C组合框上的数据绑定

我不能理解下面给出的代码的两个问题。我将一个组合框映射到一个自定义对象,每次组合框上选定的值更改时,自定义对象也会更改。

public partial class MainForm : Form
{
    private Person _person;
    public MainForm()
    {
        InitializeComponent();
        _person = new Person();

        //Populating the combox, we have this.comboBoxCities.DataSource = this.cityBindingSource;
        cityBindingSource.Add(new City("London"));
        cityBindingSource.Add(new City("Paris"));
        _person.BirthCity = new City("Roma");
        cityBindingSource.Add(_person.BirthCity);
        cityBindingSource.Add(new City("Madrid"));

        //Doing the binding
        comboBoxCities.DataBindings.Add("SelectedItem", _person, "BirthCity");
    }

    private void buttonDisplay_Click(object sender, EventArgs e)
    {
        MessageBox.Show("BirthCity=" + _person.BirthCity.Name);
    }

    private int i = 0;
    private void buttonAddCity_Click(object sender, EventArgs e)
    {
        City city = new City("City n°" + i++);
        cityBindingSource.Add(city);
        comboBoxCities.SelectedItem = city;
    }

}

public class Person
{
    private City _birthCity;
    public City BirthCity
    {
        get { return _birthCity; }
        set
        {
            Console.WriteLine("Setting birthcity : " + value.Name);
            _birthCity = value;
        }
    }
}

public class City
{
    public string Name { get; set; }
    public City(string name) { Name = name; }
    public override string ToString() { return Name; }
}

1-为什么我在一行中手动选择两次(或更多)组合框上的不同值时,我只收到一个对出生城市的调用。设置最后一个选择的值(只有当组合框失去焦点时,调用才会触发)?

2-为什么当我单击按钮添加城市,然后单击按钮显示时,双面城市不是选中的城市(不是显示在Combox中的城市)