CheckBox ใน DataGridView เพื่อทำการลบข้อมูลแต่มันไม่ลบให้

 private void toolStripLabel1_Click(object sender, EventArgs e)
        {
            con = new SqlConnection(strCon);

            con.Open();

            SqlCommand com = new SqlCommand();

            for (int i = 0; i < dataGridView1.Rows.Count - 1; i++)
            {
                if (dataGridView1.Rows[i].Cells[0].Value != null) {
                    String id=dataGridView1 .Rows [i].Cells [1].Value .ToString ();
                    sql="delete from usernamepassword where id='"+id+"'";

                    com.Connection = con;
                    com.CommandType = CommandType.Text;
                    com.CommandText = sql;
                    com.ExecuteNonQuery();
                }

            }
        }