protected override void OnException(ExceptionContext filterContext)
{
    Log(filterContext);
    base.OnException(filterContext);
}

private void Log(ExceptionContext filterContext)
{
    string fileName = System.IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, exceptionFileName);
    StreamWriter sw = System.IO.File.AppendText(fileName);

    System.DateTime dt = DateTime.Now;
    string ip = RY.Common.IPHelp.ClientIP;
    string url = Request.Url.ToString();
    sw.Write(string.Format("Time:{0:G}; ClientIp:{1}; URL:{2}; Message:{3}\n", dt, ip, url, filterContext.Exception.Message));
    sw.Close();
}

public class ExceptionLogAttribute:HandleErrorAttribute
{
       public override void OnException(ExceptionContext filterContext)
       {
            base.OnException(filterContext);
       }
}