LINQ partition (specialization of GroupBy)

gistfile1.c

// @hammett: "Linq doesn't have a Partition operation that returns two sets? (filtered/complement).. sad!"

// GroupBy can be seen as a partition. We specialize it to have exactly two groups:

public static class PartitionExtension
{
    public static Tuple<IEnumerable<T>, IEnumerable<T>> Partition<T>(this IEnumerable<T> enumeration, Func<T, bool> criteria)
    {
        var whole = enumeration.GroupBy(criteria);

        return new Tuple<IEnumerable<T>, IEnumerable<T>>(
            whole.Where(x => x.Key).SelectMany(x => x),
            whole.Where(x => !x.Key).SelectMany(x => x));
    }
}

// Usage

    var numbers = Enumerable.Range(1, 100);

    // Partition into "even" numbers and "odd" numbers
    Tuple<IEnumerable<int>, IEnumerable<int>> results = numbers.Partition((x) => x % 2 == 0);

    var evens = results.Item1;
    var odds = results.Item2;

No way to do this lazy without the ToDictionary()?

Good point... updated to be lazy.