C# 使用组选择重复项

Posted 2023-03-31

【中文标题】C# 使用组选择重复项

【英文标题】：C# select duplicate using group

【发布时间】：2021-10-27 05:20:36

【问题描述】：

我只想从任何组多次包含相同 ID 的非空白名称中进行选择：

数据设置

var a1 = new  id = 3, Name = "", Group = "GroupA" ;
var a2 = new  id = 2, Name = "", Group = "GroupA" ;
var a3 = new  id = 3, Name = "", Group = "GroupA" ;
var b1 = new  id = 4, Name = "B", Group = "GroupB" ;
var b2 = new  id = 5, Name = "B", Group = "GroupB" ;
var b3 = new  id = 5, Name = "B", Group = "GroupB" ;
List<dynamic> group = new List<dynamic>();
group.Add(a1);
group.Add(a2);
group.Add(a3);
group.Add(b1);
group.Add(b2);
group.Add(b3);

查询：

var query1 = group.ToList()
    .Where(s=>s.Name != "")
    .GroupBy(x =>x.Group)
    .Where(g => g.Count() > 1)
    .SelectMany(y => y)
    .ToList();
Console.WriteLine("output\n" + string.Join("\n", query1));

返回

 id = 4, Name = B, Group = GroupB 
 id = 5, Name = B, Group = GroupB 
 id = 5, Name = B, Group = GroupB 

但我想要的是以下内容：

 id = 5, Name = B, Group = GroupB 
 id = 5, Name = B, Group = GroupB 

我做错了什么？

【问题讨论】：

具有相同id 但不同Group 的项目会发生什么情况？例如。 new id=1,Group="A" 和 new id=1,Group="B".

【参考方案1】：

根据您的需要按id 和/或Name 分组

.GroupBy(x => new x.id, x.Name)

给定

var list = new[]

   new id = 3, Name = "", Group = "GroupA",
   new id = 2, Name = "", Group = "GroupA",
   new id = 3, Name = "", Group = "GroupA",
   new id = 4, Name = "B", Group = "GroupB",
   new id = 5, Name = "B", Group = "GroupB",
   new id = 5, Name = "B", Group = "GroupB"
;

用法

var results = list
   .Where(s => s.Name != "")
   .GroupBy(x => new x.id, x.Name)
// .GroupBy(x => x.id) <- depending on your needs
   .Where(g => g.Count() > 1)
   .SelectMany(y => y);

foreach (var result in results)
   Console.WriteLine($"result.id, result.Name, result.Group");

输出

5, B, GroupB
5, B, GroupB

【讨论】：

谢谢，如果我把名字改成 A B C，我怎样才能得到同样的结果？

【参考方案2】：

您正在将GroupBy 应用于Group 属性。不要按Group 属性分组，而是使用id 属性，

输入

var list = new[]

    new id = 3, Name = "", Group = "GroupA",
    new id = 2, Name = "", Group = "GroupA",
    new id = 3, Name = "", Group = "GroupA",
    new id = 4, Name = "B", Group = "GroupB",
    new id = 5, Name = "B", Group = "GroupB",
    new id = 5, Name = "B", Group = "GroupB"
;

解决方案：

var query1 = list.ToList()
    .Where(s=>s.Name != "")
    .GroupBy(x =>x.id)  
               //^^^^^ This change you need to do  
    .Where(g => g.Count() > 1)
    .SelectMany(y => y)
    .ToList();

输出

5, B, GroupB
5, B, GroupB

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