Ispostback在girdview绑定checkbox时的应用

Ispostback在girdview中绑定checkbox中的作用
SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=Northwind;User ID=sa;Password=;");
        con.Open();

   绑定一个gridview     SqlCommand cmd3 = new SqlCommand("select top 5 OrderID from Orders ", con);
        SqlDataAdapter sda = new SqlDataAdapter();
        sda.SelectCommand = cmd3;
        DataSet dsyou = new DataSet();
        sda.Fill(dsyou, "youxiao");
                this.GridView1.DataSource = dsyou.Tables["youxiao"];
        this.GridView1.DataBind();
然后添加了一个绑定列 checkbox
 <asp:GridView ID="GridView1" runat="server">
            <Columns>
                <asp:TemplateField HeaderText="000">
                    <ItemTemplate>
                        <asp:CheckBox ID="CheckBox1" runat="server" />
                    </ItemTemplate>
                </asp:TemplateField>
            </Columns>
        </asp:GridView>

添加一个按纽，
 protected void Button1_Click(object sender, EventArgs e)
    {
        for (int i = 0; i < this.GridView1.Rows.Count; i++)
        {
            CheckBox chk = (CheckBox)this.GridView1.Rows[i].FindControl("CheckBox1");
           
            if(chk.Checked)