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    <blockquote> <p>I believe the TPL (TaskFactory.Startnew) works similar to ThreadPool.QueueUserWorkItem in that it queues up work on a thread in the thread pool.</p> </blockquote> <p><a href="https://blog.stephencleary.com/2010/08/various-implementations-of-asynchronous.html" rel="noreferrer">Pretty much</a>.</p> <blockquote> <p>From what i've been reading it seems like async/await only "sometimes" creates a new thread. </p> </blockquote> <p>Actually, it never does. If you want multithreading, you have to implement it yourself. There's a new <code>Task.Run</code> method that is just shorthand for <code>Task.Factory.StartNew</code>, and it's probably the most common way of starting a task on the thread pool.</p> <blockquote> <p>If you were dealing with IO completion ports i can see it not having to create a new thread but otherwise i would think it would have to.</p> </blockquote> <p>Bingo. So methods like <code>Stream.ReadAsync</code> will actually create a <code>Task</code> wrapper around an IOCP (if the <code>Stream</code> has an IOCP).</p> <p>You can also create some non-I/O, non-CPU "tasks". A simple example is <code>Task.Delay</code>, which returns a task that completes after some time period.</p> <p>The cool thing about <code>async</code>/<code>await</code> is that you can queue some work to the thread pool (e.g., <code>Task.Run</code>), do some I/O-bound operation (e.g., <code>Stream.ReadAsync</code>), and do some other operation (e.g., <code>Task.Delay</code>)... and they're all tasks! They can be awaited or used in combinations like <code>Task.WhenAll</code>.</p> <p>Any method that returns <code>Task</code> can be <code>await</code>ed - it doesn't have to be an <code>async</code> method. So <code>Task.Delay</code> and I/O-bound operations just use <code>TaskCompletionSource</code> to create and complete a task - the only thing being done on the thread pool is the actual task completion when the event occurs (timeout, I/O completion, etc).</p> <blockquote> <p>I guess my understanding of FromCurrentSynchronizationContext always was a bit fuzzy also. I always throught it was, in essence, the UI thread.</p> </blockquote> <p>I wrote <a href="http://msdn.microsoft.com/en-us/magazine/gg598924.aspx" rel="noreferrer">an article</a> on <code>SynchronizationContext</code>. Most of the time, <code>SynchronizationContext.Current</code>:</p> <ul> <li>is a UI context if the current thread is a UI thread.</li> <li>is an ASP.NET request context if the current thread is servicing an ASP.NET request.</li> <li>is a thread pool context otherwise.</li> </ul> <p>Any thread <em>can</em> set its own <code>SynchronizationContext</code>, so there are exceptions to the rules above.</p> <p>Note that the default <code>Task</code> awaiter will schedule the remainder of the <code>async</code> method on the current <code>SynchronizationContext</code> <em>if it is not null</em>; otherwise it goes on the current <code>TaskScheduler</code>. This isn't so important today, but in the near future it will be an important distinction.</p> <p>I wrote my own <a href="https://blog.stephencleary.com/2012/02/async-and-await.html" rel="noreferrer"><code>async</code>/<code>await</code> intro</a> on my blog, and Stephen Toub recently posted an excellent <a href="http://blogs.msdn.com/b/pfxteam/archive/2012/04/12/10293335.aspx" rel="noreferrer"><code>async</code>/<code>await</code> FAQ</a>.</p> <p>Regarding "concurrency" vs "multithreading", see <a href="https://stackoverflow.com/questions/7663101/c-sharp-5-async-await-is-it-concurrent">this related SO question</a>. I would say <code>async</code> enables concurrency, which may or may not be multithreaded. It's easy to use <code>await Task.WhenAll</code> or <code>await Task.WhenAny</code> to do concurrent processing, and unless you explicitly use the thread pool (e.g., <code>Task.Run</code> or <code>ConfigureAwait(false)</code>), then you can have multiple concurrent operations in progress at the same time (e.g., multiple I/O or other types like <code>Delay</code>) - and there is no thread needed for them. I use the term "single-threaded concurrency" for this kind of scenario, though in an ASP.NET host, you can actually end up with "<em>zero</em>-threaded concurrency". Which is pretty sweet.</p>
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