c# - Async/Await vs Threads

Question

In .Net 4.5 Microsoft has added the new Async/Await feature to simplify asynchronous coding. However, I wonder

1. Can Async/Await completely replace the old way of using Threads?
2. Is Async/Await capable of doing whatever a Thread can do asynchronously?
3. Can Async/Await only be used with some methods like WebClient.DownloadStringAsync or can I convert any synchronous method to make it use Async/Await and not to block the main thread?

Answer 1

can it completely replace the old way of using Threads ?

No. A thread can do many more useful things. Await is specifically designed to deal with something taking time, most typically an I/O request. Which traditionally was done with a callback when the I/O request was complete. Writing code that relies on these callbacks is quite difficult, await greatly simplifies it.

capable of doing what ever a Thread can do asynchronously ?

Roughly. Await just takes care of dealing with the delay, it doesn't otherwise do anything that a thread does. The await expression, what's at the right of the await keyword, is what gets the job done. Ideally it doesn't use a thread at all, it posts a driver request and once the driver completes the data transfer it generates a completion notification callback. Networking is by far the most common usage, latencies of hundreds of milliseconds are common and an inevitable side-effect of services moving from the desktop or a LAN into "the cloud". Using such services synchronously would make a UI quite unresponsive.

only can be used with some methods like WebClient.DownloadStringAsync

No. You can use it with any method that returns a Task. The XxxxAsync() methods are just precooked ones in the .NET framework for common operations that take time. Like downloading data from a web server.