如何获得不同的列表类型列表

Question

This is my code (copy and paste it into linqpad if you like)

var messageUsers = new [] { 
    new { MsgId = 2, UserId = 7 },
    new { MsgId = 2, UserId = 8 },
    new { MsgId = 3, UserId = 7 },
    new { MsgId = 3, UserId = 8 },
    new { MsgId = 1, UserId = 7 },
    new { MsgId = 1, UserId = 8 },
    new { MsgId = 1, UserId = 9 }};

messageUsers
    .GroupBy (x => x.MsgId, x => x.UserId)
    .Select (x => x.Select (y => y))
    .Distinct()
    .Dump();

The results I get back are {7,8}, {7,8}, {7,8,9}

What I want is {7,8}, {7,8,9}.

Basically I want to remove the duplicate lists. I haven't tried this but I think I could probably achieve by creating a comparer and passing it into the Distinct method. However I would like to eventually use this in a Linq to Entities query without bringing thousands of rows back to the client so that isn't a good option.

For extra clarification...I need to return a List> where the contents of each inner list is distinct in comparison to any of the other inner list.

Answer 1

The problem is that .Distinct() determines what's distinct based on the GetHashCode() and Equals() implementation of the underlying objects. In this case, the underlying object is something that implements IEnumerable<> , but which uses the default object implementation for those methods--which is based purely on whether the objects occupy the same space in memory. So as far as it can tell, the sequences are not distinct, even though they have the same values in them.

How about this?

messageUsers
    .GroupBy (x => x.MsgId, x => x.UserId)
    .GroupBy(x => string.Join(",", x))
    .Select(x => x.FirstOrDefault())
    .Dump();

The idea is to group by a key that represents the combined value of the elements in your list. You could also pass a custom IEqualityComparer<> to the Distinct method in your original code, but that seems like a fair bit of effort for something so trivial.

It's worth noting that this won't work very well if you're using LINQ to Entities or something like that.

Update

To make it a List<List<int>> , you'll need a few .ToList() s thrown in there:

messageUsers
    .GroupBy (x => x.MsgId, x => x.UserId)
    .GroupBy(x => string.Join(",", x))
    .Select(x => x.FirstOrDefault().ToList())
    .ToList()
    .Dump();

But I'm frankly not sure why that matters to you.

Answer 2

Here is an alternative answer:

messageUsers
    .GroupBy (x => x.MsgId, y=>y.UserId)
    .Select (x => new HashSet<int>(x))
    .Distinct(HashSet<int>.CreateSetComparer())
    .Dump();

Consider the following input:

var messageUsers = new [] { 
    new { MsgId = 2, UserId = 7 },
    new { MsgId = 2, UserId = 8 },
    new { MsgId = 3, UserId = 8 },
    new { MsgId = 3, UserId = 7 },
    new { MsgId = 1, UserId = 7 },
    new { MsgId = 1, UserId = 8 },
    new { MsgId = 1, UserId = 9 }};

What result do you want?

{7,8}, {7,8,9} or {7,8}, {8,7}, {7,8,9}.