blocks|key|188888|text|我不推荐使用StartNew，除非您需要这种级别的复杂性。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|188889|如果您的异步方法依赖于其他异步方法，最简单的方法是使用async关键字：|188890|private+static+async+Task<DateTime>+CountToAsync(int+num+=+10)
{
++for+(int+i+=+0;+i+<+num;+i%2B%2B)
++{
++++await+Task.Delay(TimeSpan.FromSeconds(1));
++}

++return+DateTime.Now;
}|code-block|syntax|javascript|188891|如果异步方法正在执行CPU工作，则应使用Task.Run。|188892|private+static+async+Task<DateTime>+CountToAsync(int+num+=+10)
{
++await+Task.Run(()+=>+...);
++return+DateTime.Now;
}|188893|你可能会发现我的async/await+intro很有帮助。|188894|entityMap|0|LINK|mutability|MUTABLE|url|http://blog.stephencleary.com/2012/02/async-and-await.html^0|6|8|0|R|5|0|0|K|8|0|0|8|5|E|5|8|H|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@$9|14|A|15|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|17|8|@$9|18|A|19|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|1A|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1B|8|@$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]]|D|@$9|1G|A|1H|1|1I]]|E|$]]|$1|S|3|-4|5|6|7|1J|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]]]

I don't recommend <code>StartNew</code> unless you need that level of complexity.

If your async method is dependent on other async methods, the easiest approach is to use the <code>async</code> keyword:

<pre><code>private static async Task&lt;DateTime&gt; CountToAsync(int num = 10)
{
 for (int i = 0; i &lt; num; i++)
 {
 await Task.Delay(TimeSpan.FromSeconds(1));
 }

 return DateTime.Now;
}
</code></pre>

If your async method is doing CPU work, you should use <code>Task.Run</code>:

<pre><code>private static async Task&lt;DateTime&gt; CountToAsync(int num = 10)
{
 await Task.Run(() =&gt; ...);
 return DateTime.Now;
}
</code></pre>

You may find my <a href="http://blog.stephencleary.com/2012/02/async-and-await.html"><code>async</code>/<code>await</code> intro</a> helpful.

blocks|key|190766|text|如果您不想在方法内部使用async/await，但仍然“修饰”它，以便能够从外部使用await关键字，则可以使用TaskCompletionSource.cs|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|190767|public+static+Task<T>+RunAsync<T>(Func<T>+function)
{+
++++if+(function+==+null)+throw+new+ArgumentNullException(“function”);+
++++var+tcs+=+new+TaskCompletionSource<T>();+
++++ThreadPool.QueueUserWorkItem(_+=>++++++++++
++++{+
++++++++try+
++++++++{++
+++++++++++T+result+=+function();+
+++++++++++tcs.SetResult(result);++
++++++++}+
++++++++catch(Exception+exc)+{+tcs.SetException(exc);+}+
+++});+
+++return+tcs.Task;+
}|code-block|syntax|javascript|190768|From+here和here|190769|190770|为了支持任务的这种范例，我们需要一种方法来保留任务外观和将任意异步操作引用为任务的能力，但根据提供异步的底层基础设施的规则来控制该任务的生命周期，并且以一种成本不高的方式做到这一点。这就是TaskCompletionSource的用途。|blockquote|190771|190772|190773|我看到它也用在.NET源代码中，比如WebClient.cs|190774|++++[HostProtection(ExternalThreading+=+true)]
++++[ComVisible(false)]
++++public+Task<string>+UploadStringTaskAsync(Uri+address,+string+method,+string+data)
++++{
++++++++//+Create+the+task+to+be+returned
++++++++var+tcs+=+new+TaskCompletionSource<string>(address);

++++++++//+Setup+the+callback+event+handler
++++++++UploadStringCompletedEventHandler+handler+=+null;
++++++++handler+=+(sender,+e)+=>+HandleCompletion(tcs,+e,+(args)+=>+args.Result,+handler,+(webClient,+completion)+=>+webClient.UploadStringCompleted+-=+completion);
++++++++this.UploadStringCompleted+%2B=+handler;

++++++++//+Start+the+async+operation.
++++++++try+{+this.UploadStringAsync(address,+method,+data,+tcs);+}
++++++++catch
++++++++{
++++++++++++this.UploadStringCompleted+-=+handler;
++++++++++++throw;
++++++++}

++++++++//+Return+the+task+that+represents+the+async+operation
++++++++return+tcs.Task;
++++}|190775|最后，我还发现以下内容很有用：|190776|190777|我一直被问到这个问题。这意味着一定有某个线程阻塞了对外部资源的I/O调用。因此，异步代码释放了请求线程，但只是以系统中其他地方的另一个线程为代价，对吧？没关系，您请。|190778|190779|为了理解异步请求扩展的原因，我将跟踪一个异步I/O调用的(简化的)示例。假设一个请求需要写入一个文件。请求线程调用异步写方法。WriteAsync由基类库实现，并使用完成端口进行异步I/O。因此，WriteAsync调用作为异步文件写入向下传递到操作系统。然后，OS与驱动程序堆栈进行通信，传递要写入I/O请求数据包(IRP)的数据。|190780|这就是事情变得有趣的地方:如果设备驱动程序不能立即处理IRP，它必须异步处理它。因此，驱动程序告诉磁盘开始写入，并向操作系统返回“挂起”响应。OS将“挂起”响应传递给BCL，然后BCL向请求处理代码返回未完成的任务。请求处理代码等待任务，该任务从该方法返回未完成的任务，依此类推。最后，请求处理代码最终将一个未完成的任务返回给ASP.NET，请求线程被释放以返回线程池。|190781|190782|Introduction+to+Async/Await+on+ASP.NET|190783|如果目标是提高可伸缩性(而不是响应性)，则完全依赖于外部I/O的存在，该I/O提供了实现该目标的机会。|190784|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.taskcompletionsource-1|1|https://blogs.msdn.microsoft.com/pfxteam/2009/06/02/the-nature-of-taskcompletionsourcetresult/|2|https://msdn.microsoft.com/en-us/library/hh873177(v=vs.110).aspx|3|https://referencesource.microsoft.com/#System/net/System/Net/webclient.cs,c113b360583ad964|4|https://msdn.microsoft.com/en-gb/magazine/dn802603.aspx^0|1K|N|0|0|0|0|9|1|A|4|2|0|0|0|0|0|I|C|3|0|0|0|0|0|0|0|0|0|0|12|4|0|0^^$0|@$1|2|3|4|5|6|7|1P|8|@]|9|@$A|1Q|B|1R|1|1S]]|C|$]]|$1|D|3|E|5|F|7|1T|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|1U|8|@]|9|@$A|1V|B|1W|1|1X]|$A|1Y|B|1Z|1|20]]|C|$]]|$1|K|3|-4|5|6|7|21|8|@]|9|@]|C|$]]|$1|L|3|M|5|N|7|22|8|@]|9|@]|C|$]]|$1|O|3|-4|5|6|7|23|8|@]|9|@]|C|$]]|$1|P|3|-4|5|6|7|24|8|@]|9|@]|C|$]]|$1|Q|3|R|5|6|7|25|8|@]|9|@$A|26|B|27|1|28]]|C|$]]|$1|S|3|T|5|F|7|29|8|@]|9|@]|C|$G|H]]|$1|U|3|V|5|6|7|2A|8|@]|9|@]|C|$]]|$1|W|3|-4|5|6|7|2B|8|@]|9|@]|C|$]]|$1|X|3|Y|5|N|7|2C|8|@]|9|@]|C|$]]|$1|Z|3|-4|5|6|7|2D|8|@]|9|@]|C|$]]|$1|10|3|11|5|6|7|2E|8|@]|9|@]|C|$]]|$1|12|3|13|5|6|7|2F|8|@]|9|@]|C|$]]|$1|14|3|-4|5|6|7|2G|8|@]|9|@]|C|$]]|$1|15|3|16|5|6|7|2H|8|@]|9|@$A|2I|B|2J|1|2K]]|C|$]]|$1|17|3|18|5|6|7|2L|8|@]|9|@]|C|$]]|$1|19|3|-4|5|6|7|2M|8|@]|9|@]|C|$]]]|1A|$1B|$5|1C|1D|1E|C|$1F|1G]]|1H|$5|1C|1D|1E|C|$1F|1I]]|1J|$5|1C|1D|1E|C|$1F|1K]]|1L|$5|1C|1D|1E|C|$1F|1M]]|1N|$5|1C|1D|1E|C|$1F|1O]]]]

If you didn't want to use async/await inside your method, but still &quot;decorate&quot; it so as to be able to use the await keyword from outside, <a href="https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.taskcompletionsource-1" rel="nofollow noreferrer">TaskCompletionSource.cs</a>:
<pre><code>public static Task&lt;T&gt; RunAsync&lt;T&gt;(Func&lt;T&gt; function)
{ 
 if (function == null) throw new ArgumentNullException(“function”); 
 var tcs = new TaskCompletionSource&lt;T&gt;(); 
 ThreadPool.QueueUserWorkItem(_ =&gt; 
 { 
 try 
 { 
 T result = function(); 
 tcs.SetResult(result); 
 } 
 catch(Exception exc) { tcs.SetException(exc); } 
 }); 
 return tcs.Task; 
}
</code></pre>
<a href="https://blogs.msdn.microsoft.com/pfxteam/2009/06/02/the-nature-of-taskcompletionsourcetresult/" rel="nofollow noreferrer">From here</a> and <a href="https://msdn.microsoft.com/en-us/library/hh873177(v=vs.110).aspx" rel="nofollow noreferrer">here</a>
<blockquote>
To support such a paradigm with Tasks, we need a way to retain the Task façade and the ability to refer to an arbitrary asynchronous operation as a Task, but to control the lifetime of that Task according to the rules of the underlying infrastructure that’s providing the asynchrony, and to do so in a manner that doesn’t cost significantly. This is the purpose of TaskCompletionSource.
</blockquote>
I saw it's also used in the .NET source, e.g. <a href="https://referencesource.microsoft.com/#System/net/System/Net/webclient.cs,c113b360583ad964" rel="nofollow noreferrer">WebClient.cs</a>:
<pre><code> [HostProtection(ExternalThreading = true)]
 [ComVisible(false)]
 public Task&lt;string&gt; UploadStringTaskAsync(Uri address, string method, string data)
 {
 // Create the task to be returned
 var tcs = new TaskCompletionSource&lt;string&gt;(address);

 // Setup the callback event handler
 UploadStringCompletedEventHandler handler = null;
 handler = (sender, e) =&gt; HandleCompletion(tcs, e, (args) =&gt; args.Result, handler, (webClient, completion) =&gt; webClient.UploadStringCompleted -= completion);
 this.UploadStringCompleted += handler;

 // Start the async operation.
 try { this.UploadStringAsync(address, method, data, tcs); }
 catch
 {
 this.UploadStringCompleted -= handler;
 throw;
 }

 // Return the task that represents the async operation
 return tcs.Task;
 }
</code></pre>
Finally, I also found the following useful:
<blockquote>
I get asked this question all the time. The implication is that there must be some thread somewhere that’s blocking on the I/O call to the external resource. So, asynchronous code frees up the request thread, but only at the expense of another thread elsewhere in the system, right? No, not at all.
To understand why asynchronous requests scale, I’ll trace a (simplified) example of an asynchronous I/O call. Let’s say a request needs to write to a file. The request thread calls the asynchronous write method. WriteAsync is implemented by the Base Class Library (BCL), and uses completion ports for its asynchronous I/O. So, the WriteAsync call is passed down to the OS as an asynchronous file write. The OS then communicates with the driver stack, passing along the data to write in an I/O request packet (IRP).
This is where things get interesting: If a device driver can’t handle an IRP immediately, it must handle it asynchronously. So, the driver tells the disk to start writing and returns a “pending” response to the OS. The OS passes that “pending” response to the BCL, and the BCL returns an incomplete task to the request-handling code. The request-handling code awaits the task, which returns an incomplete task from that method and so on. Finally, the request-handling code ends up returning an incomplete task to ASP.NET, and the request thread is freed to return to the thread pool.
</blockquote>
<a href="https://msdn.microsoft.com/en-gb/magazine/dn802603.aspx" rel="nofollow noreferrer">Introduction to Async/Await on ASP.NET</a>
If the target is to improve scalability (rather than responsiveness), it all relies on the existence of an external I/O that provides the opportunity to do that.

blocks|key|188922|text|使方法异步的一种非常简单的方法是使用Task.Yield()方法。如MSDN所述：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|188923|可以在异步方法中使用|188924|+Task.Yield()；强制该方法异步完成。|blockquote|188925|188926|188927|在方法的开头插入它，然后它将立即返回给调用者，并在另一个线程上完成方法的其余部分。|188928|private+async+Task<DateTime>+CountToAsync(int+num+=+1000)
{
++++await+Task.Yield();
++++for+(int+i+=+0;+i+<+num;+i%2B%2B)
++++{
++++++++Console.WriteLine("#{0}",+i);
++++}
++++return+DateTime.Now;
}|code-block|syntax|javascript|188929|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|S|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|T|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|U|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|W|8|@]|9|@]|A|$]]|$1|K|3|L|5|M|7|X|8|@]|9|@]|A|$N|O]]|$1|P|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|Q|$]]

One very simple way to make a method asynchronous is to use Task.Yield() method. As MSDN states:

<blockquote>
 You can use await Task.Yield(); in an asynchronous method to force the
 method to complete asynchronously.
</blockquote>

Insert it at beginning of your method and it will then return immediately to the caller and complete the rest of the method on another thread.

<pre><code>private async Task&lt;DateTime&gt; CountToAsync(int num = 1000)
{
 await Task.Yield();
 for (int i = 0; i &lt; num; i++)
 {
 Console.WriteLine("#{0}", i);
 }
 return DateTime.Now;
}
</code></pre>

Every blog post I've read tells you how to consume an asynchronous method in C#, but for some odd reason never explain how to build your own asynchronous methods to consume. So I have this code right now that consumes my method:

<pre class="lang-cs prettyprint-override"><code>private async void button1_Click(object sender, EventArgs e)
{
 var now = await CountToAsync(1000);
 label1.Text = now.ToString();
}
</code></pre>

And I wrote this method that is <code>CountToAsync</code>:

<pre class="lang-cs prettyprint-override"><code>private Task&lt;DateTime&gt; CountToAsync(int num = 1000)
{
 return Task.Factory.StartNew(() =&gt;
 {
 for (int i = 0; i &lt; num; i++)
 {
 Console.WriteLine("#{0}", i);
 }
 }).ContinueWith(x =&gt; DateTime.Now);
}
</code></pre>

Is this, the use of <code>Task.Factory</code>, the best way to write an asynchronous method, or should I write this another way?

How do you create an asynchronous method in C#?

设备驱动程序

我读过的每一篇博客文章都告诉你如何在C#中使用异步方法，但出于某种奇怪的原因，从来没有解释过如何构建自己的异步方法来使用。所以我现在有这个代码来使用我的方法：private async void button1_Click(object sender, EventArgs e){    var now = await ...

问如何在C#中创建异步方法？
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