public class DepartmentDto
 {
  public int Id { get; set; }   

  public int? ParentId { get; set; }

  public string Name { get; set; }

  public string TelPhone { get; set; }
 
  public string Address { get; set; }

  public string Remark { get; set; }
   
  public int EmployeeNumber { get; set; }
 }

  public List<DepartmentDto> InitDepartmentData()
  {
   List<DepartmentDto> lst = new List<DepartmentDto>();
   lst.AddRange(new DepartmentDto[] {
    new DepartmentDto() {
      Address ="一馬路XX號",
      Id=1,
      Name="一級一號科室",
      Remark="",
      TelPhone="0731-6111111",
      EmployeeNumber=3,
    },
    new DepartmentDto() {
      Address ="二馬路XX號",
      Id=2,
      Name="一級二號科室",
      Remark="",
      TelPhone="0731-6111111",
      EmployeeNumber=4,
    },
    new DepartmentDto() {
      Address ="三馬路XX號",
      Id=3,
      Name="一級三號科室",
      Remark="",
      TelPhone="0731-6222222",
      EmployeeNumber=6,
    },
    new DepartmentDto() {
      Address ="一馬路XX號",
      ParentId=1,
      Id=4,
      Name="二級一號科室",
      Remark="",
      TelPhone="0731-6222222",
      EmployeeNumber=7,
    },
    new DepartmentDto() {
      Address ="二馬路XX號",
      ParentId=2,
      Id=5,
      Name="二級二號科室",
      Remark="",
      TelPhone="0731-6222222",
      EmployeeNumber=5,
    },
   });
   return lst;
  }

 List<DepartmentDto> lstDepartItems = InitDepartmentData();
   //1、獲取未存在父級科室的科室集合 1、2、3   
   List<DepartmentDto> notExistsParentDepartmentIdLst = lstDepartItems
    .Where(p => !p.ParentId.HasValue)
    .ToList();

 List<DepartmentDto> notExistsParentDepartmentIdLst_1 = new List<DepartmentDto>();
 foreach (DepartmentDto department in lstDepartItems)
 {
   if (!department.ParentId.HasValue)
    notExistsParentDepartmentIdLst_1.Add(department);
 }

 //2、獲取存在子科室的科室集合 1、2
   List<DepartmentDto> existsParentDepartmentIdLst1 = lstDepartItems
    .Where(p => lstDepartItems.Select(k => k.ParentId).Contains(p.Id))
    .ToList();

 List<DepartmentDto> existsParentDepartmentIdLst1_1 = new List<DepartmentDto>();
 foreach (DepartmentDto parentDepart in lstDepartItems)
 {
  foreach (DepartmentDto childDepart in lstDepartItems) 
  {
   if (parentDepart.Id == childDepart.ParentId)
   {
     existsParentDepartmentIdLst1_1.Add(parentDepart);
     continue;
   }
  }
 }

 var groupDto = lstDepartItems
    .GroupBy(p => p.Address)
    .Select(p => new
    {
     Address = p.Key,
     SumEmployeeCount = p.Sum(p => p.EmployeeNumber),
     AvgEmployeeCount = p.Average(p => p.EmployeeNumber),
    }).ToList();

 public class DepartmentEqualityComparer : IEqualityComparer<DepartmentDto>
 {
  public bool Equals([AllowNull] DepartmentDto x, [AllowNull] DepartmentDto y)
  {
   // 這里可以寫比較的關鍵代碼~
   return x.Id == y.Id;
  }

  public int GetHashCode([DisallowNull] DepartmentDto obj)
  {
   return obj.ToString().GetHashCode();
  }
 }

 List<DepartmentDto> lstDepartItemsCPs = InitDepartmentData();
   lstDepartItemsCPs.Add(new DepartmentDto()
   {
    Address = "三馬路XX號",
    Id = 6,
    Name = "二級三號科室",
    Remark = "",
    TelPhone = "0731-6222222",
    EmployeeNumber = 7
   });

// 這里如果DepartmentDto為引用類型(class)則需要使用比較器DepartmentEqualityComparer才能返回我們的預期值(根據(jù)ID值判斷是否相等)
   List<DepartmentDto> diffList = lstDepartItemsCPs.Except(lstDepartItems, new DepartmentEqualityComparer()).ToList();
   // 獲取相等元素
   List<DepartmentDto> diffList1 = lstDepartItemsCPs.Intersect(lstDepartItems.Select(p => p), new DepartmentEqualityComparer()).ToList();
  // 需要添加IEqualityComparer，因為集合內的內容為引用類型！所以兩個集合的“值”是不同的，引用類型的值在這里還包含了指向的內存堆的引用地址
   bool isEqual = lstDepartItems.SequenceEqual(InitDepartmentData(), new DepartmentEqualityComparer());

 // 這里如果DepartmentDto為值類型(struct)則不需要使用比較器DepartmentEqualityComparer即可返回我們的預期值(根據(jù)ID值判斷是否相等)
   List<DepartmentDto> diffList3 = lstDepartItemsCPs.Except(lstDepartItems).ToList();
   // 獲取相等元素
   List<DepartmentDto> diffList4 = lstDepartItemsCPs.Intersect(lstDepartItems.Select(p => p)).ToList();
     // 如果把DepartmentDto的類型改為值類型則可以不需要IEqualityComparer進行判斷的結果也會為true
   isEqual = lstDepartItems.SequenceEqual(InitDepartmentData());

  public void ConvertListTest()
  {
   try
   {
    object[] ss = { 1, "2", 3, "四", "五", "7" };
    // 1、3 OfType的本質是循環(huán)集合，對每個集合項進行類型驗證(不是強轉,所以此處的結果是1、3 而不是1、2、3、7)
    var lst = ss.ToList().OfType<int>().ToList();
    // 3 
    int max = ss.OfType<int>().Max();
    // 這句代碼會提示“System.InvalidCastException:“Unable to cast object of type 'System.String' to type 'System.Int32'.”異常，原因:Cast的執(zhí)行效率會略高與OfType,因為在對集合項進行類型轉換前不會對其進行類型校驗,當你確保集合的類型是安全的則可以用Cast,但是能用到Cast和OfType的時候基本上都是用OfType了..
    int maxCast = ss.Cast<int>().Max();
   }
   catch (Exception ex)
   {
    Console.WriteLine(ex);
   }
  }