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FrameworkElement.MeasureOverride Method

When overridden in a derived class, measures the size in layout required for child elements and determines a size for the FrameworkElement-derived class.

Namespace: System.Windows
Assembly: PresentationFramework (in presentationframework.dll)

'Declaration
Protected Overridable Function MeasureOverride ( _
	availableSize As Size _
) As Size
'Usage
Dim availableSize As Size
Dim returnValue As Size

returnValue = Me.MeasureOverride(availableSize)
protected Size MeasureOverride (
	Size availableSize
)
protected function MeasureOverride (
	availableSize : Size
) : Size
You cannot use methods in XAML.

Parameters

availableSize

The available size that this element can give to child elements. Infinity can be specified as a value to indicate that the element will size to whatever content is available.

Return Value

The size that this element determines it needs during layout, based on its calculations of child element sizes.

Override MeasureOverride to implement custom layout sizing behavior for your element as it participates in the Windows Presentation Foundation (WPF) layout system. Your implementation should do the following:

  1. Iiterate your element's particular collection of children that are part of layout, call Measure on each child element.

  2. Iimmediately get DesiredSize on the child (this is set as a property after Measure is called).

  3. Compute the net desired size of the parent based upon the measurement of the child elements.

The return value of MeasureOverride should be the element's own desired size, which then becomes the measure input for the parent element of the current element. This same process continues through the layout system until the root element of the page is reached.

During this process, child elements might return a larger DesiredSize size than the initial availableSize to indicate that the child element wants more space. This might be handled in your own implementation by introducing a scrollable region, by resizing the parent control, by establishing some manner of stacked order, or any number of solutions for measuring or arranging content.

NoteImportant:

Elements should call Measure on each child during this process, otherwise the child elements will not be correctly sized or arranged.

Notes to Inheritors: The following non-compiling code shows this implementation pattern. VisualChildren represents an enumerable collection property of children that your own element should define. The property can be named anything. VisualChildren is a placeholder name for purposes of this example, VisualChildren is not an API as provided by WPF or a part of a naming pattern..

protected override Size MeasureOverride(Size availableSize)
{
    Size desiredSize = new Size();
    foreach (UIElement child in VisualChildren)
    {
        child.Measure(availableSize);
        // do something with child.DesiredSize, either sum them directly or apply whatever logic your element has for reinterpreting the child sizes
        // if greater than availableSize, must decide what to do and which size to return
    }
    // desiredSize = ... computed sum of children's DesiredSize ...;
    // IMPORTANT: do not allow PositiveInfinity to be returned, that will raise an exception in the caller!
    // PositiveInfinity might be an availableSize input; this means that the parent does not care about sizing
    return desiredSize;
}

This example shows how to override the default layout behavior of the Panel element and create custom layout elements that are derived from Panel.

The example defines a simple custom Panel element called PlotPanel, which positions child elements according to two hard-coded x- and y-coordinates. In this example, x and y are both set to 50; therefore, all child elements are positioned at that location on the x and y axes.

To implement custom Panel behaviors, the example uses the MeasureOverride and ArrangeOverride methods. Each method returns the Size data that is necessary to position and render child elements.

Public Class PlotPanel
    Inherits Panel
    'Override the default Measure method of Panel.

    Protected Overrides Function MeasureOverride(ByVal availableSize As System.Windows.Size) As System.Windows.Size
        Dim childSize As Size = CType(availableSize, Size)
        For Each child As UIElement In InternalChildren
            child.Measure(childSize)
        Next
        Return MyBase.MeasureOverride(availableSize)
    End Function
    Protected Overrides Function ArrangeOverride(ByVal finalSize As System.Windows.Size) As System.Windows.Size
        For Each child As UIElement In InternalChildren
            Dim x As Double = 50
            Dim y As Double = 50
            child.Arrange(New Rect(New System.Windows.Point(x, y), child.DesiredSize))
        Next
        Return MyBase.ArrangeOverride(finalSize)
    End Function
End Class

For the complete sample, see Create a Simple Custom Panel Behavior Sample.

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0

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