How to set DesiredSize.Width of a UIElement?

Question

I need to calculate the Width of each UIElement in a custom panel. If looping over Children:

foreach (UIElement child in Children) { ... }

I can't do child.DesiredSize.Width = myCalculatedWidth; since DesiredSize and DesiredSize.Width are read only.

The docs say that that Measure() updates the DesiredSize of the UIElement, so I tried:

Size size = new Size(myCalculatedWidth, Double.PositiveInfinity);
child.Measure(size);

But child.DesiredSize.Width is unchanged. How do I set the DesiredSize.Width?

Answer 1

If you call Measure directly there are nothing to do for WPF layout system (because it caches the state of controls and store it as flags like MeasureIsValid). You should call InvalidateMeasure() for this control (which will mark control as MeasureIsValid = false) and UpdateLayout() then to force layout to be updated.

Answer 2

One cannot change directly DesiredSize, since only UIElement can write to DesiredSize, which happens in UIElement.Measure(). However, Measure() does not calculate DesiredWidth, but calls MeasurementCore instead. But even MeasurementCore does not calculate DesiredSize directly, but uses the return value of MeasurementOverride instead. That's the reason you have to override MeasurementOverride() to change DesiredChange :-)

It looks like this:

UIElement.Measure() calls 
  FrameworkElement.MeasureCore() calls
    YourControl.MeasureOverwrite()

return value of MeasureOverwrite() becomes return value of MeasureCore(), which then is stored in UIElement.DesiredSize.

Note that the value returned by MeasureOverwrite() is not necessarily the same as stored in DesiredSize, because the Margin gets added to the returned value ! FrameworkElement.MeasurementCore() also ensures that DesiredSize is within MinWidth, MaxWidth and Height, respectively. If the Width or Height of the returned value isInfinite of isNaN, an exception gets thrown.

Answer 3

None of the other answers worked for me. What I needed to do was to implement MeasureOverride in the child control and implement the desired layout behavior there.

Answer 4

Just try with child.Width = myCalculatedWidth.

Answer 5

Call InvalidateArrange(). The rendering system will call Arrange(Rect) and update the DesiredSize of the UIElement with your size.