WPF：使用框架技术从列表框菜单页面导航？

I got a problem. I added a frame in the window xaml to load pages in. I can directly load a page into frame with Source tag of the frame. It works. I need to use the code in C# to refer to the link from listbox menu a poplulate an apropriate link when an listbox item is selected. My problem is that I cannot refer the frame in C# code, it just cannot be seen. I defined the frame with x:Name="ContentFrame". When I refer to in in C#, Intellisense tells that "The name "ContentFrame" does not exist in the current context". What I am doing wrong? I am lost here. Any ideas are highly appreciated. Here is the code:I got a problem. I added a frame in the window