global文件里Application_Error方法处理记录应用程序错误日志

在global文件里的 Application_Error方法中处理，用此方法可以记录应用程序在什么时间发生了什么错误

代码
protected void Application_Error(Object sender, EventArgs e)
        {
            Exception ex=Server.GetLastError().GetBaseException();

            string errorTime="发生时间："+DateTime.Now.ToString();
            string errorAddress="发生异常页："+Request.Url.ToString();
            string errorInfo="异常信息："+ex.Message;
            string errorSource="错误源："+ex.Source;
            string errorTrace="堆栈信息："+ex.StackTrace;
            Server.ClearError();

            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 day=DateTime.Now.Day.ToString();
                    string path=string.Empty;
                    string filename=DateTime.Now.ToString("yyyyMMdd")+".txt";
                    path=Server.MapPath("~/Error/")+year+month+day;
                    if(!Directory.Exists(path))
                    {
                        Directory.CreateDirectory(path);
                    }
                    System.IO.FileInfo file=new FileInfo(path+"/"+filename);
                    writer=new StreamWriter(file.FullName,true);//文件不在则创建，true表示追加
                    writer.WriteLine("用户IP:"+Request.UserHostAddress);
                    writer.WriteLine(errorTime);
                    writer.WriteLine(errorAddress);
                    writer.WriteLine(errorInfo);
                    writer.WriteLine(errorSource);
                    writer.WriteLine(errorTrace);
                    writer.WriteLine("-------------------------------------------------------");

                }
            }
            finally
            {
                if(writer!=null)
                {
                    writer.Close();
                }
            }
            Server.Transfer("~/ErrorPage.aspx"); //跳转到显示友好错误的页面

        }