自己写一个Log类

 public class MyLogger
    {
        public static void Log(string str)
        {
            StreamWriter sw = null;
            try
            {
                DirectoryInfo di = new DirectoryInfo(@"D:\testing");//获取D:\testing 目录的信息
                if (!di.Exists)//如果没有该目录，则直接创建一个
                {
                    di.Create();
                }
                FileInfo fileInfo = new FileInfo(@"D:\testing\test.txt");//获取 D:\testing\test.txt 文本的信息
                if (!fileInfo.Exists)//同样的，如果没有该文件，则直接创建一个，注意使用CreateText创建完同时返回 StreamWriter 流
                {
                    sw = fileInfo.CreateText();
                }
                else
                {
                    sw = fileInfo.AppendText(); //存在该文件，则对该文件流进行写入叠加操作（即不覆盖掉之前写的东西）
                }
                sw.WriteLine(str);
                sw.Flush(); //注意关闭文件流
                sw.Close();
                sw.Dispose();
            }
            catch (Exception ex)
            {
                if (sw != null)
                {
                    sw.Close();
                    sw.Flush();
                    sw = null;
                }

            }
        }
    }

调用

 MyLogger.Log("tesing......");

把整个Exception类传递给一个方法

 /// <summary>
        /// 写错误日志
        /// </summary>
        /// <param name="objErr"></param>
        /// <param name="context"></param>
        public void WriteErrorLog(Exception objErr)
        {
            HttpContext context = HttpContext.Current;
            //Exception objErr = context.Server.GetLastError().GetBaseException();
            string strError = string.Empty;
            strError += "用户IP:" + context.Request.UserHostAddress + "\r\n";
            strError += "发生时间:" + System.DateTime.Now.ToString() + "\r\n";
            strError += "发生异常页: " + context.Request.Url.ToString() + "\r\n";
            strError += "异常信息: " + objErr.Message + "\r\n";
            strError += "异常方法: " + objErr.TargetSite + "\r\n";
            strError += "错误源:" + objErr.Source + "\r\n";
            strError += "错误堆栈信息:" + objErr.StackTrace + "\r\n";
            strError += "----------------------------------------------------------------------------------------------\r\n";

            //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(),
                           month = DateTime.Now.Month.ToString();
                    string filename = DateTime.Now.Day.ToString() + ".txt";
                    string path = context.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);

                    writer = new System.IO.StreamWriter(file.FullName, true);//文件不存在就创建,true表示追加
                    writer.Write(strError);
                }
            }
            catch
            {
                context.Response.Redirect("~/Error.html");
            }
            finally
            {
                if (writer != null)
                { writer.Flush(); writer.Close(); }
            }
        }

建议：

　　如非必要，Logger不要自己写，用Nlog或者log4net等第三方成熟控件。