LINQ GroupBy子集合属性

Question

I'm trying to find a way to group a list using LINQ not only by properties of the Foo object (EmployeeId, CompanyId and GroupId) but ONLY group them if the StartDate and EndDate properties of the Baz objects are exactly the same. The goal is to group by EmployeeId, CompanyId and GroupId and combine the Bars collections to be an aggregate of all of the grouped records.

So if List has three objects, and the Bars collection on the first object are all BazId=1, the second BazId=2 and the third BazId=3, I would expect a result where List contains TWO objects one with a Bars collection that has BazIds of 1 and 3 and another that has a Bars collection with BazIds of 2 (since 1 and 3 share the same start/end dates, as outlined below).

I hope this makes sense. I've been awake for far too long so if this is unclear, let me know and I'll do my best to clarify.

What I have so far is:

var result = foos.GroupBy(g => new { g.CompanyId, g.EmployeeId, g.GroupId })
        .Select(g => new Foo(g.Key.CompanyId, g.Key.EmployeeId, g.Key.GroupId)
        {
          Bars = g.SelectMany(x => x.Bars)}
).ToList();

However, this ignores the StartDate/EndDate properties of the corresponding Baz objects. I ONLY want to group the records for Foo objects whose Bars collection has BazIds that share the same start/end dates.

EDIT: It is a business rule that all Bar objects within a given Bars collection will have the same BazId

public class Foo {
  public int EmployeeId { get; set; }
  public int CompanyId { get; set;
  public int GroupId { get; set; }

  public List<Bar> Bars { get; set; }
}


public class Bar {
  public BazId { get; set; }
  public string Name { get; set; }
}

public class Baz {
  public int Id { get; set; }
  public DateTime StartDate { get; set; }
  public DateTime EndDate { get; set; }
}

var baz = new List<Baz> {
  new Baz {
    Id = 1,
    StartDate = DateTime.Now,
    EndDate = DateTime.Now.AddMonths(1)
  },
  new Baz {
    Id = 2,
    StartDate = DateTime.Now.AddMonths(-1),
    EndDate = DateTime.Now.AddMonths(1)
  },
  new Baz {
    Id = 3,
    StartDate = DateTime.Now,
    EndDate = DateTime.Now.AddMonths(1)
  },
}

SOLUTION What I ended up with is this, based on the comment from @jdweng:

var result = foos.GroupBy(g => g, new FooComparer())
            .Select(g => new Foo(g.Key.CompanyId, g.Key.EmployeeId, g.Key.GroupId)
            {
              Bars = g.SelectMany(x => x.Bars)}
    ).ToList();

Within FooComparer , I implemented IEqualityComparer<Foo> and created Equals and GetHashCode methods that determined whether or not the two were equal by examining the Baz collections of each.

Answer 1

It seems that you have to include in grouping Start and End dates. For that you have to join with Baz.

var bazs = new List<Baz> {
    new Baz {
        Id = 1,
        StartDate = DateTime.Today,
        EndDate = DateTime.Today.AddMonths(1)
    },
    new Baz {
        Id = 2,
        StartDate = DateTime.Today.AddMonths(-1),
        EndDate = DateTime.Today.AddMonths(1)
    },
    new Baz {
        Id = 3,
        StartDate = DateTime.Today,
        EndDate = DateTime.Today.AddMonths(1)
    }
};

List<Foo> foos = new List<Foo>{
    new Foo
    {
        EmployeeId = 1,
        CompanyId = 2,
        GroupId = 3,
        Bars = new List<Bar> {
            new Bar { BazId = 1 },
            new Bar { BazId = 2 },
            new Bar { BazId = 3 }
        }
    }
};

var result = (from extendedBar in foos.SelectMany(f => f.Bars.Select(b => new { Bar = b, f.EmployeeId, f.CompanyId, f.GroupId }))
              join baz in bazs
              on extendedBar.Bar.BazId equals baz.Id
              select new { extendedBar.Bar, extendedBar.EmployeeId, extendedBar.CompanyId, extendedBar.GroupId, baz.StartDate, baz.EndDate })
                .GroupBy(o => new { o.EmployeeId, o.CompanyId, o.GroupId, o.StartDate, o.EndDate })
                .Select(g => new Foo { EmployeeId = g.Key.EmployeeId, CompanyId = g.Key.CompanyId, GroupId = g.Key.GroupId, Bars = g.Select(o => o.Bar).ToList() })
             .ToList();

Answer 2

Try adding an inheritance to the class IComparable with the method CompareTo(). The Linq Groupby will use the CompareTo method for the groupings