Why HttpContext.Current always null in user defined class in asp.net mvc ?

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ASP.NET

MVC

I've created a class named "MyLongRunningClass" that contain a method bellow:

public string ProcessLongRunningAction(IEnumerable<HttpPostedFileBase> files ,string id)
        {
            int currentIndex = 0;
            foreach (HttpPostedFileBase file in files)
            {
                currentIndex++;
                lock(syncRoot)
                {                
                 string path=HttpContext.Current.Server.MapPath("~//Content//images       //"+file.FileName);//Excecption is created here........
                  file.SaveAs(path);
                }
              Thread.Sleep(300);
            }
            return id;
        }

From the controller this method is called with list of file to save in images directory. whenever HttpContext.Current.Server.MapPath("~//Content//images//"+file.FileName) is executed NullReferenceException thrown, and HttpContext.Current is always null. Same thing happen when I use session. I don't know whats wrong with me. I need help Immediately. please tell me what should i do, sample code is more appreciable.
THANKS.......IN ADVANCE