label called "TextOutput", a few grids (each grid is a room) and a large number of images and buttons.

 

I will give an example, there is a door that can be opened.

 

So, the code for clicking on door:

 

 

 

This works perfectly - in the main class. But if I try to move the void to another class, or even make a brand new void, that is supposed to change xaml elements, I get an error:

 

 

 I don't really understand how to bypass this. Xaml elements aren't static, and I already read that I shouldn't change them to be static, even if it's possible.

 

And if I create a variable of Button\Image\Grid etc, this will allow me to change their own attributes, not of those I need.

 

 I found out this article:

http://www.c-sharpcorner.com/UploadFile/theLizard/how-to-access-a-control-in-another-form/ 

 

It says about accessing controls from other FORMS - so I am not sure if it will work in my case, since I am using WPF, and not sure that this approach will avoid "object reference required" error.

 

What can be done to solve this?

 

Thank you in advance,

Evgenie