By hidden source code file…
A hidden source code file is auto-generated for each WPF XAML file. The names used for this file and for the class it contains are based off the XAML file’s filename. In a C# application, the file MyWindow.xaml will have a hidden code-behind file named MyWindow.g.cs containing a class named MyWindow.
…containing a class…
The contained class inherits from the WPF core class corresponding to the associated XAML file’s root element. If the root element in the XAML file is <Window>, the class in the hidden code-behind file will inherit from Window.