using System.Data.SqlClient;


namespace TestDummyTWO
{
    public partial class Form1 : Form
    {
        SqlConnection sqlcon =
            new SqlConnection("Data Source=.;Initial Catalog=your_table;Integrated Security=True;Asynchronous Processing = True");

        SqlCommand sqlCom = new SqlCommand("SELECT ITEMS FROM YOUR_TABLENAME");

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            SqlDataReader reader = sqlCom .ExecuteReader();

            // Looping throuhg the list of data
            while (reader.Read())
            {
                // Fill combo Box with data
                comboBox1.Items.Add(reader["ITEMS"]);
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            // you added the new item from the tex box into the DATA TABLE
            // Not the combobox
            // use sqlCom again to do so

            sqlCom.CommandText = "Insert into blah blah";

            /* THIS IS WHAT YOU HAVE TO DO */
            sqlcon.Close();
            sqlcon.Open();
        }