添加到界面前获取尺寸

You have few options you can do this, forcing to call Window.Measure and Window.Arrange will make all values to be calculated, or you can get those values in the Window.Loaded event. This same issue is discussed already on this question.

If you are sizing to content:

window.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
window.Arrange(new Rect(0, 0, window.DesiredWidth, window.DesiredHeight));

If you are using an explicit window size:

window.Measure(new Size(Width, Height));
window.Arrange(new Rect(0, 0, window.DesiredWidth, window.DesiredHeight));

or

public MyWindow()
{
    Loaded += delegate
    {
        // access ActualWidth and ActualHeight here
    };

}

Just had the same issue and just calling textblock.Measure(new Size()); aint enough. I had to usetextblock.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity)); to get a result for textblock.DesiredSize

The key is to realize that setting it in code like this:

sp2.Margin = new System.Windows.Thickness{ Left = 5 };

is equivalent to:

sp2.Margin = new System.Windows.Thickness{ Left = 5, Top = 0, Right = 0, Bottom = 0 };