当PropertyChanged调用event PropertyChanged=null时

Question

我有一个从INotifyPropertyChanged派生的类来跟踪属性更改。这些属性被绑定到TreeView，并且是在TreeViewItems中执行搜索所必需的。当在属性setters中时，当"IsExpanded“冒泡到TreeView的根元素时，出于某种原因，this.OnPropertyChanged("IsExpanded")被调用为this.PropertyChanged == null。为什么它在这里是null，在更深层的TreeView元素上不是null？我能用什么来解决这个问题呢？我的代码cs：

    public class TreeViewItemViewModel:INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName)
        {
            if (this.PropertyChanged != null)
                this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }

    ....................

        bool _isExpanded;
        public bool IsExpanded
        {
            get { return _isExpanded; }
            set
            {
                if (value != _isExpanded)
                {
                    _isExpanded = value;
                    this.OnPropertyChanged("IsExpanded");
                }

                //Expand all till the root
                if (_isExpanded && _parent != null)
                    _parent._isExpanded = true;

                //Lazy load children, if nessesary
                if (this.HasDummyChild)
                {
                    this.Children.Remove(DummyChild);
                    this.LoadChildren();
                }
            }

        }

        bool _isSelected;
        public bool IsSelected
        {
            get { return _isSelected; }
            set
            {
                if (value != _isSelected)
                {
                    _isSelected = value;
                    this.OnPropertyChanged("IsSelected");
                }
            }
        }
    .............................
    }

我的代码XAML：

          <TreeView ItemsSource="{Binding Areas}">
                <TreeView.ItemContainerStyle>
                    <Style TargetType="{x:Type TreeViewItem}">
                        <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
                        <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
                        <Setter Property="FontWeight" Value="Normal" />
                        <Style.Triggers>
                            <Trigger Property="IsSelected" Value="True">
                                <Setter Property="FontWeight" Value="Bold" />
                            </Trigger>
                        </Style.Triggers>
                    </Style>
                </TreeView.ItemContainerStyle>
                <TreeView.Resources>
                    <HierarchicalDataTemplate 
                      DataType="{x:Type vAreas:AreaViewModel}" 
                      ItemsSource="{Binding Children}"
                      >
                        <StackPanel Orientation="Horizontal">
                            <TextBlock Text="{Binding Name}" />
                        </StackPanel>
                    </HierarchicalDataTemplate>
                </TreeView.Resources>
            </TreeView>

Answer 1

问题是我从来没有订阅过我的event PropertyChanged。一旦我订阅了它，它就开始正常工作了

this.PropertyChanged += AreaViewModel_PropertyChanged;

void AreaViewModel_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "IsExpanded")
        {
            //Expand all till the root
            if (this.IsExpanded && this.Parent != null)
                this.Parent.IsExpanded = true;

            //Lazy load children, if nessesary
            if (this.HasDummyChild)
            {
                this.Children.Remove(DummyChild);
                this.LoadChildren();
            }
        }
    }

Answer 2

我在这个问题上挣扎了一段时间。最后，我的集合提供给树视图的结果是一个ObservableCollection属性，但实际上是一个列表。

当我把它变成ObservableCollection的时候，一切都开始工作了。