fileupload control

Question

hi
by using this code
static string impath = string.Empty;

 PhotoUpload.SaveAs(Server.MapPath("~/Logos/" + PhotoUpload.FileName));

            impath = "~/Logos/" + PhotoUpload.FileName;

            ImagePreview.ImageUrl = impath;

            

            btnPhotoPreview.Visible = false;
the filename should be empty why?
 please help  me