C# 递归算法获取下级子级 2种方法（只有我观看）

实体类
public class DepartmentItem
    {
        /// <summary>
        /// 部门Id
        /// </summary>
        public int ID { get; set; }
        /// <summary>
        /// 部门名称
        /// </summary>
        public string DepartmentName { get; set; }
        /// <summary>
        /// 上级部门Id
        /// </summary>
        public int? ParentSysDepartmentId { get; set; }
        /// <summary>
        /// 企业Id
        /// </summary>
        public int SysEnterpriseId { get; set; }
        /// <summary>
        /// 部门GUID
        /// </summary>
        public string DepartmentGUID { get; set; }
　　　　　//下级
        public IEnumerable<DepartmentItem> Children { get; set; }
    }

方法
private List<DepartmentItem> GetChildren(IEnumerable<DepartmentItem> dt, int? ID)
        {
            List<DepartmentItem> nodeList = new List<DepartmentItem>();
            var children = dt.Where(q => q.ParentSysDepartmentId == ID);
            foreach (var dr in children)
            {
                DepartmentItem node = new DepartmentItem();
                node.ID = dr.ID;
                node.DepartmentGUID = dr.DepartmentGUID;
                node.SysEnterpriseId = CurrentUser.PlatformUser.userEnterInfo.EnterpriseId;
                node.DepartmentName = dr.DepartmentName;
                node.ParentSysDepartmentId = dr.ParentSysDepartmentId;
                node.Children = GetChildren(dt, node.ID);
                nodeList.Add(node);
            }
            return nodeList;
        }

直接调用
 var tt = GetChildren(apire.Data,null);

/// <summary>
    /// 行政区域表
    /// </summary>
    public class ProJ_Region :BaseModelInt
    {
        /// <summary>
        /// 上一级id
        /// </summary>
        public int ParentID { get; set; }
        /// <summary>
        /// 工程项目ID
        /// </summary>
        public Guid ProjectID { get; set; }
        /// <summary>
        /// 行政区域名
        /// </summary>
        public string RegionName { get; set; }
        /// <summary>
        /// 级次
        /// </summary>
        public int RLevel { get; set; }
    }

/// <summary>
        /// 获取包含自己的子级
        /// </summary>
        /// <param name="regionid"></param>
        /// <returns></returns>
        public IEnumerable<Model.DB.ProJ_Region> GetChildren(int regionid)
        {
            if (!_region.Contains(q => q.ID == regionid))
                yield break;
            foreach (var x in _region.Get(q => q.ID == regionid))
                yield return x;
            foreach (var x in _region.Get(q => q.ParentID == regionid))
            {
                foreach (var y in GetChildren(x.ID))
                    yield return y;
            }

        }

var re = GetChildren(RegionID);