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//Combobox füllen mit dem Wert Firma füllen
            int spalten_nr = 1; //Nummer der Spalte, in der das gewünschte Element steht
            SqlCommand cmd2 = new SqlCommand("Select mandant_nr, firma FROM tbl_mandant");
            // CommandText nach Bedarf stetzen
            cmd2.Connection = conn;
            conn.Open();
            cb.Items.Clear();
            SqlDataReader dr = cmd2.ExecuteReader();
            while (dr.Read())
            {
                cb.Items.Add(dr.GetValue(spalten_nr).ToString());
            }
            //Standardwert setzten
            cb.SelectedIndex = 0;					

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using System;
...

namespace combobox
{
    public partial class Form1 : Form
    {
        private DataView dv = null;

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

            fillListControl(comboBox1);

        }

        private void fillListControl(ListControl lc)
        {

            string CrLf = Environment.NewLine;
            SqlConnection conn = new SqlConnection("Server=.\\SQLEXPRESS;" +
                                                   "Initial Catalog=BMDBSQL;" +
                                                   "Integrated Security=SSPI;");
            SqlCommand cmd = new SqlCommand("Select * FROM tbl_mandant", conn);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable("tbl_mandant");
            da.Fill(dt);
            dv = dt.DefaultView;

            lc.DataSource = dv;
            lc.ValueMember = "mandant_nr";
            lc.DisplayMember = "firma";
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            textBox1.Text = comboBox1.SelectedValue.ToString();
        }

    }
}