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POWhy is CompositeCollection not Freezable?
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  1. POWhy is CompositeCollection not Freezable?
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    copied!<p>I am writing an application using the MVVM pattern. I am providing data to my view by setting my view's DataContext property to an instance of my ViewModel. Generally I just use Binding from there and go about my way.</p> <p>Recently, I tried to implement a ComboBox with an "extra" element beyond the collection my ViewModel provides that says "Select Item".</p> <pre><code>&lt;ComboBox&gt; &lt;ComboBox.ItemsSource&gt; &lt;CompositeCollection&gt; &lt;ComboBoxItem IsEnabled="False"&gt;Select Item&lt;/ComboBoxItem&gt; &lt;CollectionContainer Collection="{Binding MyItemsCollection}" /&gt; &lt;/CompositeCollection&gt; &lt;/ComboBox.ItemsSource&gt; &lt;/ComboBox&gt; </code></pre> <p>The problem is, CompositeCollection is not a Freezable: <a href="http://msdn.microsoft.com/en-us/library/ms750509.aspx" rel="noreferrer">Freezable Objects Overview</a>. This causes only the static ComboBoxItem to appear and none of the results from my binding expression.</p> <p>My initial reaction to the problem was to just implement my own version of CompositeCollection that <em>was</em> Freezable. This, though, begs the following question:</p> <p><strong>Why isn't CompositeCollection a Freezable in the first place?</strong></p> <p>My concern is that generally these decisions are made for a reason and I don't feel I know enough about Freezable to say why they didn't inherit from it. I know I can implement this collection, but I'm concerned there will be a measurable difference in performance if I do.</p> <p>Any help would be appreciated. Thanks!</p> <p><em>Also: please note that I realize I can insert a Null or some other special value and provide and template or valueconverter to do what I want. This is not the question I'm interested in... only the question in bold above.</em></p> <p><strong>Update:</strong></p> <p>After some further research brought on by ArsenMkrt's comment, I'm led to believe this was actually an oversight. The evidence is this:</p> <ol> <li>There is a collection that <em>is</em> freezable called <code>FreezableCollection&lt;T&gt;</code>. I does not produce CollectionViews, which makes it inappropriate for my needs directly.</li> <li>Sam Bent of MSFT says as much in the above link. I cannot find contact information for him yet, but I plan on discussing this with him if I get the chance.</li> </ol> <p>My current plan to get around this problem is to create a new collection with the properties of CompositeCollection and <code>FreezableCollection&lt;T&gt;</code>. I don't know if it'll work yet, but I'm thinking about something like this:</p> <pre><code>public class BindableCompositeCollection : FreezableCollection&lt;object&gt;, ICollectionViewFactory </code></pre> <p>If anyone has a better option, I'd like to hear it!</p>
 

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