How to group List array in LINQ

Question

I have an List<Array>, I'm using LINQ to find duplicates and count, but it's not work. See the image you can see, lstMyList[0] and lstMyList[11] have the same value in Int[]

Here is lstMyList definition:

List<Array> lstMyList = new List<Array>();

I used code, but it's not work:

var group = lstMyList.GroupBy(t => t).ToArray();

or

Dictionary<int[], int> count = lstMyList.GroupBy(x => x).ToDictionary(g => g.Key, g => g.Count());

Here is image: http://imageshack.com/a/img69/2552/h114.png

Maybe somebody can give me a hint about my problem.

Having a key that is an `int[]` is probably not going to work for you. We need the class definition. — Enigmativity, Mar 31 '14 at 05:13
declare lstMyList List lstMyList = new List(); or List lstMyList = new List(); — verygg, Mar 31 '14 at 05:13
If you have two int arrays - both `int[] { 1, 2 }` - they are not equal to each other using standard equality as arrays are reference types and they only compare if the array is the same reference. Two arrays, even with the same values, are different. — Enigmativity, Mar 31 '14 at 05:16
See about [Enumerable.GroupBy Method with IEqualityComparer](http://msdn.microsoft.com/ru-ru/library/bb534334(v=vs.110).aspx) — Grundy, Mar 31 '14 at 05:22

score 3 · Accepted Answer · answered Mar 31 '14 at 05:47

First create comparer class:

sealed class ArrayEqualityComparer : IEqualityComparer<int[]>
{
    public bool Equals(int[] x, int[] y)
    {
        if (x == null && y == null)
            return true;
        if (x != null && y != null)
            return x.SequenceEqual(y);
        return false;
    }

    public int GetHashCode(int[] obj)
    {
        return obj.Length;
    }
}

Then you can use it in GroupBy clause.

List<int[]> lstMyList = new List<int[]> { new[] { 1, 2 }, new[] { 3, 4 }, new[] { 1, 2 } };
var groups = lstMyList.GroupBy(t => t, new ArrayEqualityComparer())
                      .Select(g => new { g.Key, Count = g.Count() })
                      .ToArray();

It should be noted that this solution does have some potential drawbacks. The `g.Key` will be a reference to one of the "equal" arrays so careful if you modify the contents. Also the comparer will not equate `{1,2}` & `{2,1}` so, depending what the domain logic is, this could be an issue too. — Enigmativity, Mar 31 '14 at 10:58

score 0 · Answer 2 · answered Mar 31 '14 at 05:25

0

Try this

var group = lstMyList.GroupBy(t => t,
              StructuralComparisons.StructuralEqualityComparer).ToArray();

answered Mar 31 '14 at 05:25

Aron

15,464
3
31
64

I try but receive error: has some invalid arguments – verygg Mar 31 '14 at 05:37
Apologies. You might want to take a look at this for the fix http://stackoverflow.com/a/5601068/1808494 – Aron Mar 31 '14 at 05:51

How to group List array in LINQ

2 Answers2