没有实现接口成员'System.ComponentModel.INotifyPropertyChanged.Pr

namespace MimicCreation
{
    public class TreeManager : INotifyPropertyChanged
    {
        public TreeManager() { }
        public TreeManager(string title, string type, string filename)
        {
            this.childElementsValue.CollectionChanged += this.OnCollectionChanged;
            Title = title;
            Type = type;
            FileName = filename;
        }
        public string Title { get; set; }
        public string Type { get; set; }
        public string FileName { get; set; }
        public override string ToString()
        {
            return Title;
        }
        private ObservableCollection<TreeManager> childElementsValue = new ObservableCollection<TreeManager>();
        public ObservableCollection<TreeManager> ChildElements
        {
            get { return childElementsValue; }
            set { childElementsValue = value; }
        }
        public void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Add:
                    foreach (TreeManager item in e.NewItems)
                    {
                        ((System.ComponentModel.INotifyPropertyChanged)item).PropertyChanged += new System.ComponentModel.PropertyChangedEventHandler(OnPropertyChanged);
                    }
                    break;
            }
        }
        public void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
        }
    }
}

我得到以下错误:错误'MimicCreation。TreeManager'没有实现接口成员'System.ComponentModel.INotifyPropertyChanged. '编译时PropertyChanged。我有一个可观察集合，我希望能够访问通知时，每个项目在可观察集合被改变，我看不出我做错了什么。有什么想法吗?

谢谢。