null_challenge2.cs

using System.Runtime.CompilerServices;

#pragma warning disable CS8604
#pragma warning disable CS8600
#pragma warning disable CS0660
#pragma warning disable CS0661

C local = null;

if (local != null && local.Prop)
{
    Console.WriteLine(local.ToString());
}

internal class C
{
    public bool Prop
    {
        get
        {
            unsafe
            {
                void** rsp = stackalloc void*[1];

                C self = this;

                if (sizeof(nuint) != sizeof(void*))
                    throw new InvalidOperationException("wtf");

                var ptr = (void*) Unsafe.As<C, nuint>(ref self);

                /*
                 * Call another method to ensure that rcx != this
                 * That way when we overwrite values on the stack, 
                 * not only do we null the local in Main, but we
                 * null the caller-saved value of rcx.
                 */
                stack(rsp, ptr);
            }

            return true;
        }
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static unsafe void stack(void** rsp, void* ptr)
    {
        const int cap = 200;

        for (int i = 0; i < cap; i++)
        {
            if (*rsp == ptr)
                *rsp = (void*) 0;

            rsp++;
        }
    }

    public static bool operator !=(in C a, in C _)
    {
        Unsafe.AsRef(a) = new C();
        return true;
    }

    public static bool operator ==(C a, C b) => throw new NotImplementedException();
}