When an XAML is markup compiled, it has an associated code behind class file. The code behind is a partial class and derives from the same class, which is the root of the XAML page element:

    public partial class MainWindow : Window 
    { 
      public MainWindow() 
      { 
        InitializeComponent(); 
      } 
    } 

The constructor of the class gives a call to the InitializeComponent method available in another partial class implementation, which resides in a .g.i.cs file. For example, if your XAML page name is MainWindow.xaml, its associated code behind is MainWindow.xaml.cs and the core partial implementation is in an autogenerated file named MainWindow.g.i.cs. The .g.i.cs file can be located in the obj folder under the project. It loads your XAML page and defines the UI elements placed in the designer.

Here's how the InitializeComponent() method looks like: