ContainerExtensions.cs

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

public static class ContainerExtensions
{
    public static IEnumerable<(int, T)> Enumerate<T>(this IEnumerable<T> enumerable)
    {
        int index = 0;
        foreach (T v in enumerable)
        {
            yield return (index, v);
            index++;
        }
    }

    public static IEnumerable<T> AsEnumerable<T>(params T[] elements)
    {
        return elements.AsEnumerable();
    }

    public static V GetOrAddWith<K, V>(this Dictionary<K, V> dict, K key, Func<V> factory)
    {
        if (!dict.TryGetValue(key, out var value))
        {
            value = factory();
            dict[key] = value;
        }
        return value;
    }

    public static void AddMulti<K, V>(this Dictionary<K, List<V>> dict, K key, V value)
    {
        if (dict.TryGetValue(key, out var list))
        {
            list.Add(value);
        }
        else
        {
            dict[key] = new List<V>(1) { value };
        }
    }
}