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POComboBox ItemsSource changed => SelectedItem is ruined
 

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    <p>Ok, this has been bugging me for a while now. And I wonder how others handle the following case:</p> <pre><code>&lt;ComboBox ItemsSource="{Binding MyItems}" SelectedItem="{Binding SelectedItem}"/&gt; </code></pre> <p>The DataContext object's code:</p> <pre><code>public ObservableCollection&lt;MyItem&gt; MyItems { get; set; } public MyItem SelectedItem { get; set; } public void RefreshMyItems() { MyItems.Clear(); foreach(var myItem in LoadItems()) MyItems.Add(myItem); } public class MyItem { public int Id { get; set; } public override bool Equals(object obj) { return this.Id == ((MyItem)obj).Id; } } </code></pre> <p>Obviously when the <code>RefreshMyItems()</code> method is called the combo box receives the Collection Changed events, updates its items and does not find the SelectedItem in the refreshed collection => sets the SelectedItem to <code>null</code>. But I would need the combo box to use <code>Equals</code> method to select the correct item in the new collection.</p> <p>In other words - the ItemsSource collection still contains the correct <code>MyItem</code>, but it is a <code>new</code> object. And I want the combo box to use something like <code>Equals</code> to select it automatically (this is made even harder because first the source collection calls <code>Clear()</code> which resets the collection and already at that point the SelectedItem is set to <code>null</code>).</p> <p><strong>UPDATE 2</strong> Before copy-pasting the code below please note that it is far from perfection! And note that it does not bind two ways by default.</p> <p><strong>UPDATE</strong> Just in case someone has the same problem (an attached property as proposed by Pavlo Glazkov in his answer):</p> <pre><code>public static class CBSelectedItem { public static object GetSelectedItem(DependencyObject obj) { return (object)obj.GetValue(SelectedItemProperty); } public static void SetSelectedItem(DependencyObject obj, object value) { obj.SetValue(SelectedItemProperty, value); } // Using a DependencyProperty as the backing store for SelectedIte. This enables animation, styling, binding, etc... public static readonly DependencyProperty SelectedItemProperty = DependencyProperty.RegisterAttached("SelectedItem", typeof(object), typeof(CBSelectedItem), new UIPropertyMetadata(null, SelectedItemChanged)); private static List&lt;WeakReference&gt; ComboBoxes = new List&lt;WeakReference&gt;(); private static void SelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) { ComboBox cb = (ComboBox) d; // Set the selected item of the ComboBox since the value changed if (cb.SelectedItem != e.NewValue) cb.SelectedItem = e.NewValue; // If we already handled this ComboBox - return if(ComboBoxes.SingleOrDefault(o =&gt; o.Target == cb) != null) return; // Check if the ItemsSource supports notifications if(cb.ItemsSource is INotifyCollectionChanged) { // Add ComboBox to the list of handled combo boxes so we do not handle it again in the future ComboBoxes.Add(new WeakReference(cb)); // When the ItemsSource collection changes we set the SelectedItem to correct value (using Equals) ((INotifyCollectionChanged) cb.ItemsSource).CollectionChanged += delegate(object sender, NotifyCollectionChangedEventArgs e2) { var collection = (IEnumerable&lt;object&gt;) sender; cb.SelectedItem = collection.SingleOrDefault(o =&gt; o.Equals(GetSelectedItem(cb))); }; // If the user has selected some new value in the combo box - update the attached property too cb.SelectionChanged += delegate(object sender, SelectionChangedEventArgs e3) { // We only want to handle cases that actually change the selection if(e3.AddedItems.Count == 1) { SetSelectedItem((DependencyObject)sender, e3.AddedItems[0]); } }; } } } </code></pre>
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    1. COiv'e came across this issue and solved it in the following manner http://stackoverflow.com/questions/12337442/proper-use-of-propertychangedtrigger-and-changepropertyaction/12341649#12341649
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