using System;

using System.Linq;

using System.Collections.Generic;

​

namespace DemoApp

{

    internal class Program

    {

        private static void Main(string[] args)

        {

            var dictOfLists = new Dictionary<string, List<string>>

            {

                ["foo"] = new List<string> { "a", "b", "c" },

                ["bar"] = new List<string> { "d" },

                ["baz"] = new List<string> { "e", "a" }

            };

​

            var result = BuildDictionary(dictOfLists);

            Console.WriteLine(System.Text.Json.JsonSerializer.Serialize(result, new System.Text.Json.JsonSerializerOptions { WriteIndented = false }));

        }

​

        private static Dictionary<TKey, TValue>[] BuildDictionary<TKey, TValue>(Dictionary<TKey, List<TValue>> dict) where TKey : notnull

        {

            // Calculating the size of the result (product of the length of each entry)

            int[] lengths = dict.Select(d => d.Value.Count()).ToArray();

            int size = lengths.Aggregate(1, (prev, val) => prev * val);   // reduce in JavaScript

            var result = new Dictionary<TKey, TValue>[size];

​

            for (int i = 0; i < result.Length; i++) { result[i] = new Dictionary<TKey, TValue>(); }

​

            int repeats = 1;

            foreach (var d in dict)

            {

                var key = d.Key;

                for (int row = 0; row < size;)

                    for (int i = 0; i < repeats; i++)

                        foreach (var val in d.Value)

                        {

                            result[row].Add(key, val);

                            row++;

                        }

                repeats *= d.Value.Count;

            }

            return result;

        }

    }

}