C# 嵌套的 linq 查询，如何获取不同的值？
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nested linq queries, how to get distinct values?
提问by Alexander Taran
table data of 2 columns "category" and "subcategory"
2列“类别”和“子类别”的表格数据
i want to get a collection of "category", [subcategories] using code below i get duplicates. Puting .Distinct() after outer "from" does not help much. What do i miss?
我想使用下面的代码获取“类别”的集合，[子类别] 我得到重复。将 .Distinct() 放在外部“from”之后并没有多大帮助。我想念什么？
var rootcategories = (from p in sr.products
orderby p.category
select new
{
category = p.category,
subcategories = (
from p2 in sr.products
where p2.category == p.category
select p2.subcategory).Distinct()
}).Distinct();
sr.products looks like this
sr.products 看起来像这样
category subcategory
----------------------
cat1 subcat1
cat1 subcat2
cat2 subcat3
cat2 subcat3
what i get in results is
我得到的结果是
cat1, [subcat1,subcat2]
cat1, [subcat1,subcat2]
but i only want one entry
但我只想要一个条目
solved my problem with this code:
用这段代码解决了我的问题：
var rootcategories2 = (from p in sr.products
group p.subcategory by p.category into subcats
select subcats);
now maybe it is time to think of what was the right question.. (-:
现在也许是时候考虑什么是正确的问题了.. (-:
采纳答案by Alexander Taran
solved with this code
用这段代码解决了
var rootcategories2 = (from p in sr.products
group p.subcategory by p.category into subcats
select subcats);
thanks everyone
谢谢大家
回答by chakrit
I think you need 2 "Distinct()" calls, one for the main categories and another for the subcategories.
我认为您需要 2 个“Distinct()”调用，一个用于主类别，另一个用于子类别。
This should work for you:
这应该适合你：
var mainCategories = (from p in products select p.category).Distinct();
var rootCategories =
from c in mainCategories
select new {
category = c,
subcategories = (from p in products
where p.category == c
select p.subcategory).Distinct()
};
回答by GeekyMonkey
Your main query is on Products, so you're going to get records for each product. Switch it around so you're querying on Category, but filtering on Product.Category
您的主要查询是关于产品，因此您将获得每个产品的记录。切换它，以便您在 Category 上查询，但在 Product.Category 上进行过滤
回答by user35959
The algorithm behind Distinct() needs a way to tell if 2 objects in the source IEnumerable are equal. The default method for that is to compare 2 objects by their reference and therefore its likely that no 2 objects are "equal" since you are creating them with the "new" keyword.
Distinct() 背后的算法需要一种方法来判断源 IEnumerable 中的 2 个对象是否相等。默认方法是通过引用比较 2 个对象，因此很可能没有 2 个对象是“相等的”，因为您使用“new”关键字创建它们。
What you have to do is to write a custom class which implements IEnumerable and pass that to the Distinct() call.
您需要做的是编写一个实现 IEnumerable 的自定义类并将其传递给 Distinct() 调用。
