using System;

using System.Collections.Generic;

using System.Runtime.InteropServices;

​

public struct Teste {

    public int Numero = 0;

    public void SetNumero(int i) {

        this.Numero = i;

    }

}

public class Program {

    public static void Main() {

        var list = new List<Teste>();

        list.EnsureCapacity(100);

        for (var i = 0; i < 100; i++) list.Add(new Teste());

        var list2 = CollectionsMarshal.AsSpan(list);

        for (var i = 0; i < list.Count; i++) {

            ref var item = ref list2[i];

            item.SetNumero(i);

        }

        for (var i = 0; i < list.Count; i++) Console.WriteLine(list[i].Numero);

    }

}