Test whether two IEnumerable have the same values with the same frequencies

Question

I have two multisets, both IEnumerables, and I want to compare them.

string[] names1 = { \"tom\", \"dick\", \"harry\" };
string[] names2 = { \

Answer 1

First sort as you have already done, and then use Enumerable.SequenceEqual. You can use the first overload if your type implements IEquatable or overrides Equals; otherwise you will have to use the second form and provide your own IEqualityComparer.

So if your type does implement equality, just do:

return a.SequenceEqual(b);

Here's another option that is both faster, safer, and requires no sorting:

public static bool UnsortedSequencesEqual(
    this IEnumerable first,
    IEnumerable second)
{
    return UnsortedSequencesEqual(first, second, null);
}

public static bool UnsortedSequencesEqual(
    this IEnumerable first,
    IEnumerable second,
    IEqualityComparer comparer)
{
    if (first == null)
        throw new ArgumentNullException("first");

    if (second == null)
        throw new ArgumentNullException("second");

    var counts = new Dictionary(comparer);

    foreach (var i in first) {
        int c;
        if (counts.TryGetValue(i, out c))
            counts[i] = c + 1;
        else
            counts[i] = 1;
    }

    foreach (var i in second) {
        int c;
        if (!counts.TryGetValue(i, out c))
            return false;

        if (c == 1)
            counts.Remove(i);
        else
            counts[i] = c - 1;
    }

    return counts.Count == 0;
}