WPF MVVM TreeView SelectedItem

Question

This cannot be this difficult. The TreeView in WPF doesn't allow you to set the SelectedItem, saying that the property is ReadOnly. I have the TreeView populating, even updating when it's databound collection changes.

I just need to know what item is selected. I am using MVVM, so there is no codebehind or variable to reference the treeview by. This is the only solution I have found, but it is an obvious hack, it creates another element in XAML that uses ElementName binding to set itself to the treeviews selected item, which you must then bind your Viewmodel too. Several other questions are asked about this, but no other working solutions are given.

I have seen this question, but using the answer given gives me compile errors, for some reason I cannot add a reference to the blend sdk System.Windows.Interactivity to my project. It says "unknown error system.windows has not been preloaded" and I haven't yet figured out how to get past that.

For Bonus Points: why the hell did Microsoft make this element's SelectedItem property ReadOnly?

Answer 1

You should not really need to deal with the SelectedItem property directly, bind IsSelected to a property on your viewmodel and keep track of the selected item there.

A sketch:

<TreeView ItemsSource="{Binding TreeData}">
    <TreeView.ItemContainerStyle>
        <Style TargetType="{x:Type TreeViewItem}">
            <Setter Property="IsSelected" Value="{Binding IsSelected}" />
        </Style>
    </TreeView.ItemContainerStyle>
</TreeView>

public class TViewModel : INotifyPropertyChanged
{
    private static object _selectedItem = null;
    // This is public get-only here but you could implement a public setter which
    // also selects the item.
    // Also this should be moved to an instance property on a VM for the whole tree, 
    // otherwise there will be conflicts for more than one tree.
    public static object SelectedItem
    {
        get { return _selectedItem; }
        private set
        {
            if (_selectedItem != value)
            {
                _selectedItem = value;
                OnSelectedItemChanged();
            }
        }
    }

    static virtual void OnSelectedItemChanged()
    {
        // Raise event / do other things
    }

    private bool _isSelected;
    public bool IsSelected
    {
        get { return _isSelected; }
        set
        {
            if (_isSelected != value)
            {
                _isSelected = value;
                OnPropertyChanged("IsSelected");
                if (_isSelected)
                {
                    SelectedItem = this;
                }
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        var handler = this.PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

Answer 2

A very unusual but quite effective way to solve this in a MVVM-acceptable way is the following:

1. Create a visibility-collapsed ContentControl on the same View the TreeView is. Name it appropriately, and bind its Content to some SelectedSomething property in viewmodel. This ContentControl will "hold" the selected object and handle it's binding, OneWayToSource;
2. Listen to the SelectedItemChanged in TreeView, and add a handler in code-behind to set your ContentControl.Content to the newly selected item.

XAML:

<ContentControl x:Name="SelectedItemHelper" Content="{Binding SelectedObject, Mode=OneWayToSource}" Visibility="Collapsed"/>
<TreeView ItemsSource="{Binding SomeCollection}"
    SelectedItemChanged="TreeView_SelectedItemChanged">

Code Behind:

    private void TreeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
    {
        SelectedItemHelper.Content = e.NewValue;
    }

ViewModel:

    public object SelectedObject  // Class is not actually "object"
    {
        get { return _selected_object; }
        set
        {
            _selected_object = value;
            RaisePropertyChanged(() => SelectedObject);
            Console.WriteLine(SelectedObject);
        }
    }
    object _selected_object;