input search list throwing a stack overflow error

I want a text box on the form, which acts like a search box. when i start typing the roll numbers, it should automatically fill it up.

this text box in bind with a query which fetches around 400 roll nos from the database.

<label> Type the Registration Number</label>
<input list="rollnos" autocomplete="off" type="search" id="f" class="form-control" runat="server" onload="getStudentWise" ><span id="errmsg"></span>
<datalist id="rollnos" runat="server" > </datalist>

Below is the code in code behind.

If the query is for top 200 or something below 200 rollnos,it works fine.

but  the query is "select regdno from student_master where college_code='GIMSR' and status='S'    // this should fetch around 450 records.

the above query automatically gives a stack overflow exception error.. 

string query = "select top 250 regdno from STUDENT_MASTER where COLLEGE_CODE='GIMSR' and status='S' order by regdno";
cmd = new SqlCommand(query, conn);
rs = new SqlDataAdapter(cmd);
ds = new DataSet();
rs.Fill(ds);
table1 = ds.Tables[0];

for (int i = 0; i < table1.Rows.Count; i++)
builder.Append(String.Format("<option value='{0}'>", table1.Rows[i][0]));

rollnos.InnerHtml = builder.ToString();
f.Attributes["list"] = rollnos.ClientID;

can any one help me