public void CreateMainCategory(MainCategory m_cat)
        {
            if (m_cat == null)
            {
                throw new ArgumentNullException();
            }

            DbMainCategory new_m_cat = new DbMainCategory
            {
                Name = m_cat.Name
            };

            Db.MainCategorys.Create(new_m_cat);
        }
        public void EditMainCategory(MainCategory entity)
        {
            if (entity == null)
            {
                throw new ArgumentNullException();
            }

            DbMainCategory tmp = new DbMainCategory
            {
                Name = entity.Name
            };

            Db.MainCategorys.Update(tmp);
            Db.Save();
        }
        public void RemoveMainCategory(int id)
        {
            DbMainCategory m_cat = Db.MainCategorys.GetItem(id);
            if (m_cat == null)
            {
                throw new ArgumentNullException();
            }

            Db.SubCategorys.Delete(id);
            foreach (DbSubCategory s_cat in m_cat.SubCategorys)
            {
                s_cat.Category = null;
                s_cat.CategoryID = -1;
            }
            Db.Save();
        }