At an initial glance, it looks like the code is a trick question – the ThreadStatic attribute applies to _instance only, and not to _instanceId – so the code should execute normally and produce:

Instance 1 of 2
Instance 2 of 2

But if you run the code, you’ll get a null reference exception!

The problem lies in the initialisation of the ThreadStatic member – it only runs once, on the first thread that runs the initialisation code. So, poor old Thread 2 always has a null _instance!

The only way around the problem is to make the field non-readonly, and then modify the Instance property accessor to do the initialisation there:


[ThreadStatic]
static private Producer _instance;

static public Producer Instance
{
get { return _instance ?? (_instance = new Producer()); }
}


Hope that makes sense!