Wrong Answer #2: Your own ThreadLocal
time to read 3 min | 476 words
Originally posted at 12/15/2010

Well, last time we tried to introduce a finalizer, but we forgot that we were using our own instance as the key to the slots dictionary, which prevented us from being collected, hence we never run the finalizer.

All problems can be solved by adding an additional level of indirection… instead of using our own instance, let us use a separate instance:

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

    object holder = new object();
    public static Dictionary<object, object> Slots
    {
        get { return slots ?? (slots = new Dictionary<object, object>()); }
    }

    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 (slots != null)// intentionally using the field here, to avoid creating the instance
            slots.Remove(holder);
    }

    ~CloseableThreadLocal()
    {
        Close();
    }
}

Now it should work, right?

Expect that it doesn’t… We still see that the data isn’t cleaned up properly, even though the finalizer is run.

Why?

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