多线程操作同一个文件时会出现并发问题。解决的一个办法就是给文件加锁(lock),但是这样的话，一个线程操作文件时，其它的都得等待，这样的话性能非常差。


另外一个解决方案，就是先将数据放在队列中，然后开启一个线程，负责从队列中取出数据，再写到文件中。

using log4net;
using RedisMvcApp.Models;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;
namespace RedisMvcApp
{
    // 注意: 有关启用 IIS6 或 IIS7 经典模式的说明，
    // 请访问 http://go.microsoft.com/?LinkId=9394801
    public class MvcApplication : System.Web.HttpApplication
    {
        protected void Application_Start()
        {
            log4net.Config.XmlConfigurator.Configure();//获取Log4Net配置信息(配置信息定义在Web.config文件中)
            AreaRegistration.RegisterAllAreas();
            WebApiConfig.Register(GlobalConfiguration.Configuration);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);
            //通过线程开启一个线程，然后不停的从队列中或数据
            string filePath = Server.MapPath("/Log/");
            ThreadPool.QueueUserWorkItem(o =>
            {
                while (true)
                {
                    try
                    {
                      //  if (MyExecptionAttribute.ExceptionQueue.Count > 0)
                        if (MyExecptionAttribute.redisClent.GetListCount("errorException") > 0)
                        {
                          //  Exception  ex= MyExecptionAttribute.ExceptionQueue.Dequeue();//从队列中拿出数据
                            string errorMsg = MyExecptionAttribute.redisClent.DequeueItemFromList("errorException");//从Redis队列中取出异常数据
                            //if (ex != null)
                            if(!string.IsNullOrEmpty(errorMsg))
                            {
                                //构建成一个完整的路径
                              //  string fileName = filePath + DateTime.Now.ToString("yyyy-MM-dd").ToString() + ".txt";
                                //string execptionMsg = ex.ToString();
                             //   File.AppendAllText(fileName, errorMsg, Encoding.Default);//将异常写到文件中。
                                ILog logger = LogManager.GetLogger("czbkError");
                                logger.Error(errorMsg);//将异常信息写到Log4Net中.
                            }
                            else
                            {
                                Thread.Sleep(30);
                            }
                        }
                        else
                        {
                            Thread.Sleep(30);//避免了CPU空转。
                        }
                    }
                    catch (Exception ex)
                    {
                        //MyExecptionAttribute.ExceptionQueue.Enqueue(ex);
                        MyExecptionAttribute.redisClent.EnqueueItemOnList("errorException", ex.ToString());
                    }
                }
            }, filePath);
        }
    }
}