将Nullable值绑定到WPF ComboBox的问题

public class NullableValueConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null)
        {
            return DependencyProperty.UnsetValue;
        }
        return value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null)
        {
            return null;
        }
        return value;
    }
}

   <local:NullableValueConverter x:Key="NullableValueConverter" />
</Window.Resources><ComboBox ItemsSource="{Binding YourItemsSource}"
          SelectedValue="{Binding YourSelectedValue, Converter={StaticResource NullableValueConverter}}" />

public class YourViewModel : INotifyPropertyChanged
{
    private IEnumerable<YourItemType> yourItemsSource;
    public IEnumerable<YourItemType> YourItemsSource
    {
        get { return yourItemsSource; }
        set
        {
            yourItemsSource = value;
            OnPropertyChanged();
        }
    }

    private YourNullableType? yourSelectedValue;
    public YourNullableType? YourSelectedValue
    {
        get { return yourSelectedValue; }
        set
        {
            yourSelectedValue = value;
            OnPropertyChanged();
        }
    }

    // Implement INotifyPropertyChanged
}