source: Common/EnumIndexedArray.cs

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

namespace Common
{
    public class EnumIndexedArray<T, TIndex> : IEnumerable where TIndex : Enum // requires C# 7.3 or later
    {
        private readonly T[] array;
        private readonly int lower;

        public EnumIndexedArray()
        {
            lower = Convert.ToInt32(Enum.GetValues(typeof(TIndex)).Cast<TIndex>().Min());
            int upper = Convert.ToInt32(Enum.GetValues(typeof(TIndex)).Cast<TIndex>().Max());
            array = new T[1 + upper - lower];
        }

        public T this[TIndex key]
        {
            get => array[Convert.ToInt32(key) - lower];
            set => array[Convert.ToInt32(key) - lower] = value;
        }

        public IEnumerator GetEnumerator()
        {
            return Enum.GetValues(typeof(TIndex)).Cast<TIndex>().Select(i => this[i]).GetEnumerator();
        }

        public void Assign(EnumIndexedArray<T, TIndex> source)
        {
            for (int i = 0; i < source.array.Length; i++)
            {
                array[i] = source.array[i];
            }
        }
    }
}