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POHow does FallbackValue work with a MultiBinding?
 

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  1. POHow does FallbackValue work with a MultiBinding?
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    <p>I ask because it doesn't seem to work.</p> <p>Assume we're binding to the following object:</p> <pre><code>public class HurrDurr { public string Hurr {get{return null;}} public string Durr {get{return null;}} } </code></pre> <p>Well, it would appear that if we used a <strong>MultiBinding</strong> against this the fallback value would be shown, right?</p> <pre><code>&lt;TextBlock&gt; &lt;TextBlock.Text&gt; &lt;MultiBinding StringFormat="{}{0} to the {1}" FallbackValue="Not set! It works as expected!)"&gt; &lt;Binding Path="Hurr"/&gt; &lt;Binding Path="Durr"/&gt; &lt;/MultiBinding&gt; &lt;/TextBlock.Text&gt; &lt;/TextBlock&gt; </code></pre> <p>However the result is, in fact, <strong>" to the "</strong>. Even forcing the bindings to return <code>DependencyProperty.UnsetValue</code> doesn't work:</p> <pre><code>&lt;TextBlock xmnlns:base="clr-namespace:System.Windows;assembly=WindowsBase"&gt; &lt;TextBlock.Text&gt; &lt;MultiBinding StringFormat="{}{0} to the {1}" FallbackValue="Not set! It works as expected!)"&gt; &lt;Binding Path="Hurr" FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" /&gt; &lt;Binding Path="Durr" FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" /&gt; &lt;/MultiBinding&gt; &lt;/TextBlock.Text&gt; &lt;/TextBlock&gt; </code></pre> <p>Tried the same with <strong>TargetNullValue</strong>, which was also a bust all the way around.</p> <p>So it appears that <strong>MultiBinding</strong> will never ever use <strong>FallbackValue</strong>. Is this true, or am I missing something?</p> <hr> <p>A little more messing around and I found that a converter can return the UnsetValue I need:</p> <pre><code>class MultiValueFailConverter : IMultiValueConverter { public object Convert( object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture) { if (values == null || values.Length != 2 || values.Any(x=&gt;x == null)) return System.Windows.DependencyProperty.UnsetValue; return values; } public object[] ConvertBack( object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture) { throw new NotImplementedException("Too complex hurt brain."); } } </code></pre> <p>However, this seems like a dirty filthy hack. I'd think a scenario like this would be accounted for in the framework. I can't find anything in Reflector, however.</p>
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