What is wrong with Canvas.SetTop(element,position)

Question

I am trying to move and reposition elements with mouse and through code. But I guess I'm missing something or do something the wrong way. So I built a little sample app. It's just a empty wpf app with this MainWindow function

    public MainWindow()
    {
        InitializeComponent();
        Label lText = new Label();
        lText.Content = "this is my test label";
        lText.Height = 50;
        lText.Width = 50;
        lText.Background = Brushes.Aqua;
        // do I really need to do this? 
        lText.VerticalAlignment = System.Windows.VerticalAlignment.Top;
        lText.HorizontalAlignment = System.Windows.HorizontalAlignment.Left;
        // this part already fails
        Canvas.SetTop(lText, 20);
        Canvas.SetLeft(lText, 10);
        this.Content = lText;
    }

Answer 1

The solution was that the Left and Top properties must be set inside the Xaml before you can read it. I was always getting NaN and therefor couldn't set the correct value either.