Question

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Comparing two collections for equality

I need to verify if two IEnumerable<T> lists have the same elements, not necessarily in the same order.

I’m targetting .NET 3.5.

Here are the tests. The question is, how should HasSameElements() be implemented?

var l1 = new[]{1,2,3};
var l2 = new[]{3,1,2};

bool rez1 = l1.HasSameElements(l2);//should be true

var l3 = new[]{1,2,3,2};
var l4 = new[]{3,1,2,2};
bool rez2 = l3.HasSameElements(l4);//should be true

var l5 = new[]{1,2,3,2};
var l6 = new[]{1,2,3};
bool rez3 = l5.HasSameElements(l6);//should be false

Aditional notes:

• In the examples I’m using IEnumerable, but T could be anything. Should T necessarily implement IComparable ?

• Enumerable.SequenceEquals() by itself doesn’t work, it expects the same order for the elements.

• Here’s a template for HasElements:

[just some placeholder text as workaround for Markdown ‘code formatting’ bug]

public static class Extensions {
    public static bool HasElements(this IEnumerable<T> l1, IEnumerable<T> l2){
        throw new NotImplementedException();
    } 
}

Answer 1

Just build a dictionary mapping each object to the number of times it appears in the sequence and then check that the resulting dictionaries are equal.

Here:

static class EnumerableExtensions {
    public static bool HasSameElementsAs<T>(
        this IEnumerable<T> first,
        IEnumerable<T> second
    ) {
        var firstMap = first
            .GroupBy(x => x)
            .ToDictionary(x => x.Key, x => x.Count());
        var secondMap = second
            .GroupBy(x => x)
            .ToDictionary(x => x.Key, x => x.Count());
        return 
            firstMap.Keys.All(x =>
                secondMap.Keys.Contains(x) && firstMap[x] == secondMap[x]
            ) &&
            secondMap.Keys.All(x =>
                firstMap.Keys.Contains(x) && secondMap[x] == firstMap[x]
            );
    }
}

Obviously the repeated code can be refactored out into helper methods but that would just muddle the idea here. You could get fancy and accept an IEqualityComparer for the GroupBy operation. Also, you should productionize the code by adding null guards and what not.