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    <p>Yes, I've done this in the past with the <code>ActualWidth</code> and <code>ActualHeight</code> properties, both of which are read-only. I created an attached behavior that has <code>ObservedWidth</code> and <code>ObservedHeight</code> attached properties. It also has an <code>Observe</code> property that is used to do the initial hook-up. Usage looks like this:</p> <pre class="lang-xml prettyprint-override"><code>&lt;UserControl ... SizeObserver.Observe="True" SizeObserver.ObservedWidth="{Binding Width, Mode=OneWayToSource}" SizeObserver.ObservedHeight="{Binding Height, Mode=OneWayToSource}" </code></pre> <p>So the view model has <code>Width</code> and <code>Height</code> properties that are always in sync with the <code>ObservedWidth</code> and <code>ObservedHeight</code> attached properties. The <code>Observe</code> property simply attaches to the <code>SizeChanged</code> event of the <code>FrameworkElement</code>. In the handle, it updates its <code>ObservedWidth</code> and <code>ObservedHeight</code> properties. Ergo, the <code>Width</code> and <code>Height</code> of the view model is always in sync with the <code>ActualWidth</code> and <code>ActualHeight</code> of the <code>UserControl</code>.</p> <p>Perhaps not the perfect solution (I agree - read-only DPs <em>should</em> support <code>OneWayToSource</code> bindings), but it works and it upholds the MVVM pattern. Obviously, the <code>ObservedWidth</code> and <code>ObservedHeight</code> DPs are <em>not</em> read-only.</p> <p>UPDATE: here's code that implements the functionality described above:</p> <pre class="lang-cs prettyprint-override"><code>public static class SizeObserver { public static readonly DependencyProperty ObserveProperty = DependencyProperty.RegisterAttached( "Observe", typeof(bool), typeof(SizeObserver), new FrameworkPropertyMetadata(OnObserveChanged)); public static readonly DependencyProperty ObservedWidthProperty = DependencyProperty.RegisterAttached( "ObservedWidth", typeof(double), typeof(SizeObserver)); public static readonly DependencyProperty ObservedHeightProperty = DependencyProperty.RegisterAttached( "ObservedHeight", typeof(double), typeof(SizeObserver)); public static bool GetObserve(FrameworkElement frameworkElement) { frameworkElement.AssertNotNull("frameworkElement"); return (bool)frameworkElement.GetValue(ObserveProperty); } public static void SetObserve(FrameworkElement frameworkElement, bool observe) { frameworkElement.AssertNotNull("frameworkElement"); frameworkElement.SetValue(ObserveProperty, observe); } public static double GetObservedWidth(FrameworkElement frameworkElement) { frameworkElement.AssertNotNull("frameworkElement"); return (double)frameworkElement.GetValue(ObservedWidthProperty); } public static void SetObservedWidth(FrameworkElement frameworkElement, double observedWidth) { frameworkElement.AssertNotNull("frameworkElement"); frameworkElement.SetValue(ObservedWidthProperty, observedWidth); } public static double GetObservedHeight(FrameworkElement frameworkElement) { frameworkElement.AssertNotNull("frameworkElement"); return (double)frameworkElement.GetValue(ObservedHeightProperty); } public static void SetObservedHeight(FrameworkElement frameworkElement, double observedHeight) { frameworkElement.AssertNotNull("frameworkElement"); frameworkElement.SetValue(ObservedHeightProperty, observedHeight); } private static void OnObserveChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e) { var frameworkElement = (FrameworkElement)dependencyObject; if ((bool)e.NewValue) { frameworkElement.SizeChanged += OnFrameworkElementSizeChanged; UpdateObservedSizesForFrameworkElement(frameworkElement); } else { frameworkElement.SizeChanged -= OnFrameworkElementSizeChanged; } } private static void OnFrameworkElementSizeChanged(object sender, SizeChangedEventArgs e) { UpdateObservedSizesForFrameworkElement((FrameworkElement)sender); } private static void UpdateObservedSizesForFrameworkElement(FrameworkElement frameworkElement) { // WPF 4.0 onwards frameworkElement.SetCurrentValue(ObservedWidthProperty, frameworkElement.ActualWidth); frameworkElement.SetCurrentValue(ObservedHeightProperty, frameworkElement.ActualHeight); // WPF 3.5 and prior ////SetObservedWidth(frameworkElement, frameworkElement.ActualWidth); ////SetObservedHeight(frameworkElement, frameworkElement.ActualHeight); } } </code></pre>
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    1. USKent Boogaart
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    1. COI wonder if you could do some trickery to automatically attach the properties, without needing Observe. But this looks like a fine solution. Thanks!
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      1. USJoe White
    2. COThanks Kent. I posted a code sample below for this "SizeObserver" class.
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      1. USScott Whitlock
    3. CO+1 to this sentiment: "read-only DPs should support OneWayToSource bindings"
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      1. USTristan
 

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