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    <p>First time when I saw <strong>async</strong> and <strong>await</strong>, I thougth they were C# Syntactic sugar for Asynchronous Programming Model. I was wrong, <strong>async</strong> and <strong>await</strong> are more than that. It is a brand new asynchronous pattern Task-based Asynchronous Pattern, <a href="http://www.microsoft.com/en-us/download/details.aspx?id=19957" rel="nofollow noreferrer">http://www.microsoft.com/en-us/download/details.aspx?id=19957</a> is a good article to get start. Most of the FCL classes which inplement TAP are call APM methods (BegingXXX() and EndXXX()). Here are two code snaps for TAP and AMP:</p> <p>TAP sample:</p> <pre><code> static void Main(string[] args) { GetResponse(); Console.ReadLine(); } private static async Task&lt;WebResponse&gt; GetResponse() { var webRequest = WebRequest.Create("http://www.google.com"); Task&lt;WebResponse&gt; response = webRequest.GetResponseAsync(); Console.WriteLine(new StreamReader(response.Result.GetResponseStream()).ReadToEnd()); return response.Result; } </code></pre> <p>APM sample:</p> <pre><code> static void Main(string[] args) { var webRequest = WebRequest.Create("http://www.google.com"); webRequest.BeginGetResponse(EndResponse, webRequest); Console.ReadLine(); } static void EndResponse(IAsyncResult result) { var webRequest = (WebRequest) result.AsyncState; var response = webRequest.EndGetResponse(result); Console.WriteLine(new StreamReader(response.GetResponseStream()).ReadToEnd()); } </code></pre> <p>Finally these two will be the same, because GetResponseAsync() call BeginGetResponse() and EndGetResponse() inside. When we reflector the source code of GetResponseAsync(), we will get code like this:</p> <pre><code>task = Task&lt;WebResponse&gt;.Factory.FromAsync( new Func&lt;AsyncCallback, object, IAsyncResult&gt;(this.BeginGetResponse), new Func&lt;IAsyncResult, WebResponse&gt;(this.EndGetResponse), null); </code></pre> <p>For APM, in the BeginXXX(), there is an argument for a callback method which will invoked when the task (typically is an IO heavy operation) was completed. Creating a new thread and asynchronous, both of them will immediately return in main thread, both of them are unblocked. On performance side, creating new thread will cost more resource when process I/O-bound operations such us read file, database operation and network read. There are two disadvantages in creating new thread, </p> <ol> <li>like in your mentioned article, there are memory cost and CLR are<br> limitation on thread pool. </li> <li>Context switch will happen. On the other hander, asynchronous will not create any thread manually and it will not have context switch when the the IO-bound operations return.</li> </ol> <p>Here is an picture which can help to understand the differences:</p> <p><img src="https://i.stack.imgur.com/q3xU7.gif" alt="enter image description here"></p> <p>This diagram is from a MSDN article "<a href="http://msdn.microsoft.com/en-us/magazine/cc163725.aspx" rel="nofollow noreferrer">Asynchronous Pages in ASP.NET 2.0</a>", which explain very detail about how the old asynchronous working in ASP.NET 2.0. </p> <p>About Asynchronous Programming Model, please get more detail from Jeffrey Richter's article "<a href="http://msdn.microsoft.com/en-us/magazine/cc163467.aspx" rel="nofollow noreferrer">Implementing the CLR Asynchronous Programming Model</a>", also there are more detail on his book "CLR via Csharp 3rd Edition" in chapter 27.</p>
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    1. POHow different async programming is from Threads?
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    1. COCan you elaborate this "asynchronous will not create any thread manually and it will not have context switch when the the IO-bound operations return." little bit more please?
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    2. CO@Tarik, acturally it's thread pool not asynchronous, because of most CLR APM implementation using thread pool. For the context swith, when we using dedicate thread do synchronous IO-bound operation, during the IO operation the thread will bolked untill complete, if the operation is longer than the "time slice" then context swith will happen. In the asyncnchronous, please look carefull on the right side of picture, the first thread call BeginXxx(), it was complete all and will return back to thread pool, after the IO operation complete, it will continue on a second thread.
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