Access array items via ref

PooledArray.cs

/* - https://github.com/hsytkm/CSharpStandardSamples/blob/master/CSharpStandardSamples.Core/Spans/PooledArray.cs
 *
 * - Pooling large arrays with ArrayPool  https://adamsitnik.com/Array-Pool/
 *    that it has a default max array length, equal to 2^20 (1024*1024 = 1 048 576).
 *
 * - LitJWTに見るモダンなC#のbyte[]とSpan操作法  http://neue.cc/2019/05/27_578.html
 *      ようするに、今どきnew byte[]なんてしたら殺されるぞ！
 *
 * - https://gist.github.com/ufcpp/b0853cff5823d49306ba693aaa5c39fb
 */
#nullable enable

using System;
using System.Buffers;

var size = 1025;    // 1MByte最大っぽいので控えめに

using var array = new PooledArray<byte>(size);

// ゼロ初期化されてないのでクリア
for (var i = 0; i < array.Length; i++)
{
    array[i] = 0;
}
Console.WriteLine($"array[1]={array[1]}");

ref var x1 = ref array[1];
x1 += 1;
Console.WriteLine($"array[1]={array[1]}");

array[1] += 10;
Console.WriteLine($"array[1]={array[1]}");

int counter = 0;
foreach (ref var x in array)
{
    x += 100;
    counter++;
}
Console.WriteLine($"array[1]={array[1]}");
Console.WriteLine($"LoopCount={counter}");

/// <summary>
/// <see cref="ArrayPool{T}"/>から借りてきて、<see cref="Dispose"/>で返却するためのラッパー。
/// 小サイズなら stackalloc<T> を使った方が効率よく、バカ大なら T[] するしかない(はず)。
/// </summary>
public readonly struct PooledArray<T> : IDisposable
    where T : notnull
{
    public readonly int Length;
    private readonly ArrayPool<T>? _pool;
    private readonly T[] _array;    // 要求サイズよりも大きいキリのよいサイズが確保されています

    public PooledArray(int minimumLength) : this(minimumLength, ArrayPool<T>.Shared) { }

    private PooledArray(int minimumLength, ArrayPool<T> pool)
        => (Length, _pool, _array) = (minimumLength, pool, pool.Rent(minimumLength));

    public ref T this[int index] => ref _array[index];
    public void Dispose() => _pool?.Return(_array);

    public PooledArrayEnumerator<T> GetEnumerator() => new PooledArrayEnumerator<T>(_array, Length);
    public struct PooledArrayEnumerator<U> where U : notnull
    {
        private int _index;
        private readonly U[] _array;
        private readonly int _length;
        public PooledArrayEnumerator(U[] array, int length)
            => (_index, _array, _length) = (-1, array, length);

        public ref U Current => ref _array[_index];
        public bool MoveNext() => ++_index < _length;
    }
}