private void Window_MouseDown(object sender, MouseButtonEventArgs e)
    {
        Point point = PointToScreen(Mouse.GetPosition(this));
        RisuemPryamougolnik(point.X, point.Y);
    }

private void RisuemPryamougolnik(double x,double y)
    {
        Rectangle rect = new Rectangle();
        rect.Height = 50;
        rect.Width = 50;
        // rect.Fill = Brushes.Aqua;
        rect.Stroke = Brushes.Aqua;
        rect.StrokeThickness = 1;
        canvas.Children.Add(rect);
        Canvas.SetLeft(rect, x);
        Canvas.SetTop(rect, y);

    }