SelectedItem和SelectedValue返回错误的项

我有4个值的List<string> MyList。这些显示在ComboBox控件中。绑定在我的MVVM WPF项目中非常有效。

我还有一个string SelectedMyList，它绑定到我的XAML，并且应该显示所选的项。我遇到的问题是，无论使用SelectedItem还是SelectedValue，它总是通过MyList 中的第一个项目

private MyClass()//constructor
{
    MyList = new List<string>() {"Hi", "Bye", "Hello", "See ya"}; 
}
private string _selectedMyList;
public string SelectedMyList
{
    get
    {
        return this._selectedMyList;
    }
    set
    {
        //value is always Hi
        if (this._selectedMyList== value)
            return;
        this._selectedMyList= value;
        OnPropertyChanged("SelectedMyList");
    }
}
private List<string> _myList;
public List<string> MyList
{
    get
    {
        return this._myList;
    }
    set
    {
        if (this._myList== value)
            return;
        this._myList= value;
        OnPropertyChanged("MyList");
    }
}

还有我的XAML

<ComboBox ItemsSource="{Binding MyList}" SelectedValue="{Binding SelectedMyList, UpdateSourceTrigger=PropertyChanged}" />

输出窗口中没有错误/绑定错误等。

为什么SelectedItem/SelectedValue没有通过我认为是ComboBox中的选定项目？

  private string _selectedMyList;
    public string SelectedMyList
    {
        get
        {
            return this._selectedMyList;
        }
        set
        {
            //value is always Hi
            if (this._selectedMyList != value)
            {
                this._selectedMyList= value;
                OnPropertyChanged("SelectedMyList");
            }
        }
    }
    private List<ObservableCollection> _myList;
    public ObservableCollection<string> MyList
    {
        get
        {
            return this._myList;
        }
        set
        {
            if (this._myList== value)
            {
                this._myList= value;
                OnPropertyChanged("MyList");
            }
        }
    }

<ComboBox ItemsSource="{Binding MyList}" 
          SelectedItem="{Binding SelectedMyList}" 
          IsSynchronizedWithCurrentItem="True"/>