C# 无任务异步等待<；T>；方法

根据MSDN中的示例：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

// Add a using directive and a reference for System.Net.Http; 
using System.Net.Http;

namespace AsyncFirstExample
{
    public partial class MainWindow : Window
    {
        // Mark the event handler with async so you can use await in it. 
        private async void StartButton_Click(object sender, RoutedEventArgs e)
        {
            // Call and await separately. 
            //Task<int> getLengthTask = AccessTheWebAsync(); 
            //// You can do independent work here. 
            //int contentLength = await getLengthTask; 

            int contentLength = await AccessTheWebAsync();

            resultsTextBox.Text +=
            String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
        }


        // Three things to note in the signature: 
        //  - The method has an async modifier.  
        //  - The return type is Task or Task<T>. (See "Return Types" section.)
        //    Here, it is Task<int> because the return statement returns an integer. 
        //  - The method name ends in "Async."
        async Task<int> AccessTheWebAsync()
        { 
            // You need to add a reference to System.Net.Http to declare client.
            HttpClient client = new HttpClient();

            // GetStringAsync returns a Task<string>. That means that when you await the 
            // task you'll get a string (urlContents).
            Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");

            // You can do work here that doesn't rely on the string from GetStringAsync.
                DoIndependentWork();

            // The await operator suspends AccessTheWebAsync. 
            //  - AccessTheWebAsync can't continue until getStringTask is complete. 
            //  - Meanwhile, control returns to the caller of AccessTheWebAsync. 
            //  - Control resumes here when getStringTask is complete.  
            //  - The await operator then retrieves the string result from getStringTask. 
            string urlContents = await getStringTask;

            // The return statement specifies an integer result. 
            // Any methods that are awaiting AccessTheWebAsync retrieve the length value. 
            return urlContents.Length;
        }


        void DoIndependentWork()
        {
            resultsTextBox.Text += "Working . . . . . . .\r\n";
        }
    }
}

// Sample Output: 

// Working . . . . . . . 

// Length of the downloaded string: 41564.

现在如何返回str

像这样：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

// Add a using directive and a reference for System.Net.Http; 
using System.Net.Http;

namespace AsyncFirstExample
{
    public partial class MainWindow : Window
    {
        // Mark the event handler with async so you can use await in it. 
        private async void StartButton_Click(object sender, RoutedEventArgs e)
        {
            // Call and await separately. 
            //Task<int> getLengthTask = AccessTheWebAsync(); 
            //// You can do independent work here. 
            //int contentLength = await getLengthTask; 

            int contentLength = await AccessTheWebAsync();

            resultsTextBox.Text +=
            String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
        }


        // Three things to note in the signature: 
        //  - The method has an async modifier.  
        //  - The return type is Task or Task<T>. (See "Return Types" section.)
        //    Here, it is Task<int> because the return statement returns an integer. 
        //  - The method name ends in "Async."
        async Task<int> AccessTheWebAsync()
        { 
            // GetStringAsync returns a Task<string>. That means that when you await the 
            // task you'll get a string (urlContents).
            Task<string> getStringTask = GetMyString(); // here

            // You can do work here that doesn't rely on the string from GetStringAsync.
                DoIndependentWork();

            // The await operator suspends AccessTheWebAsync. 
            //  - AccessTheWebAsync can't continue until getStringTask is complete. 
            //  - Meanwhile, control returns to the caller of AccessTheWebAsync. 
            //  - Control resumes here when getStringTask is complete.  
            //  - The await operator then retrieves the string result from getStringTask. 
            string urlContents = await getStringTask;

            // The return statement specifies an integer result. 
            // Any methods that are awaiting AccessTheWebAsync retrieve the length value. 
            return urlContents.Length;
        }


        void DoIndependentWork()
        {
            resultsTextBox.Text += "Working . . . . . . .\r\n";
        }
    }
}

    private Task<string> GetMyString()
    {
        string str = string.Empty;
        for (int i = 0; i < 1000000000; i++)
        {
            // compute str using simple c# code
        }
        // How to return str now?
    }

private async Task<string> GetMyString()
{
    string str = string.Empty;
    for (int i = 0; i < 1000000000; i++)
    {
        // compute str using simple c# code
    }
    return str;
}

Task getStringTask=Task.Run（（）=>GetMyString（））；

那么

GetMyString

在任何有用的方面都是异步的吗？如果不是，当您将它与需要适当异步代码的代码一起使用时（即，它几乎立即返回，任务将在稍后完成）。您的问题是什么？不，

GetMyString

不是异步的。但我还是想打电话asynchronous@yazanpro，您的意思是希望它在另一个线程中运行吗？这与异步不同……是的，我似乎希望它在另一个线程中运行。我一定把这两个概念搞混了！使用单独的线程正是我所需要的。然而，为了让答案更完整，请参考“因为GetMyString的实现不是异步的”，您能否详细说明如何以异步方式实现

GetMyString

？另外，请说明如何防止

return str引起的编译器错误

@yazanpro不，您的实现思想不是异步的。例如，如果您实际上正在从web服务器接收数据，这是一个异步I/O操作，并且不会占用任何线程。您正在进行CPU工作，因此没有机会使用异步I/O，只能使用并行（最多）。这两个概念非常不同，而

Task

和

Task

的定义方式使这一点相当混乱。@yazanpro因此

Task.Run

返回的任务基本上是对刚刚创建的CPU工作线程的异步I/O，但您正在做的实际工作仍然是CPU工作，非异步I/O。在不使用Wait的情况下使用async关键字实现方法会引发编译器警告。您可以删除async关键字并使用“return Task.FromResult（str）”代替。

private string GetMyString()
{
    string str = string.Empty;
    for (int i = 0; i < 1000000000; i++)
    {
        // compute str using simple c# code
    }
    return str;
}

Task<string> getStringTask = Task.Run(() => GetMyString());