c# - The async and await keywords don't cause additional threads to be created?

Question

I'm confused. How can one or many Task run in parallel on a single thread? My understanding of parallelism is obviously wrong.

Bits of MSDN I can't wrap my head around:

The async and await keywords don't cause additional threads to be created. Async methods don't require multithreading because an async method doesn't run on its own thread. The method runs on the current synchronization context and uses time on the thread only when the method is active.

.. and:

Between starting a task and awaiting it, you can start other tasks. The additional tasks implicitly run in parallel, but no additional threads are created.

Answer 1

They don't run in parallel, they take turns. When progress is blocked for the running Task, it stores its state and yields control to a ready Task. It's cooperative multitasking, not true parallelism.

Threads operate on the sample principle. However there are several key differences I'd like to highlight.

First, simply because async/await aren't OS threads:

• Tasks won't see different Thread IDs
• Thread-local storage is not automatically context-switched when a Task yields.

Secondly, differences in behavior:

• async/await use cooperative multitasking, Win32 threads use pre-emption. So it's necessary that all blocking operations explicitly yield control using the async/await model. So you can end up blocking an entire thread and all its Tasks by making a blocking call to a function not written to yield.
• Tasks won't be executed in parallel on a multiprocessing system. Since re-entrancy is controlled, this makes keeping data structures consistent a whole lot easier.

As Stephen points out in a comment, you can get simultaneous execution in multiple OS threads (along with all the complexity and potential race conditions) if you use a multithreaded synchronization context. But the MSDN quotes were about the single-threaded context case.

Finally, other places this same design paradigm is used, you can learn a lot about good practices for async/await by studying these:

• Win32 Fibers (uses the same call style as threads, but cooperative)
• Win32 Overlapped I/O operations, Linux aio
• Coroutines