protected string conS = "Data Source=.\\SQLEXPRESS;AttachDbFilename=|DataDirectory|\\Databees.mdf;Integrated Security=True;User Instance=True";
protected SqlConnection con;

protected void Page_Load(object sender, EventArgs e)
{
    con = new SqlConnection(conS);
    try
    {
        con.Open();
        string q = "SELECT * FROM tblKLanten;";
        SqlCommand query = new SqlCommand(q, con);

        using (SqlDataReader dr = query.ExecuteReader())
        {
            bool success = dr.Read();
            if (success)
            {
                Label1.Text = dr.GetString(1);
            }
        }

        con.Close();
    }
    catch (Exception ex)
    {
        Label2.Text = "Error";
    }
}