C# 从任务返回数据

Question

您好，我有以下简短代码摘录。代码的目的是 return 来自 Task 的 person 对象并获得结果，以便它可以在 main 中访问。

我的代码如下，但是在编译时出现了两个错误。

Error 2 'System.Threading.Tasks.Task multi_threaded_tasks.Program.ExecuteAsync_GetPerson()' has the wrong return type
Error 1 An object reference is required for the non-static field, method, or property 'multi_threaded_tasks.Program.ExecuteAsync_GetPerson()'

我不确定我哪里出错了，您能提供的任何帮助都会有所帮助。 P.S 如果有更好的方法 return 对象，我愿意接受建议，但我希望函数调用是分开的。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;
namespace multi_threaded_tasks
{
    public class Person
    {
        public string first_name;
        public string last_name;
        //Constructor
        public Person()
        {
            first_name="";
            last_name="";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Main started:");
            CancellationTokenSource cancelSource;
            Task<Person> t_person = Task<Person>.Factory.StartNew(
                function: ExecuteAsync_GetPerson, 
                cancellationToken: cancelSource.Token, 
                creationOptions: TaskCreationOptions.PreferFairness, 
                scheduler: TaskScheduler.Default);

            Person main_person = t_person.Result;
            Console.WriteLine("The person first name:" + main_person.first_name);
            Console.WriteLine("The person last name:" + main_person.last_name);
            Console.WriteLine("Main Ended:");
        }

        async Task<Person> ExecuteAsync_GetPerson()
        {
            Console.WriteLine("ExecuteAsync_GetPeople started:");
            Person a_person = new Person();
            a_person.first_name="";
            a_person.last_name="";
            await Task.Delay(2000); // Wait 2 seconds
            Console.WriteLine("ExecuteAsync_GetPeople returning:");
            return a_person;
        }
    }
}

Answer 1

Task.Factory.StartNew 具有以下定义：

public Task<TResult> StartNew(Func<TResult> function...

当你试图通过时

public Task<TResult> StartNew(Func<Task<TResult>> function...

如果你想启动一个异步方法，你可以使用：

Task<Person> t_person = Task.Run(() => a());
Person main_person = t_person.Result;

Answer 2

无论如何，您的代码没有多大意义。

由于这是一个控制台应用程序，有两个方法，其中一个是应用程序入口点，您无法从异步方法中获得任何好处，因为您想在主方法中获取任务结果。这意味着，主线程将被阻塞并且无法取消任何操作。

我从您的代码中删除了一些垃圾并添加了另一个示例 async 方法。这可能就是您想要的：

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

class Program
{
    static async Task<Person> GetPersonAsync(CancellationToken cancellationToken)
    {
        Console.WriteLine("GetPersonAsync started.");

        var person = new Person
        {
            FirstName = "John",
            LastName = "Doe"
        };

        await Task.Delay(5000, cancellationToken); // Wait 5 seconds
        Console.WriteLine("GetPersonAsync ended.");
        return person;
    }

    static async Task TestGetPersonAsync(CancellationToken cancellationToken)
    {
        Console.WriteLine("TestGetPersonAsync started.");

        try
        {
            var person = await GetPersonAsync(cancellationToken);

            Console.WriteLine("The person first name:" + person.FirstName);
            Console.WriteLine("The person last name:" + person.LastName);
            Console.WriteLine("TestGetPersonAsync ended.");
        }
        catch (OperationCanceledException)
        {
            Console.WriteLine("TestGetPersonAsync cancelled.");
        }
    }

    static void Main(string[] args)
    {
        Console.WriteLine("Main started:");

        // let's get person asynchronously;
        // this object will contain our cancellation token;            
        var cts = new CancellationTokenSource();
        // dummy variable here is needed to aviod compiler warning,
        // since TestGetPersonAsync is async, and we will not (and cannot) await it
        var _ = TestGetPersonAsync(cts.Token);

        // if TestGetPersonAsync is not finished yet, we are going to cancel it;
        // wait for a new line
        Console.WriteLine("Press ENTER to cancel TestGetPersonAsync and to exit application.");
        Console.ReadLine();
        if (!_.IsCompleted)
        {
            cts.Cancel();
        }

        Console.WriteLine("Main ended.");
    }
}

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