Selecionar item no Combobox e automaticamente preencher um textbox

private void cmbFilial_SelectedIndexChanged(object sender, EventArgs e)
        {

            SqlConnection cn = new SqlConnection();
            cn = conexao.ConectarSqlServer();
            try
            {

                if (cmbFilial.SelectedIndex >= 0  ) {
 
                    int valor = Convert.ToInt32(cmbFilial.SelectedValue.ToString());
                    txtFilial.Text = Convert.ToString(valor);

                    if (cn.State == 0)
                        cn.Open();
                    SqlCommand cmd = new SqlCommand(\"select CNPFFilial from FILIAL where IdFilial= @idfilial\", cn);
                    cmd.Parameters.Add(\"@idfilial\", SqlDbType.Int).Value = Convert.ToInt32(txtFilial.Text);  
                    SqlDataReader dr = cmd.ExecuteReader();

                    if (dr.Read())
                    {
                        txtCnpfFilial.Text = dr[\"CNPFFilial\"].ToString();
                    }
         
                }
              
              
            }
            catch (Exception i)
            {

                MessageBox.Show(i.Message);
            }

            finally
            {

                cn.Close();
            
            }
        }