Assembly: System.Windows (in System.Windows.dll)
values are set by the layout system as part of calls to Measure.
values are useful if you are implementing layout override behavior, in particular during the arrange pass. Depending on the scenario, might be fully respected by your layout logic, constraints on might be applied, and such constraints might also change other characteristics of either the parent or child in layout. For example, a control that supports scrollable regions (but chooses not to derive from controls that already enable scrollable regions) could compare available size to . The control could then set an internal state that enabled scrollbars in the UI for that control. Or, could potentially also be ignored in certain layout scenarios.
You would not typically get the value of outside of a context where your code has already called Measure on that same element as part of the measure pass of layout, or called something that initiates a full layout pass such as UpdateLayout. Otherwise, there is no guarantee that the value held by is relevant.
The following example queries as part of the child iteration for an ArrangeOverride implementation.
'Second arrange all children and return final size of panel Protected Overloads Overrides Function ArrangeOverride(ByVal finalSize As Size) As Size 'Get the collection of children Dim mychildren As UIElementCollection = Children 'Get total number of children Dim count As Integer = mychildren.Count 'Arrange children 'We're only allowing 9 children in this panel. More children will get a 0x0 layout slot. Dim i As Integer For i = 0 To 8 'Get (left, top) origin point for the element in the 3x3 block Dim cellOrigin As Point = GetOrigin(i, 3, New Size(100, 100)) 'Arrange child 'Get desired height and width. This will not be larger than 100x100 as set in MeasureOverride. Dim dw As Double = mychildren(i).DesiredSize.Width Dim dh As Double = mychildren(i).DesiredSize.Height mychildren(i).Arrange(New Rect(cellOrigin.X, cellOrigin.Y, dw, dh)) Next For i = 9 To count - 1 'Give the remaining children a 0x0 layout slot mychildren(i).Arrange(New Rect(0, 0, 0, 0)) Next 'Return final size of the panel Return New Size(300, 300) End Function 'Calculate point origin of the Block you are in Protected Function GetOrigin(ByVal blockNum As Integer, ByVal blocksPerRow As Integer, ByVal itemSize As Size) As Point 'Get row number (zero-based) Dim row As Integer = CInt(Math.Floor(blockNum / blocksPerRow)) 'Get column number (zero-based) Dim column As Integer = blockNum - blocksPerRow * row 'Calculate origin Dim origin As New Point(itemSize.Width * column, itemSize.Height * row) Return origin End Function