这段代码，为什么gridview1无法显示第二条记录

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
public partial class _Default : System.Web.UI.Page
{
    public System.Data.DataTable dt = new DataTable("mytable");
   
    protected void Page_Load(object sender, EventArgs e)
    {
        System.Data.DataColumn dc = new DataColumn();
        dc.DataType = System.Type.GetType("System.String");
        dc.ColumnName = "col1";
        dc.MaxLength = 10;
        dt.Columns.Add(dc);
        System.Data.DataRow dr = dt.NewRow();
        dr["col1"] = "string1";
        dt.Rows.Add(dr);
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
    protected void Button2_Click(object sender, EventArgs e)
    {
        System.Data.DataRow dr = dt.NewRow();
        dr["col1"] = "string2";
        dt.Rows.Add(dr);
    }
}

当 dt.Rows.Add(dr);添加第二行之后，需要GridView1再次执行DataBind()，第二行的数据才能在GridView1得到体现。