Binding arbitary depth tree to a TreeView

I've got the following interface (as you can see, I'm adapting the MVVM pattern a bit):

public interface IViewModel {
    string Header { get; }
    IEnumerable<IViewModel> Nodes { get; }
}

I want to bind this datatype to a treeview, with a single top node and an arbitary depth. Header is the text to display in each node, and Nodes are the child nodes.

I've got the following XAML:

<TreeView ItemsSource="{Binding开发者_运维百科}">
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate DataType="{x:Type ViewModel:IViewModel}" ItemsSource="{Binding Nodes}">
            <TextBlock Text="{Binding Header}" />
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

and setting the top-level node to the DataContext of the Window:

IViewModel model = ViewModelFactory.GetTopLevelViewModel();
DataContext = model;

However, nothing is appearing in the treeview. I've looked at quite a few similar questions on SO and elsewhere, but none seem to solve my problem. A few questions:

1. Why do I need to set the DataContext of the parent Window, and not the TreeView itself?
2. Why is nothing appearing in the treeview?
3. How do I fix it?

Why do I need to set the DataContext of the parent Window, and not the TreeView itself?

You don't need to. You can set the data context on the TreeView itself:

<TreeView x:Name="tree"...>

-

IViewModel model = ViewModelFactory.GetTopLevelViewModel();
tree.DataContext = model;

Why is nothing appearing in the treeview?

TreeView is an ItemsControl. In order to make it display something you need to assign its ItemsSource property to a collection.

How do I fix it?

For example, you can set the DataContext to a collection of IViewModel items instead of a single item:

IViewModel model = ViewModelFactory.GetTopLevelViewModel();
tree.DataContext = new IViewModel[] { model };