// Clear response content & headers
            Response.Clear();
            Response.ClearContent();
            Response.ClearHeaders();
            Response.AddHeader("Content-Disposition", "attachment;filename=Myfilename.csv");
            Response.Charset = string.Empty;
            Response.Cache.SetCacheability(System.Web.HttpCacheability.Public);
            Response.ContentType = "text/csv";
            // Create stringWriter
            System.IO.StringWriter stringWrite = new System.IO.StringWriter();
            DataSet ds = (your data access function)
            DataView dv = ds.Tables[0].DefaultView;
            DataTable dt = dv.Table;
            string str = string.Empty;
            int i = 0;
            int j = 0;
            //write column on csv file  
            for (j = 0; j <= dt.Columns.Count - 1; j++)
            {
                str = str + dt.Columns[j].ToString() + ",";
            }
            str = str.Substring(0, str.Length - 1);
            stringWrite.WriteLine(str);
            // write data on csv file 
            for (i = 0; i <= dt.Rows.Count - 1; i++)
            {
                str = string.Empty;
                for (j = 0; j <= dt.Columns.Count - 1; j++)
                {
                    str = str + dt.Rows[i].ItemArray[j].ToString() + ",";
                }
                str = str.Substring(0, str.Length - 1);
                stringWrite.WriteLine(str);
            }
            Response.Write(stringWrite.ToString());
            Response.End();