brittle.cs

using System.Runtime.InteropServices;

unsafe class Program
{
    static void Main()
    {
        ReadOnlySpan<byte> name = stackalloc byte[]
        {
            (byte)'J',
            (byte)'b',
            (byte)' ',
            (byte)'E',
            (byte)'v',
            (byte)'a',
            (byte)'i',
            (byte)'n',
        };
        Person person = new();

        long mem;
        fixed (byte*data = &name[0])
        {
            mem = (long)data;
            Console.WriteLine("mem: " + mem);
        }

        long fna = (long)&person.FirstName;
        long lna = (long)&person.LastName;

        Console.WriteLine("fna: " + fna);
        Console.WriteLine("lna: " + lna);

        person.FirstName.offset = (int)(mem - fna);
        person.FirstName.length = 2;

        person.LastName.offset = (int)(mem + 3 - lna);
        person.LastName.length = 5;

        PrintHello(ref person);
    }

    static void PrintHello(ref Person p)
    {
        var s = p.FirstName.ToString();
        var l = p.LastName.ToString();

        Console.WriteLine($"Hello {s} {l}!");
    }
}

struct Person
{
    public BlobString FirstName;
    public BlobString LastName;
}

unsafe struct BlobString
{
    internal int offset;
    internal int length;

    public BlobString(int offset, int length)
    {
        this.offset = offset;
        this.length = length;
    }

    public readonly override string ToString()
    {
        fixed (int* data = &offset)
        {
            return Marshal.PtrToStringAnsi((nint)data + offset, length);
        }
    }
}