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    copied!<p>It should be fairly straightforward to create an <a href="http://blog.kirupa.com/?p=374" rel="noreferrer">Attached Behaviour</a> that does what you want. To detect when the Items collection in the ComboBox changed, you'd need to use the trick mentioned in this <a href="http://www.wpfmentor.com/2008/11/notification-when-items-are-added-or.html" rel="noreferrer">blog post</a>.</p> <p><strong>Update</strong>: Here's my stab at it (you'll need to add a reference to System.Windows.Interactivity to your project - you can get it from the Expression Blend SDK):</p> <pre><code>using System.Windows.Interactivity; public class SelectFirstItemComboBoxBehavior : Behavior&lt;ComboBox&gt; { protected override void OnAttached() { base.OnAttached(); (AssociatedObject.Items as INotifyCollectionChanged).CollectionChanged += HandleCollectionChanged; } protected override void OnDetaching() { base.OnDetaching(); (AssociatedObject.Items as INotifyCollectionChanged).CollectionChanged -= HandleCollectionChanged; } private void HandleCollectionChanged(object sender, NotifyCollectionChangedEventArgs e) { if (AssociatedObject.Items.Count == 1) { AssociatedObject.SelectedItem = AssociatedObject.Items.Cast&lt;object&gt;().First(); } } } </code></pre> <p>Here's how you use it:</p> <pre><code>&lt;Window x:Class="ComboBoxSelectFirstBehavior.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="clr-namespace:ComboBoxSelectFirstBehavior" xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity" Title="MainWindow" Height="350" Width="525"&gt; &lt;Grid&gt; &lt;Grid.Resources&gt; &lt;x:Array x:Key="MyObjects" Type="{x:Type local:MyType}"&gt; &lt;local:MyType Name="WithChildren"&gt; &lt;local:MyType.Children&gt; &lt;local:MyTypeCollection&gt; &lt;local:MyType Name="Child1"/&gt; &lt;local:MyType Name="Child2"/&gt; &lt;local:MyType Name="Child3"/&gt; &lt;/local:MyTypeCollection&gt; &lt;/local:MyType.Children&gt; &lt;/local:MyType&gt; &lt;local:MyType Name="WithOneChild"&gt; &lt;local:MyType.Children&gt; &lt;local:MyTypeCollection&gt; &lt;local:MyType Name="Child1"/&gt; &lt;/local:MyTypeCollection&gt; &lt;/local:MyType.Children&gt; &lt;/local:MyType&gt; &lt;local:MyType Name="WithoutChildren"&gt; &lt;local:MyType.Children&gt; &lt;local:MyTypeCollection&gt; &lt;/local:MyTypeCollection&gt; &lt;/local:MyType.Children&gt; &lt;/local:MyType&gt; &lt;/x:Array&gt; &lt;/Grid.Resources&gt; &lt;Grid.RowDefinitions&gt; &lt;RowDefinition/&gt; &lt;RowDefinition/&gt; &lt;/Grid.RowDefinitions&gt; &lt;ComboBox x:Name="FirstCombo" Grid.Row="0" ItemsSource="{StaticResource MyObjects}" DisplayMemberPath="Name"/&gt; &lt;ComboBox Grid.Row="1" ItemsSource="{Binding ElementName=FirstCombo, Path=SelectedItem.Children}" DisplayMemberPath="Name"&gt; &lt;i:Interaction.Behaviors&gt; &lt;local:SelectFirstItemComboBoxBehavior/&gt; &lt;/i:Interaction.Behaviors&gt; &lt;/ComboBox&gt; &lt;/Grid&gt; </code></pre> <p></p>
 

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