patterncsharpMinor

Faster Way of Comparing Generic Sets

Submitted by: @import:stackexchange-codereview·Mar 10, 2026·

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codereview csharp stackoverflow c#performance set

genericcomparingwayfastersets

Problem

The following extension method is the limiting factor in the performance of an application I am developing, according to Visual Studio 2012 Performance Analysis profiler.

Is there a faster way to compare two collections for set equality?

public static bool SetEqual(this IEnumerable source, IEnumerable other, IEqualityComparer comparer = null)
{
    if (source == null) {
        throw new ArgumentNullException("source");
    }

    if (other == null) {
        throw new ArgumentNullException("other");
    }

    ICollection sourceCollection = source as ICollection ?? source.ToArray();
    ICollection otherCollection = other as ICollection ?? other.ToArray();

    if (sourceCollection.Count != otherCollection.Count) {
        return false;
    }

    if (sourceCollection.Except(otherCollection, comparer).Any()) {
        return false;
    }

    if (otherCollection.Except(sourceCollection, comparer).Any()) {
        return false;
    }

    return true;
}

Solution

It seems you are iterating the sets a lot with the Except method and possibly the ToArray.

Consider iterating the sets at most once:

public static bool SetEqual(this IEnumerable source, IEnumerable other, IEqualityComparer comparer = null)
{
    if (source == null)
        throw new ArgumentNullException("source");

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

    if (source is ICollection &&
        other is ICollection &&
        ((ICollection)source).Count != ((ICollection)other).Count)
        return false;

    HashSet set;
    if (comparer == null)
        set = new HashSet(source);
    else
        set = new HashSet(source, comparer);

    return set.SetEquals(other);
}

Source IEnumerable implementation.

Code Snippets

public static bool SetEqual<T>(this IEnumerable<T> source, IEnumerable<T> other, IEqualityComparer<T> comparer = null)
{
    if (source == null)
        throw new ArgumentNullException("source");

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

    if (source is ICollection &&
        other is ICollection &&
        ((ICollection)source).Count != ((ICollection)other).Count)
        return false;

    HashSet<T> set;
    if (comparer == null)
        set = new HashSet<T>(source);
    else
        set = new HashSet<T>(source, comparer);

    return set.SetEquals(other);
}

Context

StackExchange Code Review Q#68288, answer score: 5

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