C# Linq使用的一些小技巧

var items =deptItems.Select(p=>new {
           Code =p.Id.ToString(),
           Name = P.Name,
    });   
  这里才会真正执行上面的Select表达式
 var b = items.ToList();
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private static void Test()
 {
      try
      {
          List<int> items = new List<int>();
          var a = items?.FirstOrDefault();
          var b = items?.First();
      }
      catch (Exception ex)
      {
         System.Console.WriteLine(ex.Message);
      }
 }
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 public class DepartmentRequest
    {
        public DepartmentRequest()
        {
             默认就给Itmes赋予一个默认值(没有任何元素的空集合),避免后续代码中直接对该集合操作时候出现空引用异常
            Items = new List<DepartTagDto>();
        }       
        public List<DepartTagDto> Items { get; set; }
    }


    public class DepartTagDto
    {
        public int Id { get; set; }
        public string Title { get; set; }
    }
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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; } 
}
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 public List<DepartmentDto> InitDepartmentData()
 {
      List<DepartmentDto> lst = new List<DepartmentDto>();
      lst.AddRange(new DepartmentDto[] {
                new DepartmentDto() {
                     Address ="一马路XX号",
                     Id=1,
                     Name="一级一号科室",
                     Remark="",
                     TelPhone="0731-6111111",
                     EmployeeNumber=3,
                },
                new DepartmentDto() {
                     Address ="二马路XX号",
                     Id=2,
                     Name="一级二号科室",
                     Remark="",
                     TelPhone="0731-6111111",
                     EmployeeNumber=4,
                },
                new DepartmentDto() {
                     Address ="三马路XX号",
                     Id=3,
                     Name="一级三号科室",
                     Remark="",
                     TelPhone="0731-6222222",
                     EmployeeNumber=6,
                },
                new DepartmentDto() {
                     Address ="一马路XX号",
                     ParentId=1,
                     Id=4,
                     Name="二级一号科室",
                     Remark="",
                     TelPhone="0731-6222222",
                     EmployeeNumber=7,
                },
                new DepartmentDto() {
                     Address ="二马路XX号",
                     ParentId=2,
                     Id=5,
                     Name="二级二号科室",
                     Remark="",
                     TelPhone="0731-6222222",
                     EmployeeNumber=5,
                },
            });
            return lst;
 }
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  List<DepartmentDto> lstDepartItems = InitDepartmentData();
   1、获取未存在父级科室的科室集合 1、2、3            
  List<DepartmentDto> notExistsParentDepartmentIdLst = lstDepartItems
                .Where(p => !p.ParentId.HasValue)
                .ToList();
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List<DepartmentDto> notExistsParentDepartmentIdLst_1 = new List<DepartmentDto>();
foreach (DepartmentDto department in lstDepartItems)
{
    if (!department.ParentId.HasValue)
       notExistsParentDepartmentIdLst_1.Add(department);
}
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 2、获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();
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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;
        }
    }
}
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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();
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public class DepartmentEqualityComparer : IEqualityComparer<DepartmentDto>
 {
        public bool Equals([AllowNull] DepartmentDto x, [AllowNull] DepartmentDto y)
{
             这里可以写比较的关键代码~
            return x.Id == y.Id;
        }


        public int GetHashCode([DisallowNull] DepartmentDto obj)
{
            return obj.ToString().GetHashCode();
        }
}
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 List<DepartmentDto> lstDepartItemsCPs = InitDepartmentData();
 lstDepartItemsCPs.Add(new DepartmentDto()
  {
     Address = "三马路XX号",
     Id = 6,
     Name = "二级三号科室",
     Remark = "",
     TelPhone = "0731-6222222",
     EmployeeNumber = 7
  });
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  这里如果DepartmentDto为引用类型(class)则需要使用比较器DepartmentEqualityComparer才能返回我们的预期值(根据ID值判断是否相等)
List<DepartmentDto> diffList = lstDepartItemsCPs
        .Except(lstDepartItems, new DepartmentEqualityComparer())
        .ToList();
// 获取相等元素
List<DepartmentDto> diffList1 = lstDepartItemsCPs
        .Intersect(lstDepartItems.Select(p => p), new DepartmentEqualityComparer())
        .ToList();
// 需要添加IEqualityComparer，因为集合内的内容为引用类型！所以两个集合的“值”是不同的，引用类型的值在这里还包含了指向的内存堆的引用地址
bool isEqual = lstDepartItems
        .SequenceEqual(InitDepartmentData(), new DepartmentEqualityComparer());
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 // 这里如果DepartmentDto为值类型(struct)则不需要使用比较器DepartmentEqualityComparer即可返回我们的预期值(根据ID值判断是否相等)
 List<DepartmentDto> diffList3 = lstDepartItemsCPs
       .Except(lstDepartItems)
       .ToList();
// 获取相等元素
List<DepartmentDto> diffList4 = lstDepartItemsCPs
      .Intersect(lstDepartItems.Select(p => p))
      .ToList();
 // 如果把DepartmentDto的类型改为值类型则可以不需要IEqualityComparer进行判断的结果也会为true
 isEqual = lstDepartItems.SequenceEqual(InitDepartmentData());
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  public void ConvertListTest()
  {
     try
     {
         object[] ss = { 1, "2", 3, "四", "五", "7" };
         // 1、3 OfType的本质是循环集合，对每个集合项进行类型验证(不是强转,所以此处的结果是1、3 而不是1、2、3、7)
         var lst = ss.ToList().OfType<int>().ToList();
         // 3  
         int max = ss.OfType<int>().Max();
          // 这句代码会提示“System.InvalidCastException:“Unable to cast object of type 'System.String' to type 'System.Int32'.”异常，原因:Cast的执行效率会略高与OfType,因为在对集合项进行类型转换前不会对其进行类型校验,当你确保集合的类型是安全的则可以用Cast,但是能用到Cast和OfType的时候基本上都是用OfType了..
          int maxCast = ss.Cast<int>().Max();
      }
      catch (Exception ex)
      {
         Console.WriteLine(ex);
      }
 }
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