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POWhy would putting a no-op Converter on a Binding change its behavior?
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  1. POWhy would putting a no-op Converter on a Binding change its behavior?
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    copied!<p>I'm in the midst of testing a user control I've built, and I'm encountering something that's inexplicable to me. </p> <p>The control's an extension of the ComboBox that handles values of a specific custom type. It has a dependency property of that custom type that is the target property of a Binding.</p> <p>I've got a trace statement in the setter, and I can see that the property is getting set. But it's not appearing in my user control.</p> <p>Now, ordinarily I'd say, okay, I've got a bug in my user control. I probably do, though I'm baffled about it. But this question isn't about finding the bug in my control. Read on; here is where it gets weird.</p> <p>I'm also using Bea Stollnitz's little value converter to help debug the Binding:</p> <pre><code>public class DebuggingConverter : IValueConverter { public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) { return value; // Add the breakpoint here!! } public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) { throw new NotImplementedException("This method should never be called"); } } </code></pre> <p>The idea behind this is that I add this converter to my Binding and can set a breakpoint to see what value is being pushed out to the target. Okay, that works just fine. I can see that the value is being pushed out.</p> <p>In fact, it works a little <em>too</em> fine. If the DebuggingConverter is attached to the Binding, the user control displays the value. If it's not, it doesn't.</p> <p>How is that even possible? How could a value converter that does nothing affect the behavior of a bound control?</p> <p><strong>Edit:</strong></p> <p>Not that it's likely to help, but here's the XAML for the user control:</p> <pre><code>&lt;a:CodeLookupBox Grid.Column="1" Grid.IsSharedSizeScope="True" MinWidth="100" Style="{Binding Style}"&gt; &lt;a:CodeLookupBox.CodeLookupTable&gt; &lt;Binding Path="Codes" Mode="OneWay"/&gt; &lt;/a:CodeLookupBox.CodeLookupTable&gt; &lt;a:CodeLookupBox.SelectedCode&gt; &lt;Binding Path="Value" Mode="TwoWay" ValidatesOnDataErrors="True"/&gt; &lt;/a:CodeLookupBox.SelectedCode&gt; &lt;/a:CodeLookupBox&gt; </code></pre> <p>Without the converter on the second binding, the control behaves as though I didn't set <code>SelectedCode</code>. Even though a trace statement in the <code>OnSelectedCodePropertyChanged</code> handler shows that <code>e.Value</code> does indeed contain the correct value. This happens irrespective of whether the converter's attached or not.</p> <p>I've been trying to reverse-engineer this problem with a thought experiment: if you wanted to create a bound user control whose behavior changed if a no-op converter were attached to its binding, how would you do it? I don't know enough about binding to come up with an answer.</p>
 

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