Finding symmetric difference with LINQ

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Use HashSet<T> directly – it has a SymmetricExceptWith method:

HashSet<T> data = new HashSet<T>(a);
data.SymmetricExceptWith(b);

EDIT: If you want to maintain the order, here’s an alternative:

HashSet<T> data = new HashSet<T>(a);
data.IntersectWith(b);
foreach (T t in a.Concat(b))
{
    if (!data.Contains(t))
    {
        yield return t;
    }
}

This has the following important differences:

  • Both a and b are iterated over twice. In some cases that could be a very bad thing – you could call ToList on each of them to start with to retain a buffer.

  • If there are duplicates in either a or b, they will be yielded multiple times. If you wanted to avoid this you could keep a set of already-yielded values. At this point, it would be equivalent to:

    a.Concat(b).Except(a.Intersect(b))

That’s still only two set operations instead of the three in your original code though.

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