public T AddEntity(T entity)
        {
            db.AddObject(entityName, entity);

            //EF4.0的写法   添加实体
            //db.CreateObjectSet<T>().AddObject(entity);

            //EF5.0的写法
           //db.Entry<T>(entity).State = EntityState.Added;


            db.SaveChanges();
            return entity;
        }

        /// <summary>
        /// 实现对数据库的修改功能
        /// </summary>
        /// <param name="entity"></param>
        /// <returns></returns>
        public bool UpdateEntity(T entity)
        {
           
            //EF4.0的写法
            // db.CreateObjectSet<T>().Attach(entity);
            //db.ObjectStateManager.ChangeObjectState(entity, EntityState.Modified);

            //EF5.0的写法
            //db.Set<T>().Attach(entity);
            //db.Entry<T>(entity).State = EntityState.Modified;
                    
            return db.SaveChanges() > 0;
        }
        
        /// <summary>
        /// 实现对数据库的删除功能
        /// </summary>
        /// <param name="entity"></param>
        /// <returns></returns>
        public bool DeleteEntity(T entity)
        {
            db.DeleteObject(entity);

            //EF4.0的写法
            //db.CreateObjectSet<T>().Attach(entity);
            //db.ObjectStateManager.ChangeObjectState(entity, EntityState.Deleted);

            //EF5.0的写法
            //db.Set<T>().Attach(entity);
            //db.Entry<T>(entity).State = EntityState.Deleted;

            return db.SaveChanges() > 0;
        }