c# - 如何使用 Linq 获取不止一次遇到键的字典？

Question

假设一个对象，例如：

class A {
  string Country;
  string Region;
}

IEnumerable<A>比如：

    "Denmark", "Arhus"
    "Denmark", "Bornholm"
    "Denmark", "Frederiksborg"
    "Denmark", "Fyn"
    "Denmark", "Copenhagen"
    "Denmark", "Nordjylland"
    "France", "Alsace"
    "France", "Aquitaine"
    "France", "Auvergne"
    "France", "Basse-Normandie"
    "France", "Bourgogne"
    "France", "Bretagne"
    "France", "Centre"
    "France", "Champagne-Ardenne"
    "France", "Corse"
    etc...

我如何获得一个IDictionary<string, IEnumerable<string>> reswith Linq whereres['Country']将返回一个IEnumerable<string>相应区域？

Answer 1

ToLookup应该给你你想要的。

var lookup = data.ToLookup(a => a.Country, a=> a.Region);

从技术上讲，它是不可变的，因此不实现IDictionary. 如果实现该接口很重要，那么您需要将查找转换为字典：

var dictionary = lookup.ToDictionary(group => group.Key, 
    group => group.AsEnumerable());

Answer 2

IDictionary<string, IEnumerable<string>> dictionary =
    list.GroupBy(r => r.Country)
    .ToDictionary(grp => grp.Key, grp => grp.Select(t => t.Region));