c# - How do DownloadStringTaskAsync and DownloadStringAsync manage to not block the UI thread?

Question

As far as I know, they both do not have the "async" keyword. Do they both create a new thread when downloading? I've heard people say that they do not use a new thread. Then how does it do it?

The normal DownloadString method does indeed block the UI thread, but how do the other 2 methods manage to do it without using the "await" keyword to return from the method and wait until the operation is completed

Answer 1

As far as I know, they both do not have the "async" keyword. Do they both create a new thread when downloading?

True, they do not use async. But you don't have to use async in order to be asynchronous.

They do not create new threads, if by that you mean that they just queue the blocking work to the thread pool. They do "use thread resources" when the operation completes, though I would say that documentation is misleading.

I've heard people say that they do not use a new thread. Then how does it do it?

See There Is No Thread. In summary, true asynchronous I/O uses callbacks, which generally ends at a completion port, which in .NET is part of the thread pool. However, there is not a thread that is blocked doing the I/O; rather, the thread pool's I/O threads (not worker threads) are used to service the completion port work as it completes.