CartesianExtension.cs

using System;
using System.Collections.Generic;
using System.Linq;
using MoreLinq;

public static class CartesianExtension
{
    public static IEnumerable<TResult> Cartesian<TSource, TResult>(
        this IEnumerable<IEnumerable<TSource>> enumerables,
        Func<IEnumerable<TSource>, TResult> resultSelector)
    {
        if (enumerables == null) throw new ArgumentNullException(nameof(enumerables));
        if (resultSelector == null) throw new ArgumentNullException(nameof(resultSelector));

        var enumerators = enumerables
            .Select(e => e?.GetEnumerator() ?? throw new ArgumentException("One of the enumerables is null"))
            .Pipe(e => e.MoveNext())
            .ToArray();

        do yield return resultSelector(enumerators.Select(e => e.Current)); while (MoveNext());

        foreach (var enumerator in enumerators) enumerator.Dispose();

        bool MoveNext()
        {
            for (var i = enumerators.Length - 1; i >= 0; i--)
            {
                if (enumerators[i].MoveNext()) return true;
                if (i != 0)
                {
                    enumerators[i].Reset();
                    enumerators[i].MoveNext();
                }
            }

            return false;
        }
    }
}