How to obtain screen size from xaml?

I'm using wpf on C# to design GUI, and I want to get screen size (The value of Width and Height) from xaml code.

I knew how to get them from C# code as

   Width = System.Windows.Forms.Screen.PrimaryScreen.Bounds.Width;
   Height = System.Windows.Forms.Screen.PrimaryScreen.Bounds.Height;

But I don't know how to get them from XAML code.