C#异步方法调用一直到Main
C#异步方法调用一直到Main
提问于 2015-05-26 17:26:19
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
Console.WriteLine("END");
}
public async static Task<int> 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<int> GetNumber1()
{
await Task.Run(() =>
{
for (int i = 0; i < 10; i++)
{
Console.WriteLine("GetNumber1");
System.Threading.Thread.Sleep(100);
}
});
return 1;
}我正在尝试使用"await“从GenNumberX方法中”收集“值。我想以某种方式使方法“测试”而不是异步。我不明白为什么当我使用await来获取一个值时，测试必须是异步的。这个示例让我在使用异步的每个方法上编写异步，当我向上钻取到Main时，我不能使它异步？
如何编写真实世界的示例：
public bool ReserveAHolliday()
{
bool hotelOK = await ReserveAHotel();//HTTP async request
bool flightOK = await ReserveFlight();////HTTP async request
bool result = hotelOK && flightOK;
return result;
}如何使方法ReserveAHolliday同步？我是不是遗漏了什么或者不理解异步等待机制的用法？
回答 5
Stack Overflow用户
回答已采纳
发布于 2015-05-26 18:38:49
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。
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 + a2 + a3;
return a4;
}
public static async Task<int> GetNumber1()
{
//await Task.Run(() =>
// {
for (int i = 0; i < 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<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 "+ 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;
}
}Stack Overflow用户
发布于 2015-05-26 17:35:57
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。
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：
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;
}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。
Stack Overflow用户
发布于 2015-05-26 17:46:34
感谢FrankerZ给出的正确答案！所以我的小例子应该是这样的：
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<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 + a2 + a3;
return a4;
}
public static async Task<int> GetNumber1()
{
await Task.Run(() =>
{
for (int i = 0; i < 10; i++)
{
Console.WriteLine("GetNumber1");
System.Threading.Thread.Sleep(100);
}
});
return 1;
}//省略其他方法，因为它们与GetNumber1相同}
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原文链接：
https://stackoverflow.com/questions/30454613
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