SelectedValue binding of a nested DataGrid doesn't work while ItemsSource does

Question

I have a DataGrid inside of a UserControl which in turn lies inside of another UserControl. This is due to other needs of the project and I can't change this nested architecture. I'm binding a list of Person class to this DataGrid. This is a dumbed-down version without using a VM, but in my real project I am using a VM.

My UserControl with the DataGrid:

<Grid>
    <DataGrid x:Name="MyDg"
        ItemsSource="{Binding ItemsSource, RelativeSource={RelativeSource AncestorType=local:UCDataGrid}, UpdateSourceTrigger=PropertyChanged}"
        MouseDoubleClick="MyDg_MouseDoubleClick"
        SelectedValue="{Binding SelectedValue, RelativeSource={RelativeSource AncestorType=local:UCDataGrid}, UpdateSourceTrigger=PropertyChanged}"/>
</Grid>

Code Behind:

public partial class UCDataGrid : UserControl
{
    public event RoutedEventHandler RoutedDataGridDoubleClick;

    public UCDataGrid()
    {
        InitializeComponent();
    }

    public static readonly DependencyProperty ItemsSourceProperty = DependencyProperty.Register("ItemsSource", typeof(object), typeof(UCDataGrid), new PropertyMetadata(null));
    public object ItemsSource
    {
        get { return GetValue(ItemsSourceProperty); }
        set { SetValue(ItemsSourceProperty, value); }
    }

    public static readonly DependencyProperty SelectedValueProperty = DependencyProperty.Register("SelectedValue", typeof(object), typeof(UCDataGrid), new PropertyMetadata(null));
    public object SelectedValue
    {
        get { return GetValue(SelectedValueProperty); }
        set { SetValue(SelectedValueProperty, value); }
    }

    private void MyDg_MouseDoubleClick(object sender, MouseButtonEventArgs e)
    {
        RoutedDataGridDoubleClick?.Invoke(this, new RoutedEventArgs());
    }
}

2nd UserControl that contains the above control:

<Grid>
    <ContentControl Content="{Binding MyDataGrid, ElementName=ucDisplay}"/>
</Grid>

ucDisplay is simply the Name property value of this UserControl.

Code Behind:

Nothing fancy here.

public partial class UCDisplay : UserControl
{
    public UCDisplay()
    {
        InitializeComponent();
    }

    public static readonly DependencyProperty MyDataGridProperty = DependencyProperty.Register("MyDataGrid", typeof(object), typeof(UCDisplay), new PropertyMetadata(null));
    public object MyDataGrid
    {
        get { return GetValue(MyDataGridProperty); }
        set { SetValue(MyDataGridProperty, value); }
    }
}

Main Window

In my Main Window, I bind my People list as well as SelectedPerson instance, like so:

<Grid>
    <local:UCDisplay>
        <local:UCDisplay.MyDataGrid>
            <local:UCDataGrid ItemsSource="{Binding People}"
                              SelectedValue="{Binding SelectedPerson, UpdateSourceTrigger=PropertyChanged}"
                              RoutedDataGridDoubleClick="UCDataGrid_RoutedDataGridDoubleClick"/>
        </local:UCDisplay.MyDataGrid>
    </local:UCDisplay>
</Grid>

Code Behind:

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    protected bool SetField<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
    {
        if (EqualityComparer<T>.Default.Equals(field, value)) return false;
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }

    private List<Person> people;
    public List<Person> People
    {
        get => people;
        set => SetField(ref people, value);
    }

    private Person selectedPerson;
    public Person SelectedPerson
    {
        get => selectedPerson;
        set => SetField(ref selectedPerson, value);
    }

    public MainWindow()
    {
        InitializeComponent();
        People = GetPeople();
        DataContext = this;
    }

    private void UCDataGrid_RoutedDataGridDoubleClick(object sender, RoutedEventArgs e)
    {

    }

    private List<Person> GetPeople()
    {
        return new List<Person>
        {
            new Person() { Name = "A" },
            new Person() { Name = "B" },
            new Person() { Name = "C" }
        };
    }

    public class Person
    {
        public string Name { get; set; }
    }
}

Again, in reality I'm using a VM, this is only to keep things simple.

Now when I run this I can display my list content just fine. But when I double-click an item in my DataGrid, in the corresponding in my Main Window code behind, the SelectedPerson remains null, although its binding is identical to the People list. I confirm this by using a break point in the main code behind:

But if I debug and see the value in the code behind of my innermost UserControl, you see that the SelectedValue there has the correct selected items value.

So what am I doing wrong here? Why can't I seem to bind the SelectedValue although I do it exactly the same as my ItemsSource binding, but the latter works?

Answer 1

SelectedValue is supposed to be used in conjunction with SelectedValuePath. You should use SelectedItem instead.

Besides that, you are missing a TwoWay Binding. Either explicitly declare the SelectedItem Binding TwoWay

<DataGrid x:Name="MyDg"
    ItemsSource="{Binding ItemsSource,
        RelativeSource={RelativeSource AncestorType=UserControl}}"
    SelectedItem="{Binding SelectedItem,
        RelativeSource={RelativeSource AncestorType=UserControl}, Mode=TwoWay}"/>

or register the property to bind TwoWay by default:

public static readonly DependencyProperty SelectedItemProperty =
    DependencyProperty.Register(
        nameof(SelectedItem), typeof(object), typeof(UCDataGrid),
        new FrameworkPropertyMetadata(
            null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

public object SelectedItem
{
    get { return GetValue(SelectedItemProperty); }
    set { SetValue(SelectedItemProperty, value); }
}

Also note that setting UpdateSourceTrigger=PropertyChanged is pointless in all your Bindings.