Return dataset in linq

Question

i try to return data in web method using linq .. and also i bind grid view in jquery

$(document).ready(function () {
         BindGridView();

     });
     function BindGridView() {
         $.ajax({
             type: "POST",
             url: "WebForm1.aspx/search_data",
             data: JSON.stingify(obj),
             data: "{'fromdate':'" + fromdate + "','todate':'" + todate + "','regiondrop':'" + regiondrop + "','GridView1':'" + GridView1 + "'}",
             contentType: "application/json; charset=utf-8",
             dataType: "json",
             async: true,
             cache: false,
             success: function (result) {
                 $("#GridView1").empty();
                 if (data.d.length > 0) {
                     $("#GridView1").append(
                 "<tr><th>Owner Name</th><th>Reg No</th></tr>");

                     for (var i = 0; i < data.d.length; i++) {
                         $("#GridView1").append("<tr><td>" +
                 data.d[i].OwnerName + "</td> <td>" +
                 data.d[i].RegNo + "</td> <td>" +

                     }
                 }
             }
         });
     }

webmethod

[WebMethod]
        public static string search_data(DateTime fromdate, DateTime todate, string regiondrop)
        {

            try
            {
                TrackDataEntities1 ts = new TrackDataEntities1();

                var dq = from table //here is LINQ query which is long so i did not paste .....     .ToList();
                             DataSet ds = new DataSet();             dq(ds);             return ds;



            }
            catch (Exception)
            {
                throw new Exception();
                // GridView1.Visible = false;
                // Label4.Text = ("No Data");

            }

        }

this show error
Error 3 'dq' is a 'variable' but is used like a 'method'
Error 4 Cannot implicitly convert type 'System.Data.DataSet' to 'string'

so how i return this