C# - Get Checked Checkbox While Update From Gridview?

in my project i use chekbox list...but while m get update from gridview i can't get chek chekbox.

SqlConnection Conn = new SqlConnection(ConfigurationManager.ConnectionStrings["ims"].ConnectionString);
Conn.Open();
DataSet ds1 = new DataSet();
string querry = "Select po_tax from purchase_order where po_id='" + Request.QueryString["po_id"] + "'";
// ds1 = obj1.SelectQuery(querry);
SqlCommand cmd = new SqlCommand(querry,Conn);
cmd.ExecuteNonQuery();
SqlDataReader rdr;
rdr = cmd.ExecuteReader();
if (rdr.Read() == false)
{
//No Records
rdr.Close();
Conn.Close();
Label3.Text = "No record found";
return;
}
else
{
CheckBoxList chkbx = (CheckBoxList)form1.FindControl("CheckBoxList1");
rdr.NextResult();
if (rdr.IsClosed == false)
{
while (rdr.Read() == true) [problem ocures here. it does not executes i.e does not go inside curly braces. why?]
{
ListItem currentCheckBox = chkbx.Items.FindByValue(rdr["po_id"].ToString());
if (currentCheckBox != null)
{
currentCheckBox.Selected = true;
}
}
}
rdr.Close();
//string[] items = returned_value_from_db.Split(',');
string[] items = sb.ToString().Split(',');
for (int i = 0; i <= items.GetUpperBound(0); i++)
{
ListItem currentCheckBox = chkbx.Items.FindByValue(items[i].ToString());
if (currentCheckBox != null)
{
currentCheckBox.Selected = true;
}
}
}
Conn.Close();