public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        System.Windows.Controls.Button b = new System.Windows.Controls.Button();
        System.Windows.Shapes.Rectangle r = new System.Windows.Shapes.Rectangle();
        r.Width = 40;
        r.Height = 40;
        r.Fill = System.Windows.Media.Brushes.Black;
        b.Content = r; // Make the square the content of the Button
        this.AddChild(b);
    }
}