What's your favorite LINQ to Objects operator which is not built-in? [closed]

Answer 1

Duplicates

Used in conjunction with a method like Ani's AssertCount method (I use one called CountAtLeast), it becomes very easy to find elements in a sequence that appear more than once:

public static IEnumerable Duplicates(this IEnumerable source,
    Func keySelector = null, IEqualityComparer comparer = null)
{
    source.ThrowIfNull("source");
    keySelector = keySelector ?? new Func(x => x);
    comparer = comparer ?? EqualityComparer.Default;

    return source.GroupBy(keySelector, comparer)
        .Where(g => g.CountAtLeast(2))
        .SelectMany(g => g);
}