blocks|key|2714822|text|异步需要从某个地方开始，因为您的程序有一个起始点，所以在这一点上它是一个同步方法。大多数async都是从我们所说的async+void开始的，基本上就是触发和忘记方法。它启动一个任务，并不关心它返回什么。如果你需要等待同步方法中的某些东西，你可以在任务上使用.Wait()方法，或者使用.Result来获得任务的结果。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2714823|对于您的真实示例，如果您希望同时运行这两个任务，则需要执行以下操作：|2714824|public+async+Task<bool>+ReserveAHoliday()
{
++++++++//Initialize+and+start+this+task
++++++++Task<bool>+hotelTask+=+ReserveAHotel();//HTTP+async+request
++++++++//Initialize+and+start+this+task
++++++++Task<bool>+flightTask+=+ReserveFlight();////HTTP+async+request
++++++++//Await+until+hotel+task+is+done,+and+get+the+bool
++++++++bool+hotelOK+=+await+hotelTask;
++++++++//Await+until+flight+task+is+done+(This+might+have+already+finished+while+the+hotel+was+grabbing,+and+get+the+bool
++++++++bool+flightOK+=+await+flightTask;
++++++++bool+result+=+hotelOK+&&+flightOK;
++++++++//Return+the+final+result
++++++++return+result;
}|code-block|syntax|javascript|2714825|我强烈推荐看this+video。它很好地介绍了异步是如何工作的，并且可以让你快速进入异步的奇妙世界。|2714826|entityMap|0|LINK|mutability|MUTABLE|url|http://channel9.msdn.com/Shows/Going%2BDeep/Mads-Torgersen-Inside-C-Async^0|18|5|1L|A|3L|7|3Z|7|0|0|0|6|A|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|15|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|17|8|@]|D|@$9|18|A|19|1|1A]]|E|$]]|$1|O|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

Your async needs to start somewhere, and since your program has a starting point, it's a synchronous method at this point. Most <code>async</code>'s start at what we like to call <code>async void</code>'s, which are basically fire and forget methods. It starts a task, and doesn't care what it returns. If you need to wait for something in a synchronous method, you can use the <code>.Wait()</code> method on a task, or use <code>.Result</code> to get the result of the task.

For your real world example, if you want to run both tasks at the same time, you would need to do something like this:

<pre><code>public async Task&lt;bool&gt; ReserveAHoliday()
{
 //Initialize and start this task
 Task&lt;bool&gt; hotelTask = ReserveAHotel();//HTTP async request
 //Initialize and start this task
 Task&lt;bool&gt; flightTask = ReserveFlight();////HTTP async request
 //Await until hotel task is done, and get the bool
 bool hotelOK = await hotelTask;
 //Await until flight task is done (This might have already finished while the hotel was grabbing, and get the bool
 bool flightOK = await flightTask;
 bool result = hotelOK &amp;&amp; flightOK;
 //Return the final result
 return result;
}
</code></pre>

I highly recommend watching <a href="http://channel9.msdn.com/Shows/Going+Deep/Mads-Torgersen-Inside-C-Async" rel="nofollow">this video</a>. It gives a nice introduction to how Async works, and can jump start you in the wonderful world of Async.

blocks|key|2504740|text|感谢FrankerZ给出的正确答案！所以我的小例子应该是这样的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2504741|class+Program
{
++++static+void+Main(string[]+args)
++++{
++++++++var+task+=+test();
++++++++task.Wait();+//this+stops+async+behaviour
++++++++int+result+=+task.Result;//+get+return+value+form+method+"test"
++++++++Console.WriteLine("RESULT+IS+=+"+%2B+result);
++++}

++++public+async+static+Task<int>+test()
++++{
++++++++int+a1,+a2,+a3,+a4;

++++++++//run+all+tasks
++++++++//all+tasks+are+doing+their+job+"at+the+same+time"
++++++++var+taskA1+=+GetNumber1();
++++++++var+taskA2+=+GetNumber2();
++++++++var+taskA3+=+GetNumber3();

++++++++//wait+for+task+results
++++++++//here+I+am+collecting+results+from+all+tasks
++++++++a1+=+await+taskA1;
++++++++a2+=+await+taskA2;
++++++++a3+=+await+taskA3;

++++++++//get+value+from+all+task+results
++++++++a4+=+a1+%2B+a2+%2B+a3;
++++++++return+a4;
++++}

++++public+static+async+Task<int>+GetNumber1()
++++{
++++++++await+Task.Run(()+=>
++++++++++++{
++++++++++++++++for+(int+i+=+0;+i+<+10;+i%2B%2B)
++++++++++++++++{
++++++++++++++++++++Console.WriteLine("GetNumber1");
++++++++++++++++++++System.Threading.Thread.Sleep(100);
++++++++++++++++}
++++++++++++});
++++++++return+1;
++++}|code-block|syntax|javascript|2504742|//省略其他方法，因为它们与GetNumber1相同}|2504743|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Thank you FrankerZ for the right answer!
So my litle example should look something like this:

<pre><code>class Program
{
 static void Main(string[] args)
 {
 var task = test();
 task.Wait(); //this stops async behaviour
 int result = task.Result;// get return value form method "test"
 Console.WriteLine("RESULT IS = " + result);
 }

 public async static Task&lt;int&gt; test()
 {
 int a1, a2, a3, a4;

 //run all tasks
 //all tasks are doing their job "at the same time"
 var taskA1 = GetNumber1();
 var taskA2 = GetNumber2();
 var taskA3 = GetNumber3();

 //wait for task results
 //here I am collecting results from all tasks
 a1 = await taskA1;
 a2 = await taskA2;
 a3 = await taskA3;

 //get value from all task results
 a4 = a1 + a2 + a3;
 return a4;
 }

 public static async Task&lt;int&gt; GetNumber1()
 {
 await Task.Run(() =&gt;
 {
 for (int i = 0; i &lt; 10; i++)
 {
 Console.WriteLine("GetNumber1");
 System.Threading.Thread.Sleep(100);
 }
 });
 return 1;
 }
</code></pre>

//other methods are ommited because they are the same like GetNumber1
 }

blocks|key|2504778|text|为了在方法中使用await关键字，它必须为async。这是一个设计决定，使得向后兼容变得更容易。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2504779|Main不能是async，因为它是程序的起点，所以您通常需要以某种方式阻塞以保持程序运行。您可以通过在Task上阻塞甚至调用Console.ReadLine来实现这一点。|2504780|任务通常在后台线程上执行，如果所有前台线程都停止，后台线程不会使程序保持运行。|2504781|我在async-await+here上有一篇介绍性的博客文章|2504782|entityMap|0|LINK|mutability|MUTABLE|url|http://www.nedstoyanov.com/async-await-what-is-it-and-how-do-i-use-it/^0|8|5|L|5|0|0|4|7|5|1F|4|1Q|G|0|0|2|B|E|4|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|17|8|@]|D|@]|E|$]]|$1|J|3|K|5|6|7|18|8|@$9|19|A|1A|B|C]]|D|@$9|1B|A|1C|1|1D]]|E|$]]|$1|L|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|M|$N|$5|O|P|Q|E|$R|S]]]]

In order to use the <code>await</code> keyword inside a method, it has to be <code>async</code>. This was a design decision to make it easier to be backwards compatible.

<code>Main</code> cannot be <code>async</code> as it is the starting point of the program, so you typically need to block somehow to keep the program running. You can do that by blocking on a <code>Task</code> or even by calling <code>Console.ReadLine</code>. 

Tasks usually execute on a background thread and background threads do not keep your program running if all foreground threads stop.

I've got an introductory blog post on <code>async-await</code> <a href="http://www.nedstoyanov.com/async-await-what-is-it-and-how-do-i-use-it/" rel="nofollow">here</a>

blocks|key|36535|text|如果在方法ReserveAHolliday中使用了await关键字，则无法使其同步。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|36536|可以简单地阻塞，直到两个异步方法都返回它们的值。但我不推荐这样做，因为它可能会导致死锁或其他意外错误。|36537|如果您没有同步上下文(例如，控制台应用程序)您可以简单地编写|36538|bool+hotelOK+=+ReserveAHotel().Result;|code-block|syntax|javascript|36539|但我认为你最终还是想要创建一个GUI。这就是阻塞崩溃的地方。因为UI应用程序(窗体和WPF)确实有上下文，等待异步方法会导致死锁。Read+more+here|offset|length|36540|有解决此问题的方法，但它们高度依赖于应用程序的设计。|36541|编辑：|36542|如果您的库代码需要同步上下文，那么阻塞肯定会导致死锁。|36543|如果您不需要它，可以在您的库代码中使用ConfigureAwait(false)并避免阻塞。|style|CODE|36544|我推荐的是:异步/等待，或者根本不异步/等待。还有其他的可能性可以让你的代码异步，例如:基于事件，但就像生活中的任何事情一样，都有优缺点。|36545|请阅读来自Stephen+Cleary的this+blog。他解释了为什么阻塞异步代码会死锁，以及如何避免它。|36546|entityMap|0|LINK|mutability|MUTABLE|url|https://msdn.microsoft.com/en-us/magazine/jj991977.aspx|1|http://blog.stephencleary.com/2012/07/dont-block-on-async-code.html^0|0|0|0|0|1T|E|0|0|0|0|0|J|L|0|0|K|9|1|0^^$0|@$1|2|3|4|5|6|7|1C|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1D|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|1E|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|1F|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|1G|8|@]|9|@$M|1H|N|1I|1|1J]]|A|$]]|$1|O|3|P|5|6|7|1K|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1L|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1M|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1N|8|@$M|1O|N|1P|W|X]]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|10|3|11|5|6|7|1R|8|@]|9|@$M|1S|N|1T|1|1U]]|A|$]]|$1|12|3|-4|5|6|7|1V|8|@]|9|@]|A|$]]]|13|$14|$5|15|16|17|A|$18|19]]|1A|$5|15|16|17|A|$18|1B]]]]

The method ReserveAHolliday cannot be made synchronous if you use the await keyword inside of it.

What could be possible is to simply block till both async methods return their value. I don't recommend it though because it could lead to deadlocks or other unexpected errors.

If you don't have a synchronization context (eg. console application) you can simply write

<pre><code>bool hotelOK = ReserveAHotel().Result;
</code></pre>

But I reckon you ultimately want to create a GUI. That is where blocking falls apart. Because UI application (Forms and WPF) do have a context and waiting for asynchronous methods cause deadlocks. <a href="https://msdn.microsoft.com/en-us/magazine/jj991977.aspx" rel="nofollow">Read more here</a>

There are workarounds for this problem, but they are highly dependent on the design of your application.

Edit: 

If your library code needs Synchronization Context then blocking will most certainly cause a deadlock.

If you don't need it, it's possible to use <code>ConfigureAwait(false)</code> in your library code and get away with blocking.

What I recommend is: async/await all the way or no async/await at all. There are other possibilities to make your code asynchronous, eg: event based, but like everything in life has pros and cons.

Please read <a href="http://blog.stephencleary.com/2012/07/dont-block-on-async-code.html" rel="nofollow">this blog</a> from Stephen Cleary. He explains why blocking async code will deadlock and how you can avoid it.

blocks|key|2714976|text|下面是一个完整的例子。您可以从ReserverAHoliday+(取决于您注释的行)同步(bool+r=+ReserveAHolliday().Result;)和异步(只需调用ReserveAHolliday();)运行MAIN。您可以看到效果(在预订完成之前/之后打印“END”)。我更喜欢await+Task.WhenAll()方法，它更具可读性。还要注意的是，在GetNumber1中使用await+Task.Delay(100)而不是Thread.sleep。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2714977|++++class+Program
{
++++static+void+Main(string[]+args)//main+cant'+be+async
++++{
++++++++//int+res+=+test().Result;//I+must+put+await+here
++++++++bool+r+=+ReserveAHolliday().Result;+//this+will+run+Synchronously.
++++++++//ReserveAHolliday();+//this+option+will+run+aync+:+you+will+see+"END"+printed+before+the+reservation+is+complete.
++++++++Console.WriteLine("END");
++++++++Console.ReadLine();
++++}

++++public+async+static+Task<int>+test()
++++{+//why+can't+I+make+it+just:+public+int+test()??
++++++++//becuase+you+cannot+use+await+in+synchronous+methods.+
++++++++int+a1,+a2,+a3,+a4;

++++++++a1+=+await+GetNumber1();
++++++++a2+=+await+GetNumber1();
++++++++a3+=+await+GetNumber1();

++++++++a4+=+a1+%2B+a2+%2B+a3;
++++++++return+a4;
++++}

++++public+static+async+Task<int>+GetNumber1()
++++{
++++++++//await+Task.Run(()+=>
++++++++//++++{
++++++++++++++++for+(int+i+=+0;+i+<+10;+i%2B%2B)
++++++++++++++++{
++++++++++++++++++++Console.WriteLine("GetNumber1");
++++++++++++++++++++await+Task.Delay(100);+//+from+what+I+read+using+Task.Delay+is+preferred+to+using++System.Threading.Thread.Sleep(100);
++++++++++++++++}
++++++++//++++});
++++++++return+1;
++++}

++++public+async+static+Task<bool>+ReserveAHolliday()
++++{
++++++++//bool+hotelOK+=+await+ReserveAHotel();//HTTP+async+request
++++++++//bool+flightOK+=+await+ReserveAHotel();////HTTP+async+request
++++++++var+t1+=+ReserveAHotel("FirstHotel");
++++++++var+t2+=+ReserveAHotel("SecondHotel");
++++++++await+Task.WhenAll(t1,+t2);
++++++++bool+result+=+t1.Result+&&+t1.Result;//+hotelOK+&&+flightOK;
++++++++return+result;
++++}
++++public+static+async+Task<bool>+ReserveAHotel(string+name)
++++{
++++++++Console.WriteLine("Reserve+A+Hotel+started+for+"%2B+name);
++++++++await+Task.Delay(3000);
++++++++if+(name+==+"FirstHotel")+
++++++++++++await+Task.Delay(500);+//delaying+first+hotel+on+purpose.
++++++++Console.WriteLine("Reserve+A+Hotel+done+for+"+%2B+name);
++++++++return+true;
++++}
}|code-block|syntax|javascript|2714978|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

below is a full example. you can run the ReserverAHoliday both Synchronously (bool r = ReserveAHolliday().Result;) and Asynchronously (just call ReserveAHolliday();) from MAIN (depends which line you comment). and you can see the effect ("END" gets printed before / after the reservation is complete).
I prefer the await Task.WhenAll() methods, which is more readable.
also note that it's preferred to use await Task.Delay(100) instead of Thread.sleep inside GetNumber1.

<pre><code> class Program
{
 static void Main(string[] args)//main cant' be async
 {
 //int res = test().Result;//I must put await here
 bool r = ReserveAHolliday().Result; //this will run Synchronously.
 //ReserveAHolliday(); //this option will run aync : you will see "END" printed before the reservation is complete.
 Console.WriteLine("END");
 Console.ReadLine();
 }

 public async static Task&lt;int&gt; test()
 { //why can't I make it just: public int test()??
 //becuase you cannot use await in synchronous methods. 
 int a1, a2, a3, a4;

 a1 = await GetNumber1();
 a2 = await GetNumber1();
 a3 = await GetNumber1();

 a4 = a1 + a2 + a3;
 return a4;
 }

 public static async Task&lt;int&gt; GetNumber1()
 {
 //await Task.Run(() =&gt;
 // {
 for (int i = 0; i &lt; 10; i++)
 {
 Console.WriteLine("GetNumber1");
 await Task.Delay(100); // from what I read using Task.Delay is preferred to using System.Threading.Thread.Sleep(100);
 }
 // });
 return 1;
 }

 public async static Task&lt;bool&gt; ReserveAHolliday()
 {
 //bool hotelOK = await ReserveAHotel();//HTTP async request
 //bool flightOK = await ReserveAHotel();////HTTP async request
 var t1 = ReserveAHotel("FirstHotel");
 var t2 = ReserveAHotel("SecondHotel");
 await Task.WhenAll(t1, t2);
 bool result = t1.Result &amp;&amp; t1.Result;// hotelOK &amp;&amp; flightOK;
 return result;
 }
 public static async Task&lt;bool&gt; ReserveAHotel(string name)
 {
 Console.WriteLine("Reserve A Hotel started for "+ name);
 await Task.Delay(3000);
 if (name == "FirstHotel") 
 await Task.Delay(500); //delaying first hotel on purpose.
 Console.WriteLine("Reserve A Hotel done for " + name);
 return true;
 }
}
</code></pre>

Can someone clarify this example, which of course, is not working:

<pre><code>class Program
{
 static void Main(string[] args)//main cant' be async
 {
 int res = test();//I must put await here

 Console.WriteLine("END");
 }

 public async static Task&lt;int&gt; test()
 { //why can't I make it just: public int test()??
 int a1, a2, a3, a4;

 a1 = await GetNumber1();
 a2 = await GetNumber2();
 a3 = await GetNumber3();

 a4 = a1 + a2 + a3;
 return a4;
 }

 public static async Task&lt;int&gt; GetNumber1()
 {
 await Task.Run(() =&gt;
 {
 for (int i = 0; i &lt; 10; i++)
 {
 Console.WriteLine("GetNumber1");
 System.Threading.Thread.Sleep(100);
 }
 });
 return 1;
 }
</code></pre>

I am trying to "collect" values from GenNumberX methods by using "await". I would like to make method "test" not async somehow. I dont understand why test has to be async when I am using await to get a value. This example makes me to write async on every method where I use async, and when I drill up to Main, I cannot make it async?

How to write real-world example:

<pre><code>public bool ReserveAHolliday()
{
 bool hotelOK = await ReserveAHotel();//HTTP async request
 bool flightOK = await ReserveFlight();////HTTP async request
 bool result = hotelOK &amp;&amp; flightOK;
 return result;
}
</code></pre>

How to make method ReserveAHolliday synchronous?
Am I missing something or don't understand the use of async-await mechanism?

C# Async method call all the way to Main

有人能澄清一下这个例子吗?当然，这个例子不起作用：class Program{    static void Main(string[] args)//main cant' be async    {        int res = test();//I must put await here        Cons...

问C#异步方法调用一直到Main
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