U can't initialize threadstatics

I'm not sure what this code in particular is used for, but let me guess. It breaks if the operation isn't bound to a single thread, and perhaps if the thread is reused in a thread pool. ASP.NET requests can begin on one thread, block, and then resume on another. Many years ago I had to fix one of those bugs in a production web site. Not fun, even though I didn't introduce it myself...

I think the second thread to hit it would find the value to be null.

On a separate note: If this is for S#arp Architecture, is ThreadStatic something you really want to be using? Starting with the 2.0 framework, web requests have thread mobility, under heavy load you'll often see this.

What I typically do is create a wrapper class that delegates uses HttpContext.Current.Items if the call happens under a web request, otherwise it delegates to a ThreadStatic field.

I think C-J Berg is right. It's never fun to use ThreadStatic in an ASP.NET context because you are not in control of the threads or thread pool. Under heavy load, your thread-static variable will exhibit promiscuous behavior ;)

I think Jay is right. The dictionary will be initialized only for the first thread.