我目前在使用WPF MVVM数据绑定时遇到了一个奇怪的问题。
这个想法是使用ValueConverter，如果它不是null，则显示绑定值，否则显示ConverterParameter中定义的字符串。
Person类，具有以下属性：Name (string)、Surname (string)、Country (class)。
Country类具有以下属性：CountryName (string)。
如果Person.Country为null，则TextBlock绑定到Person.Country.CountryName时，IValueConverter使用失败。IValueConverter方法Convert甚至不触发来检查绑定值是否为null。
同时IMultiValueConverter工作得很好，无论Person.Country是否为null，每次都会触发。
为什么会这样呢？已尝试搜索任何Microsoft文章，但未找到任何内容。
任何帮助都很感激。
源GitHub项目可以在这里找到：https://github.com/kevintw86/WpfBindingConverterIssue.git

绑定到IValueConverter（当Person.Country==null）：

<TextBlock 
          Text="{Binding Person.Country.CountryName, 
          UpdateSourceTrigger=PropertyChanged, 
          Converter={StaticResource PersonConverter}, 
          ConverterParameter=- Not set -}"
          FontSize="16"/>

PersonConverter：

public class PersonConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value == null)
                return parameter;

            if (value == DependencyProperty.UnsetValue)
                return parameter;

            if (string.IsNullOrWhiteSpace(value.ToString()))
                return parameter;

            return value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

绑定到IMultiValueConverter（即使Person也可以正常工作。Country==null）：

<TextBlock FontSize="16">
                <TextBlock.Text>
                        <MultiBinding Converter="{StaticResource PersonMultiConverter}"
                                  ConverterParameter="- Not set -">
                            <MultiBinding.Bindings>
                                <Binding Path="Person.Country.CountryName"
                                     UpdateSourceTrigger="PropertyChanged"/>
                            </MultiBinding.Bindings>
                        </MultiBinding>
                    </TextBlock.Text>
                </TextBlock>

PersonMultiConverter：

public class PersonMultiConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            if (values == null)
                return parameter;

            if (values[0] == DependencyProperty.UnsetValue)
                return parameter;

            if (values[0] == null)
                return parameter;

            if (string.IsNullOrWhiteSpace(values[0].ToString()))
                return parameter;

            return values[0];
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

人员类别：

public class Person : INotifyPropertyChanged
{
    private string _Name;
    public string Name
    {
        get { return _Name; }
        set 
        { 
            _Name = value;
            OnPropertyChanged();
        }
    }

    private string _Surname;
    public string Surname
    {
        get { return _Surname; }
        set 
        { 
            _Surname = value;
            OnPropertyChanged();
        }
    }

    private Country _Country;
    public Country Country
    {
        get { return _Country; }
        set 
        {
            _Country = value;
            OnPropertyChanged();
        }
    }

    #region INotifyPropertyChanged implementation

    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged([CallerMemberName] string prop = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(prop));
    }

    #endregion
}

国家类别：

public class Country : INotifyPropertyChanged
{
    private string _CountryName;
    public string CountryName
    {
        get { return _CountryName; }
        set 
        { 
            _CountryName = value;
            OnPropertyChanged();
        }
    }

    #region INotifyPropertyChanged implementation

    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged([CallerMemberName] string prop = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(prop));
    }

    #endregion
}

主窗口视图模型：

public class MainWindowViewModel : INotifyPropertyChanged
{
    public MainWindowViewModel()
    {
        this.Person = new Person
        {
            Name = "John",
            Surname = null,
            Country = null,
        };
    }

    #region Properties

    private Person _Person;
    public Person Person
    {
        get { return _Person; }
        set 
        { 
            _Person = value;
            OnPropertyChanged();
        }
    }

    #endregion

    #region INotifyPropertyChanged implementation

    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged([CallerMemberName] string prop = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(prop));
    }

    #endregion
}