如何在 Main 中调用异步方法？

Question

public class test
{
    public async Task Go()
    {
        await PrintAnswerToLife();
        Console.WriteLine("done");
    }

    public async Task PrintAnswerToLife()
    {
        int answer = await GetAnswerToLife();
        Console.WriteLine(answer);
    }

    public async Task<int> GetAnswerToLife()
    {
        await Task.Delay(5000);
        int answer = 21 * 2;
        return answer;
    }
}

if I want to call Go in main() method, how can I do that? I am trying out c# new features, I know i can hook the async method to a event and by triggering that event, async method can be called.

But what if I want to call it directly in main method? How can i do that?

I did something like

class Program
{
    static void Main(string[] args)
    {
        test t = new test();
        t.Go().GetAwaiter().OnCompleted(() =>
        {
            Console.WriteLine("finished");
        });
        Console.ReadKey();
    }


}

But seems it's a dead lock and nothing is printed on the screen.

Answer 1

Your Main method can be simplified. For C# 7.1 and newer:

static async Task Main(string[] args)
{
    test t = new test();
    await t.Go();
    Console.WriteLine("finished");
    Console.ReadKey();
}

For earlier versions of C#:

static void Main(string[] args)
{
    test t = new test();
    t.Go().Wait();
    Console.WriteLine("finished");
    Console.ReadKey();
}

This is part of the beauty of the async keyword (and related functionality): the use and confusing nature of callbacks is greatly reduced or eliminated.

Answer 2

除了等待，您最好使用new test().Go().GetAwaiter().GetResult()因为这将避免将异常包装到 AggregateExceptions 中，因此您可以使用 try catch 包围 Go() 方法(Exception ex) 像往常一样阻止。

Answer 3

Since the release of C# v7.1 async main methods have become available to use which avoids the need for the workarounds in the answers already posted. The following signatures have been added:

public static Task Main();
public static Task<int> Main();
public static Task Main(string[] args);
public static Task<int> Main(string[] args);

This allows you to write your code like this:

static async Task Main(string[] args)
{
    await DoSomethingAsync();
}

static async Task DoSomethingAsync()
{
    //...
}

Answer 4

class Program
{
    static void Main(string[] args)
    {
       test t = new test();
       Task.Run(async () => await t.Go());
    }
}

Answer 5

As long as you are accessing the result object from the returned task, there is no need to use GetAwaiter at all (Only in case you are accessing the result).

static async Task<String> sayHelloAsync(){

       await Task.Delay(1000);
       return "hello world";

}

static void main(string[] args){

      var data = sayHelloAsync();
      //implicitly waits for the result and makes synchronous call. 
      //no need for Console.ReadKey()
      Console.Write(data.Result);
      //synchronous call .. same as previous one
      Console.Write(sayHelloAsync().GetAwaiter().GetResult());

}

if you want to wait for a task to be done and do some further processing:

sayHelloAsyn().GetAwaiter().OnCompleted(() => {
   Console.Write("done" );
});
Console.ReadLine();

If you are interested in getting the results from sayHelloAsync and do further processing on it:

sayHelloAsync().ContinueWith(prev => {
   //prev.Result should have "hello world"
   Console.Write("done do further processing here .. here is the result from sayHelloAsync" + prev.Result);
});
Console.ReadLine();

One last simple way to wait for function:

static void main(string[] args){
  sayHelloAsync().Wait();
  Console.Read();
}

static async Task sayHelloAsync(){          
  await Task.Delay(1000);
  Console.Write( "hello world");

}

Answer 6

public static void Main(string[] args)
{
    var t = new test();
    Task.Run(async () => { await t.Go();}).Wait();
}

Answer 7

Use .Wait()

static void Main(string[] args){
   SomeTaskManager someTaskManager  = new SomeTaskManager();
   Task<List<String>> task = Task.Run(() => marginaleNotesGenerationTask.Execute());
   task.Wait();
   List<String> r = task.Result;
} 

public class SomeTaskManager
{
    public async Task<List<String>> Execute() {
        HttpClient client = new HttpClient();
        client.BaseAddress = new Uri("http://localhost:4000/");     
        client.DefaultRequestHeaders.Accept.Clear();           
        HttpContent httpContent = new StringContent(jsonEnvellope, Encoding.UTF8, "application/json");
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        HttpResponseMessage httpResponse = await client.PostAsync("", httpContent);
        if (httpResponse.Content != null)
        {
            string responseContent = await httpResponse.Content.ReadAsStringAsync();
            dynamic answer = JsonConvert.DeserializeObject(responseContent);
            summaries = answer[0].ToObject<List<String>>();
        }
    } 
}

Answer 8

try "Result" property

class Program
{
    static void Main(string[] args)
    {
        test t = new test();
        t.Go().Result;
        Console.ReadKey();
    }
}

Answer 9

C# 9 Top-level statements simplified things even more, now you don't even have to do anything extra to call async methods from your Main , you can just do this:

using System;
using System.Threading.Tasks;

await Task.Delay(1000);
Console.WriteLine("Hello World!");

For more information see What's new in C# 9.0, Top-level statements :

The top-level statements may contain async expressions. In that case, the synthesized entry point returns a Task , or Task<int> .