GlobalFilters.Filters.Add(new ExceptionFilter());



  public class ExceptionFilter:IExceptionFilter
    {


        public void OnException(ExceptionContext filterContext)
        {
            Exception ex = filterContext.Exception;
           string path= filterContext.HttpContext.Server.MapPath("/");
            File.AppendAllText(path,ex.ToString(),System.Text.Encoding.Default);
        }
    }