[ghodss]

"By studying the output of the TraceThreadId method we see that in ASP.NET/GUI it’s the same thread that enters ReadTask and that exits ReadTask ie no problems. When we run it as a Console application we see that ReadTask is entered by one thread and exited by another ie readingFiles is accessed by two separate threads with no synchronization primitives which mean we have a race-condition."

This is not entirely true - the code as written does not have a race condition because the two accesses are run sequentially. Accessing the same variable by two separate threads with no synchronization primitives is actually okay if those two threads never run in parallel. Now, if you called many ReadTask()'s in a row and you had thread_pool > 1 (as in the GUI/ASP.NET application), then you would have a race condition. But if you're accessing a shared variable from a multithreaded context that should be somewhat obvious. It would depend on the programmer's understanding of the async/await paradigm, which I think is the author's point. ;)