Select all columns but group only one item in linq

Question

I was looking for a way to get multiple columns, but only group in SQL, and I found some information. However, I cannot think of a way to do this in linq.

I have the following table of toy examples:

| Id | Message | GroupId | Date |
|-------------------------------|
| 1  | Hello   | 1       | 1:00 |
| 2  | Hello   | 1       | 1:01 |
| 3  | Hey     | 2       | 2:00 |
| 4  | Dude    | 3       | 3:00 |
| 5  | Dude    | 3       | 3:01 |

And I would like to restore all columns for even rows GroupIdas follows (with desc order of Date order):

| Id | Message | GroupId | Date |
|-------------------------------|
| 1  | Hello   | 1       | 1:00 |
| 3  | Hey     | 2       | 2:00 |
| 4  | Dude    | 3       | 3:00 |

It doesn’t matter to me which row is selected from the grouped ones (first, second ...), as long as this is the only one specified in the Id group.

I have come out with the following code so far, but it does not do what it should:

List<XXX> messages = <MyRep>.Get(<MyWhere>)
                            .GroupBy(x => x.GroupId)
                            .Select(grp => grp.OrderBy(x => x.Date))
                            .OrderBy(y => y.First().Date)
                            .SelectMany(y => y).ToList();

Answer 1

This will give you one element for each group:

List<dynamic> data = new List<dynamic>
{
    new {ID  = 1, Message = "Hello", GroupId = 1, Date = DateTime.Now},
    new {ID  = 2, Message = "Hello", GroupId = 1, Date = DateTime.Now},
    new {ID  = 3, Message = "Hey",   GroupId = 2, Date = DateTime.Now},
    new {ID  = 4, Message = "Dude",  GroupId = 3, Date = DateTime.Now},
    new {ID  = 5, Message = "Dude",  GroupId = 3, Date = DateTime.Now},
};

var result = data.GroupBy(item => item.GroupId)
                 .Select(grouping => grouping.FirstOrDefault())
                 .OrderByDescending(item => item.Date)
                 .ToList();

//Or you can also do like this:
var result = data.GroupBy(item => item.GroupId)
                 .SelectMany(grouping => grouping.Take(1))
                 .OrderByDescending(item => item.Date)
                 .ToList();

OrderBy, :

var result = data.GroupBy(item => item.GroupId)
                 .SelectMany(grouping => grouping.OrderBy(item => item.Date).Take(1))
                 .OrderByDescending(item => item.Date)
                 .ToList();