asp.net mvc3中的会话项始终为null

上传图像的功能：

        [HttpPost]
        public String UploadSingle(HttpPostedFileBase imageFile)
    {
        try
        {
            String fileSavePath = Server.MapPath("~/Content/Temp/" +
               imageFile.FileName);
            imageFile.SaveAs(fileSavePath);
            List<String> uploadedFile = (List<String>)HttpContext.Session["UploadedFiles"];
            if (uploadedFile == null)
            {
                uploadedFile = new List<string>();
            }
            uploadedFile.Add(fileSavePath);
            HttpContext.Session["UploadedFiles"] = uploadedFile;
            return "Success";
        }
        catch (Exception e)
        {
            return "Error";
        }
    }

此功能正常工作，图像已上传。但是当我尝试查看上传的图片时。

 public String UploadResult()
    {
        List<String> uploadedFile = (List<String>)HttpContext.Session["UploadedFiles"];
        return uploadedFile.Count + "";
    }

调试时，HttpContext.Session["UploadedFiles"]总是返回null。请给我一些帮助。