WPF TreeView通过ICommand更新后丢失子节点结构问题求助

开发WPF项目时，实现了ComboBox、Button、ICommand和TreeView组件，TreeView通过Button的ICommand生成数据。首次选择ComboBox项点击Button后TreeView显示正常，但TreeView展开后再次点击Button（无论ComboBox选项是否改变），仅显示更新后的父节点，子节点丢失。另外，若XAML中TreeViewItem.IsExpanded设为True，首次运行也不显示子节点；设为False且未手动展开时，点击Button能保持结构并显示更新的子节点。未使用代码后置，需定位问题原因。

XAML代码 #

<Window.Resources>
        <vm:RevitVM x:Key="viewmodel"/>
        <HierarchicalDataTemplate DataType="{x:Type vm:ParentNew}"
                                  ItemsSource="{Binding Children}">
            <TextBlock Text="{Binding Name}"/>
        </HierarchicalDataTemplate>
        <DataTemplate DataType="{x:Type vm:ChildNew}">
            <TextBlock Text="{Binding Name}"/>
        </DataTemplate>
        
    </Window.Resources>

    <Grid DataContext="{StaticResource viewmodel}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="2*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="6*"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="1">
            <ComboBox ItemsSource="{Binding UniqueAreaTypes}"
                      SelectedItem="{Binding SelectedAreaType}">
                <ComboBox.ItemTemplate>
                    <DataTemplate>
                        <StackPanel>
                            <TextBlock Text="{Binding AreaType}"/>
                        </StackPanel>
                    </DataTemplate>
                </ComboBox.ItemTemplate>
            </ComboBox>
        </StackPanel>
        <Button Content="Search" 
            Command="{Binding SearchCommand}"
            Grid.Row="5"
            Grid.Column="0"
            Margin="4"/>
        <TreeView
            Grid.Row="1"
            Grid.Column="1">
            <TreeViewItem ItemsSource="{Binding FamilyList}" Header="Test"
                         IsExpanded="False"/>
        </TreeView>
    </Grid>

ViewModel相关代码 #

public RevitVM()
{
    SearchCommand = new SearchCommand(this);
    FamilyList = new ObservableCollection<ParentNew>();
}

public void SetTree()
{
    FamilyList.Clear();
    foreach (var a in AreasToShow)
    {
        ParentNew parent = new ParentNew(); 
        parent.Name = a.Name;
        FamilyList.Add(parent); 
        ObservableCollection<ChildNew> children = new ObservableCollection<ChildNew>();
        foreach (var r in RoomsToShow)
        {
            if (r.Position == a.Position) 
            {
                ChildNew child = new ChildNew();
                child.Name = r.Name;
                children.Add(child);
            }
        }
        parent.Children = children;
    }
}

public void MakeQuery()
{
    SetTree();
}

ICommand实现代码 #

public class SearchCommand : ICommand
{
    public RevitVM VM { get; set; }

    public SearchCommand(RevitVM revitVM)
    {
        VM = revitVM;
    }

    public event EventHandler? CanExecuteChanged;

    public bool CanExecute(object? parameter)
    {
        return true;
    }

    public void Execute(object? parameter)
    {
        VM.MakeQuery();
    }
}

ParentNew类代码 #

public class ParentNew
{
    public string Name { get; set; }
    public int Id { get; set; }
    public ObservableCollection<ChildNew> Children { get; set; }
}

问题原因 #

核心问题在于ParentNew类未实现INotifyPropertyChanged接口，当你在SetTree方法中为ParentNew的Children属性赋值时，WPF的绑定系统无法感知到这个属性变化：

• 当TreeView未展开时，WPF还未生成子项的可视化元素，后续赋值后展开会重新读取Children属性，因此能正常显示；
• 一旦TreeView展开过，WPF已缓存父节点的绑定上下文，此时修改Children属性但没有通知，绑定系统不会更新子节点显示，导致子节点丢失；
• XAML中TreeViewItem.IsExpanded设为True时，首次加载就会尝试读取Children属性，但此时赋值操作无通知，因此也不会显示子节点。

解决方案 #

1. 让ParentNew实现INotifyPropertyChanged接口 #

在Children属性赋值时触发PropertyChanged事件，让WPF绑定系统感知到属性变化：

public class ParentNew : INotifyPropertyChanged
{
    private string _name;
    private int _id;
    private ObservableCollection<ChildNew> _children;

    public string Name
    {
        get => _name;
        set
        {
            _name = value;
            OnPropertyChanged(nameof(Name));
        }
    }

    public int Id
    {
        get => _id;
        set
        {
            _id = value;
            OnPropertyChanged(nameof(Id));
        }
    }

    public ObservableCollection<ChildNew> Children
    {
        get => _children;
        set
        {
            _children = value;
            OnPropertyChanged(nameof(Children));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

2. 优化SetTree方法逻辑（可选，更规范） #

先创建好ParentNew及其Children集合，再添加到FamilyList中，避免集合变更与属性变更的交叉影响：

public void SetTree()
{
    FamilyList.Clear();
    foreach (var a in AreasToShow)
    {
        var children = new ObservableCollection<ChildNew>();
        foreach (var r in RoomsToShow)
        {
            if (r.Position == a.Position) 
            {
                children.Add(new ChildNew { Name = r.Name });
            }
        }
        var parent = new ParentNew
        {
            Name = a.Name,
            Children = children
        };
        FamilyList.Add(parent); 
    }
}

修改后，无论TreeView是否展开，点击Button后父节点和子节点的变化都会被绑定系统感知，从而正确更新显示。

内容的提问来源于stack exchange，提问作者origo