How to set autocomplete for a webservice in Asp.Net with C#?

<pre lang="xml">[System.Web.Script.Services.ScriptMethod()]
    [System.Web.Services.WebMethod]
    public static List<string> GetCategory(string prefixText)
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["connection"].ToString());
        con.Open();
        SqlCommand cmd = new SqlCommand("select distinct product from ProductAdd where product like @Name+'%'", con);
        cmd.Parameters.AddWithValue("@Name", prefixText);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        da.Fill(dt);
        List<string> Cid = new List<string>();
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            Cid.Add(dt.Rows[i]["product"].ToString());
        }
        return Cid;
    }