7/25/2011 Don’t modify other controls during a WPF layout pass

Unlike WinForms or native Win32 development, WPF provides a rich layout model which allows developers to easily create complicated UIs that resize to fit their contents or the parent window.

However, when developing custom controls, it can be necessary to layout child controls manually by overriding the MeasureOverride and ArrangeOverride methods.  To quote MSDN,

Measure allows a component to determine how much size it would like to take. This is a separate phase from Arrange because there are many situations where a parent element will ask a child to measure several times to determine its optimal position and size. The fact that parent elements ask child elements to measure demonstrates another key philosophy of WPF – size to content. All controls in WPF support the ability to size to the natural size of their content. This makes localization much easier, and allows for dynamic layout of elements as things resize. The Arrange phase allows a parent to position and determine the final size of each child.

Overriding these methods gives your custom control full power over the layout of its child element(s).

Be careful what you do when overriding these methods.  Any code in MeasureOverride or ArrangeOverride runs during the WPF layout passes.  in these methods, you should not modify any part of the visual tree outside of the control you’re overriding in.  If you do, you’ll be changing the visuals between Measure() and Arrange(), which will have unexpected results.

It is safe to modify your own child controls during the layout pass.  Before you call Measure() on a child control, its layout pass has not started.  Therefore, any changes will be seen by the child’s layout code.  Similarly, after you Arrange() a child control, its layout pass is finished, so it is safe to modify again (although you may end up triggering another layout pass to see the changes).

If you do need to modify an outside control during the layout pass, you should call Dispatcher.BeginInvoke() to run code asynchronously during the next message loop.  This way, your code will run after the layout pass finishes, and it will be able to safely modify whatever it wants.

Note that Measure() can be called multiple times during a single layout pass (if a parent needs to iteratively determine the best fit for a child).