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POUpdating ObservableCollection causes "The Parameter is incorrect" exception
 

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    <p>I've got a weird issue I don't understand. This is in Silverlight/WP7.</p> <p>I'm filling an ObservableCollection with items, and later I want to update each of the items. </p> <p>I've managed to strip down the code to reproduce the error. My XAML is just a ListBox and a Button. </p> <pre><code> private ObservableCollection&lt;int&gt; Words = new ObservableCollection&lt;int&gt;(); public MainPage() { InitializeComponent(); listBox1.ItemsSource = Words; } private void button1_Click(object sender, RoutedEventArgs e) { List&lt;int&gt; numbers = new List&lt;int&gt;() { 1,2,3 }; foreach (var number in numbers) { var index = Words.IndexOf(number); if (index &gt; -1) Words[index] = number; else Words.Add(number); } } </code></pre> <p>The first time I run the code it fills the ObservableCollection with the numbers 1, 2 and 3, and they are displayed in the ListBox.</p> <p>The second time it is run all the code is executed, but then an unhandled exception with the message "The parameter is incorrect" is thrown.</p> <p>The weird thing is that if I remove my line in the constructor, the one where I set up the ItemsSource, the error isn't thrown. The observable collection is updated as it should.</p> <p>Also, if I comment out the "Words[index] = number" line it also works. So for some reason, when my ObservableCollection is set as the datasource to a ListBox I can't replace the item.</p> <p>Can someone explain why? (Or suggest a workaround?)</p> <p><strong>My solution;</strong> I changed my codebehind from</p> <pre><code>if (index &gt; -1) Words[index] = number; </code></pre> <p>to</p> <pre><code>if (index &gt; -1) { Words.RemoveAt(index); Words.Add(number); } </code></pre> <p>That made the problem go away.</p>
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