IEnumerable extension for all but the last n elements of a sequence.

ExceptLastLinq.cs

namespace System.Linq
{
    public static class ExceptLastLinq
    {
        public static IEnumerable<T> ExceptLast<T>(
            this IEnumerable<T> source)
        {
            if (source == null)
                throw new ArgumentNullException("source");

            return InternalExceptLast(source);
        }

        private static IEnumerable<T> InternalExceptLast<T>(
            IEnumerable<T> source)
        {
            T buffer = default(T);
            bool buffered = false;

            foreach (T x in source)
            {
                if (buffered)
                    yield return buffer;

                buffer = x;
                buffered = true;
            }
        }

        public static IEnumerable<T> ExceptLast<T>(
            this IEnumerable<T> source, int n)
        {
            if (source == null)
                throw new ArgumentNullException("source");

            if (n < 0)
                throw new ArgumentOutOfRangeException("n",
                    "Argument n should be non-negative.");

            return InternalExceptLast(source, n);
        }

        private static IEnumerable<T> InternalExceptLast<T>(
            IEnumerable<T> source, int n)
        {
            Queue<T> buffer = new Queue<T>(n + 1);

            foreach (T x in source)
            {
                buffer.Enqueue(x);

                if (buffer.Count == n + 1)
                    yield return buffer.Dequeue();
            }
        }
    }
}