Asp.net - GridView中的DropDownLists

Question

我试图用C＃代码在GridView中创建一个DropDownLists列，但无济于事。这是我使用的代码：

var manager=Context.GetOwinContext().GetUserManager<ApplicationUserManager>();
            DataTable dt = new DataTable();
            DataRow dr = null;
            dt.Columns.Add("Email", typeof(String));
            dt.Columns.Add("Username", typeof(String));
            dt.Columns.Add("Roles", typeof(DropDownList));
            var a = manager.Users.ToArray();

            for (int i = 0; i < a.Length; i++ )
            {
                dr = dt.NewRow();
                dr["Email"] = a[i].Email;
                dr["Username"] = a[i].UserName;
                DropDownList lista = new DropDownList();
                var b = manager.Users.ToArray();
                lista.DataSource = b;
                lista.DataBind();
                dr["Roles"] = (DropDownList)lista;
                dt.Rows.Add(dr);
            }

                GridView1.AutoGenerateSelectButton = true;

            dt.AcceptChanges();
            GridView1.DataSource = dt;
            GridView1.DataBind();

前两个列显示，最后一个（应该有DropDownLists）不显示。为什么呢？