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I'm trying to bind the expanded event to the viewmodel (not the *.xaml.cs file) to extend the treeview only after expanding a node.

My approach is:

<TreeView x:Name="TreeViewTest" ItemsSource="{Binding Items}">
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding Children, Mode=OneTime}">
            <StackPanel>
                <Label Content="{Binding Title, Mode=OneTime}" />
            </StackPanel>
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="TreeViewItem.Expanded">
            <i:InvokeCommandAction Command="{Binding DataContext.ExpandedCommand, Source={x:Reference TreeViewTest}}">       </i:InvokeCommandAction>
        </i:EventTrigger>
    </i:Interaction.Triggers>
</TreeView>
I get following error message:
  
Cannot call MarkupExtension.ProvideValue because of a cyclical dependency. Properties inside a MarkupExtension cannot reference objects that reference the result of the MarkupExtension. The affected MarkupExtensions are: 
  System.Windows.Data.Binding
Can someone help me how to solve this error or is there another way to bind the event to a command in the viewmodel?
As I can understand you try to attach a command to TreeViewItem.Expanded event.
I've done a small research concerning this issue and here is the result I've end up with.
There is  a small problem with the tree view items and this event.
There are several approaches to solve the problem: 
First is AttachedProperties in style of the TreeViewItem, here is the link:
Invoke Command when TreeViewItem is Expanded
Second is Behavior involved solution:
XAML:
<TreeView x:Name="TreeViewTest" ItemsSource="{Binding Items}" TreeViewItem.Expanded="TreeViewTest_OnExpanded">
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate DataType="{x:Type soTreeViewHelpAttempt:TreeObject}" 
                                  ItemsSource="{Binding Children, Mode=OneTime}">
            <HierarchicalDataTemplate.ItemTemplate>
                <DataTemplate>
                    <StackPanel>
                        <Label Content="{Binding }" />
                    </StackPanel>
                </DataTemplate>
            </HierarchicalDataTemplate.ItemTemplate>
            <StackPanel>
                <Label Content="{Binding Title, Mode=OneTime}" />
            </StackPanel>
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
    <i:Interaction.Behaviors>
        <soTreeViewHelpAttempt:TreeViewItemExpandBehavior OnExpandedAction="{Binding OnExpandedActionInViewModel}"/>
    </i:Interaction.Behaviors>
</TreeView></Grid>
ViewModel:
    public MainDataContext()
    OnExpandedActionInViewModel = new Action<object, RoutedEventArgs>(OnExpanded);
public Action<object, RoutedEventArgs> OnExpandedActionInViewModel
    get { return _onExpandedActionInViewModel; }
    private set
        _onExpandedActionInViewModel = value;
        OnPropertyChanged();
Behavior Command property:
public static readonly DependencyProperty OnExpandedActionProperty = DependencyProperty.Register(
    "OnExpandedAction", typeof (Action<object,RoutedEventArgs>), typeof (TreeViewItemExpandBehavior), new PropertyMetadata(default(Action<object,RoutedEventArgs>)));
public Action<object,RoutedEventArgs> OnExpandedAction
    get { return (Action<object,RoutedEventArgs>) GetValue(OnExpandedActionProperty); }
    set { SetValue(OnExpandedActionProperty, value); }
Behavior code:
    protected override void OnAttached()
    base.OnAttached();
    AssociatedObject.Loaded += AssociatedObjectOnLoaded;
private void AssociatedObjectOnLoaded(object sender, RoutedEventArgs routedEventArgs)
    var treeView = sender as TreeView;
    if(treeView == null) return;
    treeView.ItemsSource.Cast<object>().ToList().ForEach(o =>
        var treeViewItem = treeView.ItemContainerGenerator.ContainerFromItem(o) as TreeViewItem;
        if(treeViewItem == null) return;
        treeViewItem.Expanded += TreeViewItemOnExpanded;
        _list.Add(treeViewItem);
private void TreeViewItemOnExpanded(object sender, RoutedEventArgs routedEventArgs)
    if(OnExpandedAction == null)
        return;
    OnExpandedAction(sender, routedEventArgs);
protected override void OnDetaching()
    base.OnDetaching();
    AssociatedObject.Loaded -= AssociatedObjectOnLoaded;
    _list.ForEach(item => item.Expanded -= TreeViewItemOnExpanded);
    _list.Clear();
I hope It will help you.
Regards,
To create Binding to TreeView you must use RelativeSource Self, but not Source with x:Reference.
For example here i'm binding Width of TreeView to Height:
<TreeView Name="tv" Width="{Binding Height, RelativeSource={RelativeSource Self}}">
</TreeView>
Or also you can use ElementName, like here:
<TreeView Name="tv" Width="{Binding Height, ElementName=tv}">
</TreeView>
I know this question is old, but there's an obvious solution that does not require any custom code.
When using i:EventTrigger, you can specify the object to listen the event from as below, where I'm using RelativeSource binding to walk up the visual tree and find a TreeViewItem.
Also, you can bind to an element by name using ElementName, which doesn't cause circular dependencies because bindings work on the dispatcher when the controls are already created and initialized (unlike x:Reference, which has to be resolved during XAML deserialization).
<i:Interaction.Triggers>
    <i:EventTrigger SourceObject="{Binding RelativeSource={RelativeSource AncestorType=TreeViewItem}}" EventName="Expanded">
        <i:InvokeCommandAction Command="{Binding ElementName=TreeView, Path=DataContext.ExpandedCommand}" />
    </i:EventTrigger>
</i:Interaction.Triggers>
        
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