using System.Globalization;

    /// summary>
    /// 用于将错误信息输出到txt文件
    /// /summary>
    /// param name="errorMessage">错误详细信息/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)
    {
        // 在出现未处理的错误时运行的代码
        Exception objErr = Server.GetLastError().GetBaseException();
        //记录出现错误的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() + "】";
        //记录错误
        FN.WriteError(err);
    }

system.web>
    customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
     !--可以指定其他错误页面...-->
    /customErrors>
    /system.web>