LINQ通过循环另一个列表来过滤列表 [英] LINQ Filtering a list by looping on another list

问题描述

大家好，



我有 IEnumerable< Principle> 说用户其中包含100个项目。我有 IEnumerable> DataRow> 说 groupUsers ，其中包含10个项目。



原则包含身份证，姓名，姓氏，电子邮件地址

Datarow包含公司，姓名，姓氏，电子邮件等字段/>


我想循环用户和users.mailaddress上的每个项目等于groupUsers.Email我想添加用户的项目为IEnumerable< principle>。有人可以帮我写LINQ声明吗。





到目前为止我得到了什么;

Hello all,

I have a IEnumerable<Principle> say users which contains 100 items. And I have IEnumerable>DataRow> say groupUsers which contains 10 items.

"Principle" contains fields of id, name, surname, emailaddress
"Datarow" contains fields of company, name, surname, email

I want to loop on users and for each item on users.mailaddress equals to groupUsers.Email I want to add the users's item to a IEnumerable<principle>. Can someone help me write the LINQ statement.


What I have came up so far;

IEnumerable<Principle> result = users.ForEach(x => x.Emailaddress.Select(//another loop here for matching emails));

提前谢谢。

Thanks in advance.

推荐答案

首先从内部测试中查看它：

A 原则应该包含在如果 Emailaddress 与groupUsers中的电子邮件值的任何匹配，则输出：

Look at it from the inner test first:
A Principle should be included in the output if the Emailaddress matches any of the Email values in groupUsers:

groupUsers.Any(gu => gu.Email == u.Emailaddress)

现在为每个用户元素执行此操作并收集结果：

Now do that for each element of users and collect the result:

IEnumerable<Principle> result = users.Where(u => groupUsers.Any(gu => gu.Email == u.Emailaddress));

这当然是非常低效的。时间与两个列表的长度的产品成比例增长。因此，让内部测试更快：

This, of course, is very inefficient. Time grows proportional to the product of the lengths of the two lists. So make the inner test faster:

HashSet<string> groupEmails = new HashSet<string>(groupUsers.Select(gu => gu.Email));
IEnumerable<Principle> result = users.Where(u => groupEmails.Contains(u.Emailaddress));

现在需要时间与两个列表的长度的总和成比例。

创建和填充 groupEmails 是一个单次传递 groupUsers 集合。

查找结果是单次传递用户集合。

优点是 groupEmails.Contains（...）是（近似）恒定时间，与元素数量无关在 groupEmails HashSet 。



*从字面上看，你可能意味着校长不是原则......查找差异：委托人与原则 [ ^ ]

This now takes time proportional to the sum of the lengths of the two lists.
Creating and populating the groupEmails is a single pass over the groupUsers collection.
Finding the result is a single pass over the users collection.
The advantage is that the groupEmails.Contains(...) is (approximately) constant time, independent of the number of elements in the groupEmails HashSet.

* Gramatically, you probably mean "Principal" not "Principle"... lookup the difference: Principal vs Principle[^]

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