c#でヒープに乗らないリストっぽいものを作ってみた

使い方

Unityで使う用の一時的な処理で使うリストを作ってみました．
初期化時に下のコードの様にstackalloc T[maxsize]で確保したスパンを渡します．
初期サイズがmaxsizeでサイズを拡張しない，本家Listのような動きを再現しました.

ListedSpan<T> list = new ListedSpan<T>(stackalloc T[maxsize]);

コード

using System;
public ref struct ListedSpan<T>
{
    private int count;
    private Span<T> sparray;

    public int BufferCapacity => sparray.Length;
    public int Count => count;

    /// <summary>
    /// = new ListedSpan<T>(stackalloc T[maxcount]);と使う
    /// </summary>
    public ListedSpan(in Span<T> stackalloced)
    {
        sparray = stackalloced;
        count = 0;
    }
    public Span<T> AsSpan() => sparray.Slice(0, count);
    public bool ContainsIndex(int i) => 0 <= i && i < count;


    public void Clear() => count = 0;
    public void Add(in T add)
    {
        if (BufferCapacity < count + 1) throw new IndexOutOfRangeException("確保バッファを超過");
        sparray[count++] = add;
    }

    public void RemoveRange(int start, int range)
    {
        if (!ContainsIndex(start)) throw new IndexOutOfRangeException("削除開始インデックスがリスト外");
        if (!ContainsIndex(start + range - 1)) throw new IndexOutOfRangeException("削除開始インデックスから指定範囲を行くとはみ出ます");
        count -= range;
        int backCount = BufferCapacity - start - range;
        Span<T> copyFrom = sparray.Slice(start + range, backCount);// 削除範囲含まない後ろの全部
        Span<T> copyTo = sparray.Slice(start, backCount);// 削除開始を含む後ろ
        copyFrom.CopyTo(copyTo);
    }
    public void Insert(int index,in T insert)
    {
        int add = 1;
        int newCount = count + add;
        if (!ContainsIndex(index)) throw new IndexOutOfRangeException();
        if (BufferCapacity < newCount) throw new IndexOutOfRangeException();
        count = newCount;

        int backCount = BufferCapacity - index - add;
        Span<T> copyFrom = sparray.Slice(index, backCount);
        Span<T> copyTo = sparray.Slice(index + add, backCount);
        copyFrom.CopyTo(copyTo);
        sparray[index] = insert;
    }
    public void Insert(int index, in T t0, in T t1, in T t2)
    {
        int add = 3;
        int newCount = count + add;
        if (!ContainsIndex(index)) throw new IndexOutOfRangeException();
        if (BufferCapacity < newCount) throw new IndexOutOfRangeException();
        count = newCount;

        int backCount = BufferCapacity - index - add;
        Span<T> copyFrom = sparray.Slice(index, backCount);
        Span<T> copyTo = sparray.Slice(index + add, backCount);
        copyFrom.CopyTo(copyTo);
        sparray[index++] = t0;
        sparray[index++] = t1;
        sparray[index] = t2;
    }
}