how to insert data from combobox to sql?when i inserting the data from combobox to sql so it is showing blank field in sql tabel

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data;
using System.Data.SqlClient;
namespace studentMS
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }
        SqlConnection conn = new SqlConnection("Data Source=TAHIR-PC;Initial Catalog=StudentMS;Integrated Security=True");
        SqlCommand cm;
 
        private void Form2_Load(object sender, EventArgs e)
        {
         
           
 
        }
 
        private void bnt_save_Click(object sender, EventArgs e)
        {
            //string s = "";
            //if (cmb_SCourse.SelectedIndex >= 0)
            //    s = cmb_SCourse.Items[cmb_SCourse.SelectedIndex].ToString();

            try
            {
               
                string sql="Insert into Students(Name,FatherName,address,PhoneNo,Email,Course,Batch,Teacher)values('"+txt_SName.Text+"','"+txt_SFname.Text+"','"+txt_SAddress.Text+"','"+txt_SPhoneNo.Text+"','"+txt_SEmail.Text+"','"+cmb_SCourse.SelectedItem.text+"','"+cmb_SCourse.SelectedItem.text+"','"+cmb_Steacher.SelectedItem.text+"')";
                conn.Open();
 
                cm = new SqlCommand(sql, conn);
                cm.ExecuteNonQuery();
                conn.Close();
                label1.Visible = true;
                label1.Text = "saved Successfully";
                txt_SAddress.Text = txt_SEmail.Text = txt_SFname.Text = txt_SName.Text = txt_SPhoneNo.Text = "";
            }
            catch(Exception ex)
            {
                label1.Text = ex.Message;
            }
        }
    }
}