Wrong Answer #3: Your own Thread Local

Last time, we forgot that the slots dictionary is a thread static variable, and that the finalizer is going to run on another thread… Let us fix this, too:

public class CloseableThreadLocal
{
    [ThreadStatic]
    public static Dictionary<object, object> slots;

    private readonly object holder = new object();
    private Dictionary<object, object> capturedSlots;

    private Dictionary<object, object> Slots
    {
        get
        {
            if (slots == null)
                slots = new Dictionary<object, object>();
            capturedSlots = slots;
            return slots;
        }
    }


    public /*protected internal*/ virtual Object InitialValue()
    {
        return null;
    }

    public virtual Object Get()
    {
        object val;

        if (Slots.TryGetValue(holder, out val))
        {
            return val;
        }
        val = InitialValue();
        Set(val);
        return val;
    }

    public virtual void Set(object val)
    {
        Slots[holder] = val;
    }

    public virtual void Close()
    {
        GC.SuppressFinalize(this);
        if (capturedSlots != null)
            capturedSlots.Remove(this);
    }

    ~CloseableThreadLocal()
    {
        if (capturedSlots == null)
            return;
        capturedSlots.Remove(holder);
    }
}

And now it works!

Except… Under some very rare scenarios, it will not do so.

What are those scenarios? Why do we care? And how do we fix this?