public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        SqlConnection connection = new SqlConnection("Data Source=.;Initial Catalog=email_client;Integrated Security=True");
        SqlCommand command = new SqlCommand("Select * FROM address_book ", connection);

        try
        {
            connection.Open();
            {
                SqlDataReader drd = command.ExecuteReader();

                while (drd.Read())
                {
                    this.checkedListBox1.Items.Add(drd.GetString(0).ToString());
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message.ToString());
        }

        connection.Close();
    }

    private void checkedListBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=email_client;Integrated Security=True");
        con.Open();
        SqlCommand cmd = new SqlCommand("select * from address_book where name='" + checkedListBox1.Text + "'", con);
        SqlDataReader dr;
        dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            textBox1.Text += Convert.ToString(dr["id"] + ",");
        }
        dr.Close();
    }

    private void textBox1_Enter(object sender, EventArgs e)
    {
        ToolTip tt = new ToolTip();
        tt.SetToolTip(textBox1, "sorry");
    }
}

private void checkedListBox1_ItemCheck(object sender, ItemCheckEventArgs e)

private void checkedListBox1_ItemCheck(object sender, ItemCheckEventArgs e)
{
    StringBuilder stringBuilder = new StringBuilder();

    foreach (var item in checkedListBox1.CheckedItems)
    {
        SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=email_client;Integrated Security=True");
        con.Open();
        SqlCommand cmd = new SqlCommand(string.Format("select * from address_book where name='{0}'", item), con);
        SqlDataReader dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            stringBuilder.Append(Convert.ToString(dr["id"] + ","));
        }
        dr.Close();
    }

    textBox1.Text = stringBuilder.ToString().TrimEnd(',');
}

textBox1.Text = textBox1.Text.Substring(0, textBox1.Text.Length - 1);