How to access window elements from another class in C#?

Question

I have a WPF application, and I need to access a textbox from another class. In actionscript, I had a similar problem with accessing the stage, and it is solved by doing this:

var stageRef:Object;
//and then do
stageRef=this; //in the constructor

Then I can access anything on the stage, like this:

className.stageRef.textField.text = "text";

It can't get this approach to work in C#. Is there a simple way to do it?