How to put a variable in the name of contorols, txt[i]?

Question

Hi everyone
i have some Textboxes and lables. their names is txt1,txt2,...and lbl1,lbl2,.... I want to put 'i' rather than 1 ro 2... fro ex: lbl[i](=lbl1). i have to put i rather than 1 when i can send all lbl or txt . but when i run, in highlight line has this eeror "eeror Sequence contains no elements"

 SqlCommand cmd = new SqlCommand("ClerkEvaluation_Insert", conn);

                    cmd.CommandType = CommandType.StoredProcedure;

                    for (int i = 1; i <= 10; i++)

                    {

                        cmd.Parameters.AddWithValue("@clrk_Name", txtNameClerkEvaluation.Text);

                        cmd.Parameters.AddWithValue("@clrk_Family", txtFamilyClerkEvaluation.Text);

                        cmd.Parameters.AddWithValue("@clrk_MelliCode", txtMelliCodeClerkEvaluation.Text);

                        //cmd.Parameters.AddWithValue("@epcnt_Name", "?????");

                        //cmd.Parameters.AddWithValue("@clrke_Percent", lbl1MadrakeTahsili[i].Name.su

                        //cmd.Parameters.AddWithValue("@clrke_Tozihat", txtRaftar[i].);

                        cmd.Parameters.AddWithValue("@i", i);

                        MessageBox.Show(Convert.ToString(i));

                        cmd.Parameters.AddWithValue("@clrke_Percent", this.Controls.Find(string.Format("lbl{0}", i), false).First().Text);

                        cmd.Parameters.AddWithValue("@clrke_Percent", this.Controls.Find(string.Format("txt{0}", i), false).First().Text);

                        cmd.ExecuteNonQuery();

                        conn.Close();