c# - LINQ：分组子组

Question

如何对子组进行分组以创建大陆列表，其中每个大陆都有自己的县，每个国家都有自己的城市，如下表所示

这是t-sql：

select Continent.ContinentName, Country.CountryName, City.CityName 
from  Continent
left join Country
on Continent.ContinentId = Country.ContinentId

left join City
on Country.CountryId = City.CountryId

以及 t-sql 的结果：

我试过这个，但它以错误的方式对数据进行分组，我需要像上表一样进行分组

  var Result = MyRepository.GetList<GetAllCountriesAndCities>("EXEC sp_GetAllCountriesAndCities");

    List<Continent> List = new List<Continent>();


    var GroupedCountries = (from con in Result
                             group new
                             {


                                 con.CityName,

                             }

                             by new
                             {

                                 con.ContinentName,
                                 con.CountryName
                             }

            ).ToList();

    List<Continent> List = GroupedCountries.Select(c => new Continent()
    {

        ContinentName = c.Key.ContinentName,
        Countries = c.Select(w => new Country()
        {
            CountryName = c.Key.CountryName,

            Cities = c.Select(ww => new City()
            {
                CityName = ww.CityName
            }
            ).ToList()

        }).ToList()


    }).ToList();

score 11 · Accepted Answer

您需要按大陆、国家和城市对所有内容进行分组：

List<Continent> List = MyRepository.GetList<GetAllCountriesAndCities>("EXEC sp_GetAllCountriesAndCities")
    .GroupBy(x => x.ContinentName)
    .Select(g => new Continent 
    {
        ContinentName = g.Key,
        Countries = g.GroupBy(x => x.CountryName)
                     .Select(cg => new Country 
                     {
                         CountryName = cg.Key,
                         Cities = cg.GroupBy(x => x.CityName)
                                    .Select(cityG => new City { CityName = cityG.Key })
                                    .ToList()
                     })
                     .ToList()
    })
    .ToList();

score 2 · Accepted Answer

我知道这是旧的，但我想提一个更简单的方式，每个微软都更易读。这是一个只有 2 个级别的示例，但它很可能适用于访问此页面的其他人（比如我）

 var queryNestedGroups =
        from con in continents
        group con by con.ContinentName into newGroup1
        from newGroup2 in
            (from con in newGroup1
             group con by con.CountryName)
        group newGroup2 by newGroup1.Key;

相关文档位于 https://docs.microsoft.com/en-us/dotnet/csharp/linq/create-a-nested-group 并以 Student 对象为例

我想提一下，他们用于打印的方法比仅在您的大陆对象上创建快速打印功能更难

score 1 · Accepted Answer

您应该两次应用分组

var grouped = Result
    .GroupBy(x => x.CountryName)
    .GroupBy(x => x.First().ContinentName);

var final = grouped.Select(g1 => new Continent
{
    ContinentName = g1.Key,
    Countries = g1.Select(g2 => new Country
    {
        CountryName = g2.Key,
        Cities = g2.Select(x => new City { CityName = x.CityName }).ToList()
    }).ToList()
});

c# - LINQ：分组子组

3 回答 3

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