public void RemoveRecord(ImageRecord _ir)
 {
     if (_ir == null)
     {
         return;
     }
     this.images_on_record.Remove(_ir);
     if (_ir.ID == this.nr_images)
     {
         // recalculate
         this.nr_images = 0;
         foreach (ImageRecord i in this.images_on_record)
         {
             this.nr_images = Math.Max(this.nr_images, i.ID);
         }
     }
 }
        public void AddRecord(string _file)
        {
            ImageRecord ir = new ImageRecord((++this.nr_images), _file);

            this.images_on_record.Add(ir);
        }
 static ImageRecord()
 {
     EMPTY = new ImageRecord();
 }