这段代码为何不行

这段代码为什么不行？
简单说明一下，DataGridView的数据设置为一个BingList<A> 类型，A中两个字段，我希望第二个字段从下拉列表中选择，
请看下面代码：

C# code

namespace TestWinForm{    public partial class Form1 : Form    {        public Form1()        {            InitializeComponent();            BindingList<A> datasource = new BindingList<A>();            dataGridView1.AutoGenerateColumns = false;            dataGridView1.DataSource = datasource;            DataGridViewTextBoxColumn c = new DataGridViewTextBoxColumn();            dataGridView1.Columns.Add(c);            c.DataPropertyName = "FieldA";            DataGridViewComboBoxColumn combo = new DataGridViewComboBoxColumn();            listBox1.DataSource = dataGridView1.DataSource;            combo.Items.AddRange(                new Pack[] {                    new Pack(){                        Value= new B(){Field1="F10",Field2="F20"}                                     },                   new Pack(){                        Value=  new B(){Field1="F11",Field2="F21"}                   }                });            combo.DisplayMember = "Name";            combo.ValueMember = "Value";            combo.ValueType = typeof(B);            //如果注释掉下面这行，不会报错，但是值也当然不会设置到数据源中去            combo.DataPropertyName = "B";            dataGridView1.Columns.Add(combo);            dataGridView1.CellValueChanged += new DataGridViewCellEventHandler(dataGridView1_CellValueChanged);        }        void dataGridView1_CellValueChanged(object sender, DataGridViewCellEventArgs e)        {            if (e.ColumnIndex == 0)            {                DataGridViewTextBoxCell tc = dataGridView1[e.ColumnIndex, e.RowIndex] as DataGridViewTextBoxCell;                Console.WriteLine(tc.Value.ToString());            }            else if (e.ColumnIndex == 1)            {                DataGridViewComboBoxCell bc = dataGridView1[e.ColumnIndex, e.RowIndex] as DataGridViewComboBoxCell;                Console.WriteLine(bc.Value.ToString());                        }            Console.WriteLine();            Console.WriteLine("==dataGridView1.DataSource==Begin==");            BindingList<A> list = dataGridView1.DataSource as BindingList<A>;            foreach (A a in list)            {                Console.WriteLine(a);            }            Console.WriteLine("==dataGridView1.DataSource==End===");        }    }    public class A    {        public A()        {            B = new B() { Field1 = "default1", Field2 = "default2" };        }        public String FieldA { get; set; }        public B B { get; set; }        public override string ToString()        {            return FieldA + " " + B.ToString();        }    }    public class B    {        public String Field1 { get; set; }        public String Field2 { get; set; }        public override string ToString()        {            return Field1 + " " + Field2;        }    }    public class Pack    {        public String Name { get { return Value.Field1; } }        public B Value { get; set; }    }}