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
– Enigmativity, Commented Mar 31, 2014 at 5:13
declare lstMyList List<int[]> lstMyList = new List<int[]>(); or List<Array> lstMyList = new List<Array>(); — verygg
– verygg, Commented Mar 31, 2014 at 5:13
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
– Enigmativity, Commented Mar 31, 2014 at 5:16
See about Enumerable.GroupBy Method with IEqualityComparer<TKey> — Grundy
– Grundy, Commented Mar 31, 2014 at 5:22

Ulugbek Umirov · Accepted Answer · 2014-03-31 05:47:23Z

3

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();

answered Mar 31, 2014 at 5:47

Ulugbek Umirov

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1 Comment

Enigmativity Over a year ago

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.

Aron · Accepted Answer · 2014-03-31 05:25:12Z

0

Try this

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

answered Mar 31, 2014 at 5:25

Aron

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2 Comments

verygg Over a year ago

I try but receive error: has some invalid arguments

Aron Over a year ago

Apologies. You might want to take a look at this for the fix stackoverflow.com/a/5601068/1808494

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