Web Forms :: How To Check Condition In Check Box

i have one problem. i wrote the coding for the gridview binding and the checkbox. but at the run time, we cannot able to get

the value in the checkbox..

for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
{
if (((CheckBox)GridView1.Rows[i].FindControl("CheckBox1")).Checked == true)
{
c = "true";
cmd = new SqlCommand("insert into attenancedetail values(" + Studentid_txt.Text + ",'" + Studentname_txt.Text + "','" + DepartmentDdl1.SelectedItem.ToString() + "','" + Sectionddl2.SelectedIndex.ToString() + "','" + Label2.Text + "','"
+ c + "')", con);
pg();
cmd.ExecuteNonQuery();