Change WPF window background image in C# code

Question

I have a couple of Images configured as application resources.

When my application starts, the background of the main window is set via XAML:

Answer 1

I have been trying all the answers here with no success. Here is the simplest way to do it with ms-appx

        ImageBrush myBrush = new ImageBrush();
        Image image = new Image();
        image.Source = new BitmapImage(new Uri(@"ms-appx:///Assets/background.jpg"));
        myBrush.ImageSource = image.Source;
        TheGrid.Background = myBrush;

Assets folder is in the first level of my project, so make sure to change the path as convenient.