LINQ: Grouping SubGroup

问题

How to group SubGroup to create list of Continents where each Continent has it own counties and each country has its own cities like this table

Here is the 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

and the result of t-sql:

I tried this but it groups the data in wrong way i need to group exactly like the above table

  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();

回答1:

You need to group everything by continent, these by country and the countries by city:

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();

回答2:

You should apply grouping twice

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()
});

回答3:

I know this is old, but I wanted to mention a much easier way per microsoft that is a bit more readable. This is an example with only 2 levels though but it will most likely work for others who reach this page (like me)

 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;

The documentation for that is at https://docs.microsoft.com/en-us/dotnet/csharp/linq/create-a-nested-group and is using a Student object as an example

I do want to mention that the method they use for printing is harder than just creating a quick print function on your continent object

来源：https://stackoverflow.com/questions/39081806/linq-grouping-subgroup