Applying combo template issue

            DataTemplate dataTemplate = new DataTemplate(); // DataTemple for Combo box
            FrameworkElementFactory parentFactory = new FrameworkElementFactory(typeof(Grid));
            FrameworkElementFactory childFactoryTxtBox = new FrameworkElementFactory(typeof(TextBox));
            childFactoryTxtBox.SetBinding(TextBox.TextProperty, new Binding("Value") { RelativeSource = new RelativeSource() { AncestorType = typeof(IPropertyDataAccessor) }, Mode = BindingMode.TwoWay, ValidatesOnExceptions = true, NotifyOnValidationError = true });
            Binding comboBinding = new Binding("ItemCollection");
            FrameworkElementFactory cmbBoxFactory = new FrameworkElementFactory(typeof(ComboBox));
cmbBoxFactory.SetBinding(ComboBox.ItemsSourceProperty, comboBinding);
            cmbBoxFactory.SetBinding(ComboBox.SelectedItemProperty, new Binding("Value") { RelativeSource = new RelativeSource() { AncestorType = typeof(IPropertyDataAccessor) } });
            parentFactory.AppendChild(childFactoryTxtBox);
            parentFactory.AppendChild(cmbBoxFactory);
            dataTemplate.VisualTree = parentFactory;
            propertyGridItem.ValueTemplate = dataTemplate;

cmbBoxFactory.SetBinding(ComboBox.SelectedItemProperty, new Binding("Value") { RelativeSource = new RelativeSource() { AncestorType = typeof(IPropertyDataAccessor) } });