//定义 set 列表 
private HashSet<string> ids = new HashSet<string>();

/// <summary>
        /// 递归获取区域id
        /// </summary>
        /// <param name="adId">用户的属地</param>
        public void getAdId(string adId)
        {
            ids.Add(adId);
            var ad2 = _administrativeDivisionRepository.GetAll().Where(x => x.ParentId == adId).ToList();
            foreach (AdministrativeDivision a in ad2)
            {
                ids.Add(a.Id);
                getAdId(a.Id);
            }
        }

public void SeachAdministrativeDivision() {
            var ad = _administrativeDivisionRepository
                .GetAll()
                .Where(x => ids.Contains(x.Id));//这里就是 In 查询
        }

 　　　　　　string[] array = ids.ToArray();
            
            string str = "(";
            for (int i = 0; i < ids.Count; i++)
            {
                if (i == array.Length - 1)
                {
                    str += "'" + array[i] + "')";
                }
                else
                {
                    str += "'" + array[i] + "',";
                }
            }

　　　　　　var queryAdministrativeDivision = _administrativeDivisionRepository
                .GetAll();

            var query = from ad in queryAdministrativeDivision
                        where str.Contains(ad.Id) //这里是 in 查询
                        select ad;