SkipLast.cs

public static class EnumerableExt
{
    public static IEnumerable<T> SkipLast<T>(this IEnumerable<T> source)
    {
        if (source == null)
        {
            throw new ArgumentNullException(nameof(source));
        }

        return SkipLastCore();
        IEnumerable<T> SkipLastCore()
        {
            using (var iterator = source.GetEnumerator())
            {
                if(!iterator.MoveNext())
                {
                    yield break;
                }

                var previous = iterator.Current;
                while (iterator.MoveNext())
                {
                    yield return previous;
                    previous = iterator.Current;
                }
            }
        }
    }
}