ZipLongest returns a projection of tuples, where each tuple contains the N-th element from each of the argument sequences. The resulting sequence will always be as long as the longest of input sequences.

EnumerableExtensions.cs

public static class EnumerableExtensions
{
    public static IEnumerable<TResult> ZipLongest<TResult>(this IEnumerable<TResult> first, IEnumerable<TResult> second)
    {
        if (first == null)
        {
            throw new ArgumentNullException(nameof(first));
        }

        if (second == null)
        {
            throw new ArgumentNullException(nameof(second));
        }

        using var iter1 = first.GetEnumerator();
        using var iter2 = second.GetEnumerator();

        while (iter1.MoveNext())
        {
            yield return iter1.Current;

            if (iter2.MoveNext())
            {
                yield return iter2.Current;
            }
        }

        while (iter2.MoveNext())
        {
            yield return iter2.Current;
        }
    }
}