Castom ComboBox and lose binding

Question

Hello. I'm trying to write my own ComboBox and I ran into a problem.
public class TestComboBox : ComboBox
public static readonly DependencyProperty SelectedCarProperty = DependencyProperty.Register("SelectedCar", typeof(string), typeof(TestComboBox), new FrameworkPropertyMetadata(null, OnSelectedCarPropertyChanged));
public TestComboBox()
this.Cars = new ObservableCollection<string>() { "Select Car", "BMW", "Mersedes", "Audi" };
this.ItemsSource = this.Cars;
this.SelectedItem = this.Cars[0];
public string SelectedCar
get { return (string)GetValue(SelectedCarProperty); }
set { this.SetValue(SelectedCarProperty, value); }
public IList<string> Cars { get; private set; }
protected override void OnSelectionChanged(SelectionChangedEventArgs e)
base.OnSelectionChanged(e);
this.SelectedCar = (string)SelectedItem;
private static void OnSelectedCarPropertyChanged(DependencyObject source, DependencyPropertyChangedEventArgs e)
var car = (string)e.NewValue;
var editor = (TestComboBox)source;
editor.SelectedItem = car;
When I try to install the default value, I lose the binding. If I commented this code, everything works fine.
this.SelectedItem = this.Cars[0];
How can I set the default value without losing binding to the VM
P.S
TestApp

Answer 1

I use this code.
<propgrid:PropertyGrid Margin="0,0,11,7">
<propgrid:PropertyGrid.Items>
<propgrid:PropertyGridCategoryItem DisplayName="Main">
<propgrid:PropertyGridPropertyItem Value="{Binding Car, Mode=TwoWay}"
DisplayName="Car">
<propgrid:PropertyGridPropertyItem.ValueTemplate>
<DataTemplate>
<wpfApplication1:TestComboBox SelectedCar="{Binding Value, Mode=TwoWay, RelativeSource={RelativeSource AncestorType={x:Type propgrid:IPropertyDataAccessor}}}" />
</DataTemplate>
</propgrid:PropertyGridPropertyItem.ValueTemplate>
</propgrid:PropertyGridPropertyItem>
</propgrid:PropertyGridCategoryItem>
</propgrid:PropertyGrid.Items>
</propgrid:PropertyGrid>
I found a solution my problem Ijust replacedthe code
this.SelectedItem = this.Cars[0];
with this
this.Loaded += (sender, args) =>
if (this.SelectedItem == null)
this.SelectedItem = this.Cars[0];
And now everything works fine