効率のよくない List.partition

Partition.cs

public static Tuple<IList<T>, IList<T>> Partition<T>(this IList<T> source, Func<T, bool> predicate)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (predicate == null) throw new ArgumentNullException(nameof(predicate));

    var list = source.ToList();
    return new Tuple<IList<T>, IList<T>>(
        list.Where(x => predicate(x)).ToList(),
        list.Where(x => predicate(x) == false).ToList());
}