.Net web service that should return Json string, but returns only brackets
<p>I'm trying to create web service in .Net that should return Json string, but it returns only brackets [[]]. Please help me, here is my code:</p>

<pre><code>    [WebMethod(Description = "ovoj metod treba da gi dade podatocite za kniga spored vnesen naslov")]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]

    public string VratiKnigi(string vneseno)
    {
       // StringBuilder sb = new StringBuilder();
        SqlConnection konekcija = new SqlConnection();
        konekcija.ConnectionString = "Data Source=majuska-pc;Initial             Catalog=biblioteka;Integrated Security=true";
        string sqlString = "SELECT Naslov, Avtor FROM Kniga WHERE Naslov LIKE '%" +      vneseno + "%'";
        SqlCommand komanda = new SqlCommand(sqlString, konekcija);
        DataSet ds = new DataSet();
        SqlDataAdapter adapter = new SqlDataAdapter(komanda);

        konekcija.Open();
        adapter.Fill(ds, "Kniga");
        konekcija.Close();

        JArray ja = new JArray(ds.Tables[0].Rows.Count);
        int i = 0;
        foreach (DataRow rs in ds.Tables[0].Rows)
        {
            JObject obj=new JObject();
            obj.Add("Naslov", rs["Naslov"].ToString());
            obj.Add("Avtor", rs["Avtor"].ToString());
            ja.Add(obj);

            i = i + 1;
        }
        System.Web.Script.Serialization.JavaScriptSerializer oSerializer =
        new System.Web.Script.Serialization.JavaScriptSerializer();
        string sJSON = oSerializer.Serialize(ja);
        return sJSON.ToString();
</code></pre>
