linq - 使用 Fluent 语法的 LINQ 嵌套 GroupBy

Question

我正在尝试使用 fluent(即“方法”)语法编写嵌套的 GroupBy LINQ 表达式。

这是我的类(class)和数据:

class Person
{
    public String ZipCode, Gender, Name;
}

private static List<Person> people = new List<Person>
{
    new Person { ZipCode= "11111", Gender = "M", Name = "Tom" },
    new Person { ZipCode= "11111", Gender = "M", Name = "Bob" },
    new Person { ZipCode= "11111", Gender = "F", Name = "Nancy" },
    new Person { ZipCode= "11111", Gender = "F", Name = "Lisa" },
    new Person { ZipCode= "22222", Gender = "M", Name = "Dan" },
    new Person { ZipCode= "33333", Gender = "F", Name = "Mary" },
    new Person { ZipCode= "44444", Gender = "F", Name = "Joan" },
    new Person { ZipCode= "44444", Gender = "F", Name = "Jane" },
    new Person { ZipCode= "44444", Gender = "M", Name = "Bill" }
};

我想要实现的是一个按 ZipCode 分组的对象然后通过 Gender内 ZipCode .就对象类型而言，我正在寻找:

IEnumerable<IGrouping<string, IEnumerable<IGrouping<string, Person>>>>

这将允许我像这样访问查询结果:

foreach(var byZip in mainQuery) {
   Console.WriteLine(byZip.Key);  // print the ZipCode
   foreach(var byGender in byZip) {
      Console.WriteLine(byGender.Key) // print the Gender
      foreach (Person p in byGender)
         Console.WriteLine(p.Name);
   }
}

同样，我希望使用流畅的符号。

根据下面杰夫的回答，这是查询和访问:

IEnumerable<IGrouping<string, IEnumerable<IGrouping<string, Person>>>> query = 
    people
        .GroupBy(p => p.ZipCode)
            .GroupBy(
                keySelector: g => g.Key,
                elementSelector: g => g.GroupBy(p => p.Gender)
            );


foreach (IGrouping<string, IEnumerable<IGrouping<string, Person>>> byZip in query)
{
    Console.WriteLine(byZip.Key);  // print the ZipCode
    foreach (IEnumerable<IGrouping<string, Person>> byGender in byZip)
    {
        foreach (IGrouping<string, Person> t in byGender)
        { 
            Console.WriteLine(t.Key); // print the Gender
            foreach (Person y in t)
                Console.WriteLine(y.Name);
        }

    }
}

Answer 1

在查询语法中，你可以这样写:

var query =
    from p in people
    group p by p.ZipCode into g
    let gender =
        from p in g
        group p by p.Gender
    group gender by g.Key;

并使用“流畅”语法将其转换为:

var query2 = people.GroupBy(p => p.ZipCode)
    .GroupBy(
        g => g.Key,
        g => g.GroupBy(p => p.Gender)
    );

但是，您想要访问它的方式与实际类型不匹配。您的访问模式使您的类型实际上是:

IEnumerable<IGrouping<string, IGrouping<string, Person>>>

所以查询应该是:

var query =
    from p in people
    group p by p.ZipCode into g
    let gender =
        from p in g
        group p by p.Gender
    from g2 in gender
    group g2 by g.Key;

而转换……有点复杂。

var query2 = people.GroupBy(p => p.ZipCode)
    .SelectMany(g =>
        g.GroupBy(p => p.Gender).GroupBy(g2 => g.Key)
    );