Gets a value indicating whether the computed size and position of child elements in this element's layout are valid.
Assembly: PresentationCore (in PresentationCore.dll)
You can force arrangement to be invalidated by calling InvalidateArrange on this element (or its parents). This flags the layout for recomposition at such time as the layout system determines. Alternatively, an immediate call to UpdateLayout could be made, but this should only be done if it is certain that no further invalidations are pending (a large number of unnecessarily forced updates has performance consequences).
cannot be true unless IsMeasureValid is also true (in the layout process, arrangement cannot be valid without measurement first being valid).