在Global.asax的Application_Error中的代码如下：

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();
        string error = string.Empty;
        string errortime = string.Empty;
        string erroraddr = string.Empty;
        string errorinfo = string.Empty;
        string errorsource = string.Empty;
        string errortrace = string.Empty;

        error += "发生时间:" + System.DateTime.Now.ToString() + "<br>";
        errortime = "发生时间:" + System.DateTime.Now.ToString();

        error += "发生异常页: " + Request.Url.ToString() + "<br>";
        erroraddr = "发生异常页: " + Request.Url.ToString();

        error += "异常信息: " + objErr.Message + "<br>";
        errorinfo = "异常信息: " + objErr.Message;
        errorsource = "错误源:" + objErr.Source;
        errortrace = "堆栈信息:" + objErr.StackTrace;
        error += "--------------------------------------<br>";
        Server.ClearError();
        Application["error"] = error;
        //独占方式，因为文件只能由一个进程写入.
        System.IO.StreamWriter writer = null;
        try
        {
            lock (this)
            {
                // 写入日志
                string year = DateTime.Now.Year.ToString();
                string month = DateTime.Now.Month.ToString();
                string path = string.Empty;
                string filename = DateTime.Now.Day.ToString() + ".txt";
                path = Server.MapPath("~/Error/") + year + "/" + month;
                //如果目录不存在则创建
                if (!System.IO.Directory.Exists(path))
                {
                    System.IO.Directory.CreateDirectory(path);
                }
                System.IO.FileInfo file = new System.IO.FileInfo(path + "/" + filename);
                //if (!file.Exists)
                //    file.Create();
                //file.Open(System.IO.FileMode.Append);       
                writer = new System.IO.StreamWriter(file.FullName, true);//文件不存在就创建,true表示追加
                writer.WriteLine("用户IP:" + Request.UserHostAddress);
                //if (Session["UserName"] != null)
                //{
                //    writer.WriteLine("用户名" + System.Web.HttpContext.Current.Session["UserName"].ToString());
                //}
                writer.WriteLine(errortime);
                writer.WriteLine(erroraddr);
                writer.WriteLine(errorinfo);
                writer.WriteLine(errorsource);
                writer.WriteLine(errortrace);
                writer.WriteLine("--------------------------------------------------------------------------------------");
                //writer.Close();
            }
        }
        finally
        {
            if (writer != null)
                writer.Close();

        }
        Response.Redirect("~/Error/ErrorPage.aspx");

    }