是否存在“不等于"?在 linq 连接中 [英] Is there a "not equal" in a linq join

问题描述

我正在尝试完成下面的 LINQ 查询，但我需要一个不等于"而不是等于，以便过滤员工拥有 groupA 减去 groupB 的所有员工.

I am trying accomplish the LINQ query below but I need a "not equal" instead of equal, so that filteredEmployees has all employees from groupA minus groupB.

List<Employee> groupA = getEmployeeA();
List<Employee> groupB = getEmployeeB();        

var filteredEmployees = from a in groupA 
                        join b in groupB on a.Name equals b.Name
                        select a;

推荐答案

你不需要加入:

var filteredEmployees = groupA.Except(groupB);

请注意，这将是一系列唯一员工 - 因此，如果 groupA 中有任何重复项，则它们只会在 filteredEmployees 中出现一次.当然，它还假设您有一个合理的相等比较器¹.如果您需要专门针对名称，可以使用 ExceptBy 来自 MoreLINQ:

Note that this will be a sequence of unique employees - so if there are any duplicates in groupA, they will only appear once in filteredEmployees. Of course, it also assumes you've got a reasonable equality comparer¹. If you need to go specifically on name, you can use ExceptBy from MoreLINQ:

var filteredEmployees = groupA.ExceptBy(groupB, employee => employee.Name);

或者不进入第三方库:

var groupBNames = new HashSet<string>(groupB.Select(x => x.Name));
var filteredEmployees = groupA.Where(x => !groupBNames.Contains(x.Name));

<小时>

¹ 正如评论中所指出的，您可以将 IEqualityComparer 作为参数传递给 Except.我在 MiscUtil 中有一个 ProjectionEqualityComparer 类，这使得构建一个您需要的那种比较器:

---

¹ As pointed out in the comments, you can pass in an IEqualityComparer<T> as an argument to Except. I have a ProjectionEqualityComparer class in MiscUtil which makes it easy to build a comparer of the kind you need:

// I can't remember the exact method name, but it's like this :)
var comparer = ProjectionEqualityComparer<Employee>.Create(x => x.Name);
var filteredEmployees = groupA.Except(groupB, comparer);

这篇关于是否存在“不等于"?在 linq 连接中的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！