使用复选框删除gridview中的多行

我已经使用以下代码来执行我的查询删除多个值在gridview使用复选框，但当我执行以下它说checkbox1。Checked是假的，但是我检查了。它不会删除我选择的值

这是我的代码和脚本部分

 protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        grd_bnd();
    }
}
 private void grd_bnd()
{
 SqlConnection con = new        SqlConnection(ConfigurationManager.ConnectionStrings["cn"].ConnectionString);
    SqlCommand cmd = new SqlCommand("select * from student", con);
    con.Open();
    SqlDataAdapter adp = new SqlDataAdapter(cmd);
    DataSet ds = new DataSet();
    adp.Fill(ds);
    GridView1.DataSource = ds;
    GridView1.DataBind();
    con.Close();
}
protected void Button3_Click(object sender, EventArgs e)
{
    foreach (GridViewRow row in GridView1.Rows)
    {
        CheckBox checkbox1 = (CheckBox)row.FindControl("checkboxdelete");
        if (checkbox1.Checked)
        {
            int rollno = Convert.ToInt32(GridView1.DataKeys[row.RowIndex].Value.ToString());
            //CheckBox checkbox1 = (CheckBox)row.FindControl("checkbox1");
            SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["cn"].ConnectionString);
            SqlCommand cmd = new SqlCommand("delete from student where rollno = @rollno ", con);
            cmd.CommandType = CommandType.Text;
            cmd.Parameters.Add("@rollno", SqlDbType.Int).Value = rollno.ToString();
            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
            cmd.Dispose();
        }
    }
    grd_bnd();
}
}

，这里是脚本

      <asp:GridView ID="GridView1" runat="server" DataKeyNames="rollno" AllowSorting="true" AutoGenerateColumns="False" BackColor="White" BorderColor="#CC9966" BorderStyle="None" BorderWidth="1px" CellPadding="4" >
        <Columns>
            <asp:TemplateField HeaderText="Select">
                <ItemTemplate>
                    <asp:CheckBox ID="checkboxdelete" runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Name">
                <ItemTemplate>
                    <asp:Label ID="lblname" runat ="server" Text='<%#Eval("name") %>' /> 
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Roll No.">
                <ItemTemplate>
                    <asp:Label ID="lblrollno" runat ="server" Text='<%#Eval("rollno") %>' /> 
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Batch">
                <ItemTemplate>
                    <asp:Label ID="lblbatch" runat ="server" Text='<%#Eval("batch") %>' /> 
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Course">
                <ItemTemplate>
                    <asp:Label ID="lblcourse" runat ="server" Text='<%#Eval("course") %>' /> 
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
        </gridview>
        <asp:Button ID="Button3" runat="server" Font-Bold="True" Text="Delete Selected" ForeColor="#000066" OnClick="Button3_Click" />
    <asp:Label ID="Label1" runat="server" ForeColor="#666666"></asp:Label>

       for (int i = 0; i < GridView1.Rows.Count; i++)
       {
        CheckBox checkboxdelete = ((CheckBox)GridView1.Rows[i].FindControl("checkboxdelete"));
        if (checkboxdelete.Checked == true)
        {
            Label lblrollno = ((Label)GridView1.Rows[i].FindControl("lblrollno"));
            int rolNo = Convert.ToInt32(lblrollno.Text);
            SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["cn"].ConnectionString);
            SqlCommand cmd = new SqlCommand("delete from student where rollno = @rollno ", con);
            cmd.CommandType = CommandType.Text;
            SqlParameter param = new SqlParameter("@rollno", SqlDbType.Int);
            param.Value = rolNo;
            cmd.Parameters.Add(param);
            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
            cmd.Dispose();
        }
      } 

    foreach (GridViewRow row in GridView1.Rows)
    {
        CheckBox checkboxdelete = (CheckBox)row.FindControl("checkboxdelete");
        if (checkboxdelete.Checked == true)
        {
            using (SqlConnection scon = new SqlConnection(ConfigurationManager.ConnectionStrings["CS"].ConnectionString))
            {
                scon.Open();
                Label LblId = (Label)row.FindControl("rollno");
                int rollno = Convert.ToInt32(LblId.Text);
                SqlCommand scmd = new SqlCommand("delete from student where rollno = @rollno", scon);
                scmd.Parameters.AddWithValue("@rollno", rollno);
                scmd.ExecuteNonQuery();
                scon.Close();
                BindGrid();
                Label4.Text = "";
            }
        }
        else
        {
            Label4.Text = "No record is selected to delete !";
            Label4.ForeColor = System.Drawing.Color.Red;
        }
    }