WPF call method parent from usercontrol

You'll need to cast the Window object to the specific window type you're using - which in your case is MainWindow:

MainWindow win = (MainWindow)Window.GetWindow(this);
win.getList();

However, it's not wise to have such coupling between the user control and the window it's hosted in, since that means you will only be able to use it in a window of type MainWindow. It would be better to expose a dependency property in the user control and bind the list to that property - this way the user control will have the data it requires and it will also be reusable in any type of window.