Program.cs

namespace ConsoleApp1;

internal unsafe class Program
{
    private struct StructWithFixedBuffer
    {
        public StructWithFixedBuffer()
        {
        }

        public fixed long buffer[1];
    }

    public static void Main(string[] args)
    {
        for (int i = 0; i < 10; i++)
        {
            var a = new StructWithFixedBuffer();
            Console.WriteLine($"{a.buffer[0]} ");
            a.buffer[0]++;
        }
    }
}