.NET async/await in a single picture

The main power behind async/await is that while we “await” on an ongoing I/O operation, the calling thread may be released for doing other work. And this provides a great thread re-usability. Thus, better scalability – much smaller number of threads is able to handle the same amount of operations comparing to asynchronous/waiting approach. The main role here plays so-called overlapped I/O (in case of Windows) which allows to asynchronously delegate the I/O operation to the operating system, and only after completion the provided callback will notify us about the result. The main workforce here is so-called I/O completion port (IOCP).