WPF 双向绑定下拉框ComboBox

 <ComboBox
            Width="210"
            Height="28"
            Margin="102,20,0,0"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            pu:ComboBoxHelper.CornerRadius="4"
            DisplayMemberPath="Name"
            FontFamily="PingFangSC-Regular"
            FontSize="18"
            FontWeight="Regular"
            ItemsSource="{Binding MTs}"
            SelectedItem="{Binding SelectedMaterialType,Mode=TwoWay}"
            SelectedValue="{Binding SelectedMaterialType.id}"
            SelectedValuePath="id" 
            SelectionChanged="ComboBox_SelectionChanged" 
            Visibility="Visible" />