WPF PropertyChanged代码错误:无法实现'System.ComponentModel.INoti

我有这样的代码:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.ComponentModel;
using System.Collections.Specialized;
using System.Collections.ObjectModel;
namespace Bix
{
    public class SettingsDataObject
    {
        private int id;
        public int Id
        {
            get { return id; }
            set { id = value == 0 ? Db.GetNextSettingsId() : value; }
        }
        private string adminEmail; public string AdminEmail { 
            get { return adminEmail; } 
            set { adminEmail = value; } 
        }
        private int state; public int State { get { return state; } set { state = value == 0 ? 1 : value; } }
        public object[] GetArray()
        {
            return new object[] { id, adminEmail, state };
        }
        public SettingsDataObject()
        {
        }
    }
    public class SettingsUIObjects : ObservableCollection<SettingsUIObject>,INotifyPropertyChanged
    {
        protected override void InsertItem(int index, SettingsUIObject item)
        {
            base.InsertItem(index, item);
            // handle any EndEdit events relating to this item
            item.ItemEndEdit += new SettingsUIObject.ItemEndEditEventHandler(ItemEndEditHandler);
            item.PropertyChanged += new SettingsUIObject.PropertyChangedEventHandler(PropertyChanged);
        }
        public void ItemEndEditHandler(IEditableObject sender)
        {
            // simply forward any EndEdit events
            if (ItemEndEdit != null)
            {
                ItemEndEdit(sender);
            }
        }
        public event SettingsUIObject.ItemEndEditEventHandler ItemEndEdit;
        public event SettingsUIObject.PropertyChangedEventHandler PropertyChanged;
    }
    public class SettingsDataProvider
    {
        private DataAccessLayer dl;
        public SettingsDataProvider()
        {
            dl = new DataAccessLayer();
        }
        public SettingsUIObjects GetSettings()
        {
            try
            {
                SettingsUIObjects objs = new SettingsUIObjects();
                List<SettingsDataObject> objDataObjects = dl.GetSettings();
                foreach (SettingsDataObject obj in objDataObjects)
                {
                    objs.Add(new SettingsUIObject(obj));
                }
                objs.ItemEndEdit += new SettingsUIObject.ItemEndEditEventHandler(SettingsItemEndEdit);
                objs.CollectionChanged += new
                  NotifyCollectionChangedEventHandler(SettingsCollectionChanged);
                objs.PropertyChanged += new SettingsUIObject.PropertyChangedEventHandler(SettingsPropertyChanged);
                return objs;
            }
            catch (Exception) { return new SettingsUIObjects(); }
        }
        void SettingsItemEndEdit(IEditableObject sender)
        {
            SettingsUIObject obj = sender as SettingsUIObject;
            // use the data access layer to update the wrapped data object
            dl.UpdateSettings(obj.GetDataObject());
        }
        void SettingsPropertyChanged(INotifyPropertyChanged sender, PropertyChangedEventArgs e)
        {
            SettingsUIObject obj = sender as SettingsUIObject;
            // use the data access layer to update the wrapped data object
            dl.UpdateSettings(obj.GetDataObject());
        }
        void SettingsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            if (e.Action == NotifyCollectionChangedAction.Remove)
            {
                foreach (object item in e.OldItems)
                {
                    SettingsUIObject obj = item as SettingsUIObject;
                    // use the data access layer to delete the wrapped data object
                    dl.DeleteSettings(obj.GetDataObject());
                }
            }
            if (e.Action == NotifyCollectionChangedAction.Add)
            {
                foreach (object item in e.NewItems)
                {
                    SettingsUIObject obj = item as SettingsUIObject;
                    // use the data access layer to delete the wrapped data object
                    dl.UpdateSettings(obj.GetDataObject());
                }
            }
        }
    }
    public class SettingsUIObject : IEditableObject, INotifyPropertyChanged
    {
        private SettingsDataObject obj;
        public SettingsUIObject(SettingsDataObject o)
        {
            obj = o;
        }
        public SettingsDataObject GetDataObject()
        {
            return obj;
        }
        public int Id { get { return obj.Id; } set { obj.Id = value; } }
        public string AdminEmail { 
            get { return obj.AdminEmail; } 
            set { obj.AdminEmail = value; } 
        }
        public delegate void ItemEndEditEventHandler(IEditableObject sender);
        public event ItemEndEditEventHandler ItemEndEdit;
        #region IEditableObject Members
        public void BeginEdit() { }
        public void CancelEdit() { }
        public void EndEdit()
        {
            if (ItemEndEdit != null)
            {
                ItemEndEdit(this);
            }
        }
        #endregion
        public delegate void PropertyChangedEventHandler(INotifyPropertyChanged sender, PropertyChangedEventArgs e);
        public event PropertyChangedEventHandler PropertyChanged;
        private void NotifyPropertyChanged(string info)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(info));
            }
        }
    }
}

和我一直得到编译错误:

Bix。setingsuiobject '没有实现接口成员'System.ComponentModel.INotifyPropertyChanged.PropertyChanged'。"Bix.SettingsUIObject。不能实现System.ComponentModel.INotifyPropertyChanged。PropertyChanged'，因为它没有匹配的返回类型'System.ComponentModel.PropertyChangedEventHandler'

有谁能告诉我为什么吗?

感谢

奥森

    public delegate void PropertyChangedEventHandler(INotifyPropertyChanged sender, PropertyChangedEventArgs e);
    public event PropertyChangedEventHandler PropertyChanged;