Writing Value to Managed Object at an Offset

Program.cs

using System.Runtime.CompilerServices;

class Program
{
    static void Main(string[] args)
    {
        TestSetIntAtOffset();
    }

    public static void TestSetIntAtOffset()
    {
        var boxedInt = (object)10;
        var offset = 8;
        SetIntAtOffset(boxedInt, 8, 20);
        Console.WriteLine(boxedInt);
    }

    public unsafe static void SetIntAtOffset(object o, int offset, int value)
    {
        const int kObjectHeader = 8;
        fixed (byte* data = new ObjectPinner(o))
        {
            var target = data + offset - kObjectHeader;
            int oldValue = Unsafe.AsRef<int>(target);

            Unsafe.AsRef<int>(target) = value;

            Console.WriteLine(oldValue);
        }
    }
    unsafe struct ObjectPinner
    {
        public ObjectPinner(object o)
        {
            this.o = o;
        }

        public ref byte GetPinnableReference()
        {
            return ref Unsafe.As<ObjectWrapper>(o).GetPinnableReference();
        }

        class ObjectWrapper
        {
            public byte Data;
            public ref byte GetPinnableReference() => ref Data;
        }

        object o;
    }
}