OrderByDescending 通过 Lambda 表达式的项目计数

Question

我有 MyClass 类型：

 MyClass
{
 public string Name { get;set; }
 public string LastName { get;set; }
}

我也得到了我的collection：

List<MyClass> _Lst = new List<MyClass>();

我在其中添加了一些这样的项目：
更新：

_Lst.Add(new MyClass{Name = "Petya" , LastName = "sergeevi4"};
_Lst.Add(new MyClass{Name = "Vasya" , LastName = "petrovi4"};
_Lst.Add(new MyClass{Name = "Igori" , LastName = "alibertovi4"};
_Lst.Add(new MyClass{Name = "Vasya" , LastName = "iakubovi4"};
_Lst.Add(new MyClass{Name = "Vasya" , LastName = "ianukovi4"};

那么我如何通过 Lambda 表达式对我的 collection 进行排序（降序）以获得类似这样的结果：

_Lst[0] = Name = "Vasya" , LastName = "iakubovi4"; //Where [n] - just index of mylist;
_Lst[1] = Name = "Vasya" , LastName = "petrovi4";
_Lst[2] = Name = "Vasya" , LastName = "ianukovi4";
_Lst[3] = Name = "Petya" , LastName = "sergeevi4";
_Lst[4] = Name = "Igori" , LastName = "alibertovi4";

目前我正在做这样的事情：

var NewList = _Lst.OrderByDescending(x => x.Name.Count());

但是，不能正常工作。

Answer 1

只需将您的代码更改为，

var NewList = _Lst.OrderByDescending(x => x.Name);

Answer 2

GroupBy 方法完全符合您的要求。组是带有键（在本例中为名称）的 collection。您可以订购分组，然后使用 SelectMany:

展平您的 collection

var newList = _Lst.GroupBy(m => m.Name)
    .OrderByDescending(g => g.Count())
    .SelectMany(g => g)
    .ToList();

一条建议：使用 ToArray() 比 ToList() 更好，因为结果 collection 是不可变的，事后无法修改，导致棘手的错误在你的代码的其他地方。

奖金：

按名字匹配排序后，您可能想按姓氏字母顺序排序。同样，这很容易，因为每个组都是 collection。您可以在链中添加 Select 以实现此目的：

var newList = _Lst.GroupBy(m => m.Name)
    .OrderByDescending(g => g.Count())
    .Select(g => g.OrderBy(m => m.LastName))
    .SelectMany(g => g)
    .ToList();

Answer 3

 List<MyClass> _Lst = new List<MyClass>();
 _Lst.Add(new MyClass { Name = "Vasya", LastName = "iakubovi4" });
 _Lst.Add(new MyClass { Name = "Vasya", LastName = "petrovi4" });
 _Lst.Add(new MyClass { Name = "Vasya", LastName = "ianukovi4" });
 _Lst.Add(new MyClass { Name = "Petya", LastName = "sergeevi4" });
 _Lst.Add(new MyClass { Name = "Igori", LastName = "alibertovi4" });

 var result = _Lst.GroupBy(m => m.Name).OrderByDescending(x => x.Count()).SelectMany(x => x).ToList();

OrderByDescending 通过 Lambda 表达式的项目计数

OrderByDescending by count of items via Lambda expressions

c#

linq

lambda

sql-order-by