如何使用Linq獲得字典，在字典中多次遇到鍵？

Question

假設一個對象如：

class A {
  string Country;
  string Region;
}

和IEnumerable<A>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...

如何使用Linq獲取IDictionary<string, IEnumerable<string>> res ，其中res['Country']將返回對應區域的IEnumerable<string> ？

Answer 1

ToLookup應該給您您想要的。

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

從技術上講，它是不可變的，因此不會實現IDictionary 。 如果必須實現該接口，則需要將查找轉換為字典：

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));