InterlockingLock.cs

using System;
using System.Threading;

struct InterlockingLock : IDisposable
{
    object _lock;

    public InterlockingLock(object @lock)
    {
        _lock = @lock;
        bool lockWasTaken = false;
        try { Monitor.Enter(_lock, ref lockWasTaken); }
        catch { if (lockWasTaken) Monitor.Exit(_lock); throw; }
    }

    public void Transfer(object @new)
    {
        bool lockWasTaken = false;
        try { Monitor.Enter(@new, ref lockWasTaken); }
        catch { if (lockWasTaken) Monitor.Exit(@new); throw; }
        var old = _lock;
        _lock = @new;
        Monitor.Exit(old);
    }

    public void Dispose() => Monitor.Exit(_lock);
}