Question

我有一个学校数据库，每个学校都在一个城市。现在我想创建一个包含每个schoool的所有城市的字典。为此，我尝试了这种方法：

var schoolCities = schoolsWithAddresses.Where(school => school.Address.City != null).ToDictionary(sc => sc.Address.City.Name.ToLower());

现在，问题在于一个城市可以拥有多所学校。所以，当我创建我的字典时，我最终会遇到异常＆＃34;重复密钥＆＃34;。

我想创建一个dicitonary，因为它可以让我快速查找有学校的城市（这就是为什么我没有使用List，例如）。

如何以某种方式克服这个问题，让我仍能进行有效的查找？

Answer 1

您应该先对项目进行分组，以便拥有独特的城市。

schoolsWithAddresses.Where(school => school.Address.City != null)
.GroupBy(s => s.Address.City, (k, v) => new { City = k, Schools = v })
.ToDictionary(d => d.City, e => e.Schools)
;

Answer 2

使用ToLookUp扩展方法而不是

var schoolCities = schoolsWithAddresses
.Where(school => school.Address.City != null)
.ToLookup(sc => sc.Address.City.Name.ToLower());

Answer 3

我认为你想要的是Lookup：

表示每个映射到一个或多个值的键集合。

使用示例：

Lookup<string, School> schoolCities = schoolsWithAddresses
    .Where(school => school.Address.City != null)
    .ToLookup(school => school.Address.City);

IEnumerable<School> schoolsInLondon = schoolCities["London"];

Answer 4

这样的事情：

Dictionary<string, List<School>> schoolCities = schoolsWithAddresses
            .Where(school => school.Address.City != null)
            .GroupBy(school => school.Address.City)
            .ToDictionary(group => group.Key, group => group.ToList());

Answer 5

试试这个

var schoolCities = schoolsWithAddresses
.GroupBy(x => x.city!=null)
.ToDictionary(x => x.Key, x => x.ToList());

使用重复键从DB查询创建C＃字典

5 个答案: