using System.Globalization;

    /// <summary>
    /// 用于將錯誤信息輸出到txt文件
    /// </summary>
    /// <param name="errorMessage">錯誤詳細信息</param>
    public static void WriteError(string errorMessage)
    {
        try
        {
            string path = "~/Error/" + DateTime.Today.ToString("yyMMdd") + ".txt";
            if (!File.Exists(System.Web.HttpContext.Current.Server.MapPath(path)))
            {
                File.Create(System.Web.HttpContext.Current.Server.MapPath(path)).Close();
            }
            using (StreamWriter w = File.AppendText(System.Web.HttpContext.Current.Server.MapPath(path)))
            {
                w.WriteLine("\r\nLog Entry : ");
                w.WriteLine("{0}", DateTime.Now.ToString(CultureInfo.InvariantCulture));
                w.WriteLine(errorMessage);
                w.WriteLine("________________________________________________________");
                w.Flush();
                w.Close();
            }
        }
        catch (Exception ex)
        {
            WriteError(ex.Message);
        }
    }

void Application_Error(object sender, EventArgs e)
    {
        // 在出現(xiàn)未處理的錯誤時運行的代碼
        Exception objErr = Server.GetLastError().GetBaseException();
        //記錄出現(xiàn)錯誤的IP地址
        string strIP = Request.UserHostAddress;
       string err = "Ip【" + strIP + "】" + Environment.NewLine + "Error in【" + Request.Url.ToString() +
                          "】" + Environment.NewLine + "Error Message【" + objErr.Message.ToString() + "】";
        //記錄錯誤
        FN.WriteError(err);
    }

<system.web>
    <customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
     <!--可以指定其他錯誤頁面...-->
    </customErrors>
    </system.web>