string name = System.Configuration.ConfigurationSettings.AppSettings["downloadurl"].ToString();
FileStream fs = new FileStream(name, FileMode.Create, FileAccess.Write);
StreamWriter sw = new StreamWriter(fs, System.Text.Encoding.GetEncoding("gb2312"));

sw.WriteLine(" 自動編號,姓名,年齡");
foreach (DataRow dr in dt.Rows)
{
sw.WriteLine(dr["ID"] + "," + dr["vName"] + "," + dr["iAge"]);
}
sw.Close();
Response.AddHeader("Content-Disposition", "attachment; filename=" + Server.UrlEncode(name));
Response.ContentType = "application/ms-excel";// 指定返回的是一個不能被客戶端讀取的流，必須被下載
Response.WriteFile(name); // 把文件流發送到客戶端
Response.End();

重點為紅色標記處！

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string strFile = "FileName" + DateTime.Now.ToString("yyyyMMddhhmmss") + ".csv";

//這一部分替換為你從DataTable/GridView獲取的內容
StringBuilder sb = new StringBuilder();
sb.AppendLine("id,name");
sb.AppendLine("1,鄒俊才");
sb.AppendLine("2,才俊鄒");
sb.AppendLine("3,Jon");

StringWriter sw = new StringWriter(sb);
sw.Close();

Response.AddHeader("Content-Disposition", string.Format("attachment;filename={0}", strFile));
Response.Charset = "gb2312";
Response.ContentType = "application/ms-excel";
Response.ContentEncoding = System.Text.Encoding.GetEncoding("gb2312");
Response.Write(sw);
Response.Flush();
Response.End();