无法在ASP.NET中执行SQL查询

Question

我已经被困在这几个小时了，而且我很生气。

    public static SqlConnection GetConnection()
    {
        string conString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        SqlConnection con = new SqlConnection(conString);
        con.Open();
        return con;
    }

public static DataTable SelectAll(string zoeknaam)
{
    string sql = "SELECT Achternaam, " + 
                 "       Voornaam, " + 
                 "       Bestelnummer, " +
                 "       Datum, " +
                 "       Artikelen.Artikelnr, " + 
                 "       Omschijving, " +
                 "       Prijs " +
                 "  FROM Klanten, Bestellingen, Artikelen, Bestelregel " +
                 " WHERE Klanten.Klantnr = Bestellingen.Klantnummer " +
                 "   AND Bestellingen.Bestelnummer = Bestelregel.Bestelnr " + 
                 "   AND Bestelregel.Artikelnr = Artikelen.Artikelnr " + 
                 "   AND Klanten.Achternaam = '" + zoeknaam + "';";

    DataTable dt = new DataTable();
    using (SqlConnection cn = GetConnection())
    {
        ConnectionState st = cn.State;
        SqlDataAdapter da = new SqlDataAdapter(sql, cn);
        da.Fill(dt);
        return dt;
    }
}

由于某种原因，dt总是空的。无论我使用什么查询。当我执行与存储过程完全相同的查询时，它完美地工作。

ConnectionState始终是Open，因此不应该是问题

有什么想法吗？