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POHow to Subclass WPF's TreeViewItem and use it in a treevview
 

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  1. POHow to Subclass WPF's TreeViewItem and use it in a treevview
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    <p>I have created a simple dependency property that I want to attach to a treeViewitem, I have done similar things for other controls such as buttons but cannot figure out how to use the TreeViewItem in a treeview without loosing my defined style. With the code below I get "A style intended for type 'ErrorTreeViewItem' cannot be applied to type 'TreeViewItem'."</p> <pre><code>public class ErrorTreeViewItem : TreeViewItem { static ErrorTreeViewItem() { } public bool ErrorState { get { return (bool)GetValue(ErrorStateProperty); } set { base.SetValue(ErrorStateProperty, value); } } public static readonly DependencyProperty ErrorStateProperty = DependencyProperty.Register("ErrorState", typeof(bool), typeof(ErrorTreeViewItem), new UIPropertyMetadata(false)); } </code></pre> <p>The style of my tree view looks like:</p> <pre><code> &lt;Style TargetType="me:ErrorTreeViewItem"&gt; &lt;Style.Resources&gt; ... &lt;/Style.Resources&gt; &lt;Setter Property="HorizontalContentAlignment" Value="Stretch"/&gt; &lt;Setter Property="Template"&gt; &lt;Setter.Value&gt; &lt;ControlTemplate TargetType="TreeViewItem"&gt; ... &lt;/Setter.Value&gt; &lt;/Setter&gt; </code></pre> <p>I am using it like:</p> <pre><code> &lt;TreeView Name="ApplicationTree" ItemsSource="{Binding Applications}" HorizontalContentAlignment="Stretch" Background="#E8E8E8" &gt; &lt;TreeView.ItemContainerStyle&gt; &lt;Style TargetType="me:ErrorTreeViewItem" BasedOn="{StaticResource {x:Type TreeViewItem}}"&gt; &lt;Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" /&gt; &lt;Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" /&gt; &lt;Setter Property="HorizontalContentAlignment" Value="Stretch"/&gt; &lt;/Style&gt; &lt;/TreeView.ItemContainerStyle&gt; </code></pre>
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    1. COAre you not using the built in validation services because you don't understand them (they're tough to get right!) or because you didn't know they exist? IDataErrorInfo.
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    2. CONo its not that I just want to change the colour of a treeview item based on the value of my dependency property. Didn't think it would be this hard!
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    3. COWell, there are a number of ways to do this. IDataErrorInfo on your Models (if you are using MVVM) will automatically trigger validation and show the invalid template on the standard WPF controls. You could also try using an attached DependencyProperty on the TreeViewItem, so you don't even *need* to extend the base class, etc. I've never done it this way in WPF, but I know to accomplish this in Winforms you have to extend the TreeView class as well, so when you create a new TVI you return an instance of your type instead. That's where your error is cropping up.
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