ObservableCollection<T> 类和 IndexOf 函数

下午好，我遇到了这样一个问题：ObservableCollection 类的 IndexOf 函数总是返回-1。这是代码：

ObservableCollection<Item> FilterMask = new ObservableCollection<Item>();
foreach(Worker w in ListWorker)
    {
        if(w.Directions.Count == 0) continue;
        Item ForFilter = new Item() { Text = w.Directions[0], IsSelected = true };
        if (FilterMask.IndexOf(ForFilter) == -1)
            FilterMask.Add(ForFilter);
    }

项目类示例：

public class Item : INotifyPropertyChanged
{
    string _Text;
    public string Text
    {
        get { return _Text; }
        set
        {
            _Text = value;
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("Text"));
        }
    }

    bool _IsSelected;
    public bool IsSelected
    {
        get { return _IsSelected; }
        set
        {
            _IsSelected = value;
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("IsSelected"));
        }
    }

    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    #endregion
}

FilterMask出现重复，应该不是按照代码来的=）