C＃How to GroupBy如何将一个项目放入多个组中 [英] C# How to GroupBy so that an item can be put into multiple groups

问题描述

我有一堆文章，我想按类别整理。这对于GroupBy（x => x.Category）来说很简单，除了一些文章有多个类别。如何分组我的文章，以便如果他们有两个类别，它们将被放到两个类别分组中？

例如：

 第1条，类别：苹果，橙子
第2条，类别：苹果
第3条，cateogry：橙子
  
 
 
 
 苹果：第1条第2款
橙色：第1条第3款
  
 
 
解决方案
如果我已正确理解您的问题，则可以执行以下操作： 
 
 
  var groups = 
 products.SelectMany（p => p.Categories，
（p，c）=> new {Product = p，Category = c}）
 .GroupBy （p => p.Category，p => p.Product）; 
  
相关的 SelectMany 过载是 this one 。
 
 
 
等价查询语法更好： 
 
 
  var groups = from p in products 
从c。p.Categories 
 group p by c into g 
 select g; 
  
感谢 Servy 指出了将流利版翻译为查询语法的显而易见的方法（我试图将其转换为字面意思）。
 
I have a bunch of articles and I want to organize them by category. This would be simple with a GroupBy(x => x.Category), except that some articles have multiple categories. How can I group my articles so that if they have two categories, they are put into both category groupings?

example: 
Article 1, category:apples, oranges
Article 2, category:apples
Article 3, cateogry:oranges
I would end up with two categories, apples and oranges:
apples: Article 1, Article 2
orange: Article 1, Article 3

解决方案
If I've understood your problem correctly, you can do the following:
var groups = 
    products.SelectMany(p => p.Categories,
                        (p, c) => new { Product = p, Category = c })
            .GroupBy(p => p.Category, p => p.Product);
The relevant SelectMany overload is this one.

The equivalent query syntax is much nicer:
var groups = from p in products
             from c in p.Categories
             group p by c into g
             select g;
Thanks to Servy for pointing out the obvious way to translate the fluent version to the query syntax one (I was trying to convert it way too literally).

                        
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