Implementing Smalltalk "become" in C#

become.cs

using System;
using System.Runtime.CompilerServices;
using System.Threading;

class Program
{
    static void Main()
    {
        var x = new Person();
        var y = new Dog();

        Monitor.Enter(x);
        {
            Console.WriteLine($"x = {x}; y = {y}");
            Swap(x, y);
            Console.WriteLine($"x = {x}; y = {y}");
        }
        Monitor.Exit(y);
    }

    unsafe static void Swap<T, U>(T left, U right)
        where T: class
        where U: class
    {
        if (Unsafe.SizeOf<T>() != Unsafe.SizeOf<U>())
        {
            throw new ArgumentException("Too unsafe, even for me!");
        }

        int size = Unsafe.SizeOf<T>() + sizeof(int) * 3;

        byte* buffer = stackalloc byte[size];
        fixed (char* cLeft = Unsafe.As<string>(left))
        fixed (char* cRight = Unsafe.As<string>(right))
        {
            const int three = 4;
            void* leftStart = ((int*)cLeft) - three;
            void* rightStart = ((int*)cRight) - three;
            var usize = (uint)size;

            Unsafe.CopyBlock(buffer, leftStart, usize);
            Unsafe.CopyBlock(leftStart, rightStart, usize);
            Unsafe.CopyBlock(rightStart, buffer, usize);
        }
    }
}

class Person { int i = 1; public override string ToString() => $"Person {i}"; }
class Dog { int i = 2; public override string ToString() => $"Dog {i}"; }