Stackpanel: Height vs ActualHeight vs ExtentHeight vs ViewportHeight vs DesiredSize vs RenderSize

As you know the StackPanel is a [Panel] object. Each panel communicates with its children by two methods to determine the final sizes and positions.The first method is MeasureOverride and the second is ArrangeOverride.

The MeasureOveride asks each child for desired size with a given amount of space available.ArrangeOverride arranges the children after measurement has been completed.

Let's create a stackpanel:

public class AnotherStackPanel : Panel{    public static readonly DependencyProperty OrientationProperty =        DependencyProperty.Register(“Orientation”, typeof(Orientation),        typeof(SimpleStackPanel), new FrameworkPropertyMetadata(        Orientation.Vertical, FrameworkPropertyMetadataOptions.AffectsMeasure));    public Orientation Orientation    {        get { return (Orientation)GetValue(OrientationProperty); }        set { SetValue(OrientationProperty, value); }    }    protected override Size MeasureOverride(Size availableSize)    {        Size desiredSize = new Size();        if (Orientation == Orientation.Vertical)            availableSize.Height = Double.PositiveInfinity;        else            availableSize.Width = Double.PositiveInfinity;        foreach (UIElement child in this.Children)        {            if (child != null)            {                child.Measure(availableSize);                if (Orientation == Orientation.Vertical)                {                    desiredSize.Width = Math.Max(desiredSize.Width,                    child.DesiredSize.Width);                    desiredSize.Height += child.DesiredSize.Height;                }                else                {                    desiredSize.Height = Math.Max(desiredSize.Height,                    child.DesiredSize.Height);                    desiredSize.Width += child.DesiredSize.Width;                }            }        }        return desiredSize;    }    protected override Size ArrangeOverride(Size finalSize)    {        double offset = 0;        foreach (UIElement child in this.Children)        {            if (child != null)            {                if (Orientation == Orientation.Vertical)                {                                   child.Arrange(new Rect(0, offset, finalSize.Width,                    child.DesiredSize.Height));                                     offset += child.DesiredSize.Height;                }                else                {                                       child.Arrange(new Rect(offset, 0, child.DesiredSize.Width,                    finalSize.Height));                    offset += child.DesiredSize.Width;                }            }        }        return finalSize;    }}

• The DesiredSize (the sizereturned by MeasureOverride) is sumof child sizes in the direction ofStackPanel and the size of largestchild in the other direction.

• RenderSize represents the finalsize of the StackPanel after layoutis complete.

• ActualHeight is exactly same asRenderSize.Height.

For rely these properties you should access them only within an event handler for LayoutUpdated event.

• for ViewPortHeight lookhere
• for ExtentHeight lookhere

Above answer is correct, except that RenderSize and ActualHeight can have temporarily different values. RenderSize gets set before OnRender, whereas ActualHeight gets set once WPF has finished the layout and render processing for that control. At the very end, LayoutUpdated gets raised.

Therefore, RenderSize can be used within OnRender, but ActualHeight will still have the old value from before the layout started.

The sequence looks like this:

MeasureOverride() => sets DesiredSizeArrangeOverride() => sets RenderSizeOnRender()

WPF might execute this sequence several times (recursion). Once everything is settled, the following gets executed:

ActualHeight = RenderSize.Height

ActualHeight can be accessed any time (!) after the first layout is done, except during the layout process itself of measure, arrange and render. WPF ensures that any code gets completed before layout processing runs.