WPF TreeView绑定多类异常：CADModel子节点无法正常显示

已成功将WPF TreeView绑定到ClassModel，但部分ClassModel需要包含子CADModel，当前层级数据模板无法正常工作。预期每个ClassModel节点下显示名为"Bob"的CADModel叶子节点，实际未达成该效果。

相关代码 #

XAML代码 #

<TreeView.Resources>
    <!-- HierarchicalDataTemplate for ClassModel -->
    <HierarchicalDataTemplate DataType= "{x:Type models:ClassModel}" ItemsSource = "{Binding Path=ChildClasses}">
        <TextBlock Text="{Binding Path=ClassName}"/>
    </HierarchicalDataTemplate>
    <DataTemplate DataType="{x:Type models:CADModel}">
        <TextBlock Text="{Binding Path=PartNumber}"/>
    </DataTemplate>
</TreeView.Resources>

C# 实体类代码 #

IClassModel接口与ClassModel实现

public interface IClassModel : ITreeNode
{
    ObservableCollection<ClassModel> ChildClasses { get; set; }
    ObservableCollection<CADModel>? CADModels { get; set; }
    int ClassId { get; set; }
    int ParentClassId { get; set; }
    string ClassName { get; set; }
    bool HasChildClass { get; set; }
    LibraryModel? ParentLibrary { get; set; }
}

public class ClassModel : TreeNode, IClassModel
{
    public LibraryModel? ParentLibrary { get; set; } = null;
    public int ParentClassId { get; set; } = 0;
    public int ClassId { get; set; } = 0;
    public string ClassName { get; set; } = string.Empty;
    public bool HasChildClass { get; set; } = false;

    public ObservableCollection<ClassModel> ChildClasses { get; set; } = new ObservableCollection<ClassModel>();
    public ObservableCollection<CADModel>? CADModels { get; set; } = null;

    public ClassModel()
    {
        if (HasChildClass == false)
        {
            CADModels = new ObservableCollection<CADModel>()
            {
                new CADModel()
                {
                    Id = 0,
                    PartNumber = "Bob",
                    Revision = "A.1"
                }
            };
        }
    }
}

ICADModel接口与CADModel实现

public interface ICADModel : ITreeNode
{
    string PartNumber { get; set; }
    string Revision { get; set; }
}
public class CADModel :TreeNode, ICADModel
{
    public string PartNumber { get; set; } = string.Empty;
    public string Revision { get; set; } = string.Empty;
}

当前TreeView显示效果 #

问题核心是当前HierarchicalDataTemplate仅绑定了ChildClasses集合，TreeView无法自动识别ClassModel中的CADModels子节点。以下两种方法可解决该问题：

方法1：在ClassModel中添加合并后的子集合 #

给ClassModel新增一个只读属性，合并ChildClasses和CADModels两个集合：

public class ClassModel : TreeNode, IClassModel
{
    // 保留原有所有属性...

    public IEnumerable<object> Children
    {
        get
        {
            foreach (var childClass in ChildClasses)
                yield return childClass;
            if (CADModels != null)
            {
                foreach (var cadModel in CADModels)
                    yield return cadModel;
            }
        }
    }
}

修改XAML中的HierarchicalDataTemplate，将ItemsSource绑定到新的Children属性：

<HierarchicalDataTemplate DataType="{x:Type models:ClassModel}" ItemsSource="{Binding Path=Children}">
    <TextBlock Text="{Binding Path=ClassName}"/>
</HierarchicalDataTemplate>

方法2：在XAML中使用CompositeCollection合并集合 #

无需修改C#代码，直接在XAML中通过CompositeCollection合并两个子集合：

<HierarchicalDataTemplate DataType="{x:Type models:ClassModel}">
    <TextBlock Text="{Binding Path=ClassName}"/>
    <HierarchicalDataTemplate.ItemsSource>
        <CompositeCollection>
            <CollectionContainer Collection="{Binding Path=ChildClasses}"/>
            <CollectionContainer Collection="{Binding Path=CADModels}"/>
        </CompositeCollection>
    </HierarchicalDataTemplate.ItemsSource>
</HierarchicalDataTemplate>

注意：使用该方法时，建议将ClassModel中CADModels的初始值设为空集合而非null，避免绑定错误。

内容的提问来源于stack exchange，提问作者Marc