Enumerable.ZipAll

Enumerable.ZipAll

// https://dotnetfiddle.net/INhbdg

public static class EnumerableEx
{
    public static IEnumerable<TReturn> ZipAll<T1, T2, TReturn>(
        this IEnumerable<T1> first,
        IEnumerable<T2> second,
        Func<T1, T2, TReturn> f,
        T1 seed1,
        T2 seed2)
    {
        var iter1 = first.GetEnumerator();
        var iter2 = second.GetEnumerator();

        while(iter1.MoveNext())
        {
            if(iter2.MoveNext())
                yield return f(iter1.Current, iter2.Current);
            else
                yield return f(iter1.Current, seed2);
        }

        while(iter2.MoveNext())
            yield return f(seed1, iter2.Current);
    }

    public static IEnumerable<TReturn> ZipAll<T1, T2, TReturn>(
        this IEnumerable<T1> first,
        IEnumerable<T2> second,
        Func<T1, T2, TReturn> f)
    {
        return first.ZipAll(second, f, default(T1), default(T2));
    }
}