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POWPF ListBox & Items with Changing Hashcode
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  1. POWPF ListBox & Items with Changing Hashcode
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    copied!<p>I have a <code>ListBox</code> bound to a collection of items that have an ID that is used to generate the result of <code>GetHashCode()</code>. When a new item is added, it has ID 0 until it is first saved to our database. This is causing my <code>ListBox</code> to complain; I believe the reason is because when an item is first used by a <code>ListBox</code> it is stored in an internal <code>Dictionary</code> that does not expect the hashcode to change.</p> <p>I can fix this by removing the unsaved item from the collection (I <strong>must</strong> notify the UI at this stage to remove it from the dictionary), save to the DB, and add it back to the collection. This is messy and I do not always have access to the collection from my <code>Save(BusinessObject obj)</code> method. Does anyone have an alternative solution to this problem?</p> <p>EDIT In respose to Blam's answer:</p> <p>I am using MVVM so I have modified the code to use bindings. To reproduce the problem click add, select the item, click save, repeat, then try to make a selection. I think this demonstrates that the <code>ListBox</code> is still holding onto the old hashcodes in it's internal <code>Dictionary</code>, hence the conflicting keys error.</p> <pre><code>&lt;Window x:Class="ListBoxHashCode.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="MainWindow" Height="350" Width="525"&gt; &lt;Grid&gt; &lt;Grid.RowDefinitions&gt; &lt;RowDefinition Height="Auto"/&gt; &lt;RowDefinition Height="*"/&gt; &lt;/Grid.RowDefinitions&gt; &lt;StackPanel HorizontalAlignment="Center" Orientation="Horizontal"&gt; &lt;Button Click="Button_Click_Add" Content="Add"/&gt; &lt;Button Click="Button_Click_Save" Content="Save Selected"/&gt; &lt;/StackPanel&gt; &lt;ListBox Grid.Row="1" ItemsSource="{Binding List}" DisplayMemberPath="ID" SelectedItem="{Binding Selected}"/&gt; &lt;/Grid&gt; &lt;/Window&gt; public partial class MainWindow : Window { public ObservableCollection&lt;ListItem&gt; List { get; private set; } public ListItem Selected { get; set; } private Int32 saveId; public MainWindow() { this.DataContext = this; this.List = new ObservableCollection&lt;ListItem&gt;(); this.saveId = 100; InitializeComponent(); } private void Button_Click_Add(object sender, RoutedEventArgs e) { this.List.Add(new ListItem(0)); } private void Button_Click_Save(object sender, RoutedEventArgs e) { if (Selected != null &amp;&amp; Selected.ID == 0) { Selected.ID = saveId; saveId++; } } } </code></pre> <p>EDIT 2 After some testing, I have discovered a few things:</p> <ul> <li><p>Changing the hash code of an item in a <code>ListBox</code> seems to work ok.</p></li> <li><p>Changing the hash code of <em>the selected item</em> in a <code>ListBox</code> breaks<br> it's functionality.</p></li> </ul> <p>When a selection made (single or multiple selection mode) the <code>IList ListBox.SelectedItems</code> is updated. Items that are added to the selection are added to <code>SelectedItems</code> and items that are no longer included in the selection are removed.</p> <p>If the hash code of an item is changed while it is selected, there is no way to remove it from <code>SelectedItems</code>. Even manuallty calling <code>SelectedItems.Remove(item)</code>, <code>SelectedItems.Clear()</code> and setting <code>SelectedIndex</code> to -1 all have no effect, and the item remains in the <code>IList</code>. This causes exceptions to be thrown after the next time it is selected as I believe it is once again added to <code>SelectedItems</code>.</p>
 

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