List<T>モドキ作る時用のヘルパクラス

ListCore.cs

using System;
using System.Runtime.CompilerServices;

namespace Nandemoii
{
    public struct ListCore<T>
    {
        public Memory<T> Memory;
        public int Count;

        public bool IsFull => this.Memory.Length == this.Count;

        public ListCore(Memory<T> memory, int count)
        {
            this.Memory = memory;
            this.Count = count;
        }

        public void Add(T item)
        {
            ValidateFull();

            this.Memory.Span[this.Count] = item;
            this.Count++;
        }

        public void Insert(int index, T item)
        {
            ValidateFull();

            if(index == this.Count)
            {
                Add(item);
                return;
            }

            ValidateIndex(index);

            var start = index;
            var end = this.Count - 1;

            var span = this.Memory.Span;
            for(int i = end; i >= start; i--)
            {
                span[i + 1] = span[i];
            }

            span[index] = item;
            this.Count++;
        }

        public void RemoveAt(int index)
        {
            ValidateIndex(index);

            var start = index + 1;

            var span = this.Memory.Span;
            for (int i = start; i < this.Count; i++)
            {
                span[i - 1] = span[i];
            }

            span[this.Count - 1] = default;
            this.Count--;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void ValidateFull()
        {
            if (IsFull)
                throw new InvalidOperationException("Full");
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void ValidateIndex(int index)
        {
            if (index < 0 || this.Count <= index)
                throw new ArgumentOutOfRangeException(nameof(index));
        }
    }
}

Array.Resizeなどは勝手にやらないけど、ArrayPoolなど使い始めたら逆に邪魔になるので外でやってくれたほうが良かろうかと