c# - C# 示例中的异步和等待

Question

我从 Microsoft 网站上找到了以下关于 Async 和 await 的示例( https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/ ):

// 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("https://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;  
}  

我的第一个问题来自网页“如果 AccessTheWebAsync 在调用 GetStringAsync 和等待其完成之间没有任何它可以做的工作，您可以通过在以下单个语句中调用和等待来简化代码。”似乎暗示以下语句确实在做某事:

    Task<string> getStringTask = client.GetStringAsync("https://msdn.microsoft.com");  

这是真的吗？我虽然它只是一个任务的定义，但它还没有开始/运行。

第二个问题是我尝试如下更改实现以检查我的想法:

async Task<int> AccessTheWebAsync()  
{    
    HttpClient client = new HttpClient();  

    //Task<string> getStringTask = client.GetStringAsync("https://msdn.microsoft.com");
        Task<string> getStringTask = DoPriorWork();  

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

    string urlContents = await getStringTask;  

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

    Task<string> DoPriorWork()
    {
        return new Task<string>(
            () =>
            {
                string retStr = "good";
                Debug.Assert(true, "running dopriorwork");
                System.Threading.Thread.Sleep(10000);
                return retStr;
            }
        );
    }

但是，方法 DoPriorWork() 根本没有执行。这个实现有什么问题？谢谢!

Answer 1

Is that true? I thought it's just a definition of a task, but it has not yet started/run.

的确如此。您调用的操作立即返回，给您的任务是“热的”——即已经在运行。这就是您实际获得任务的原因，因此您可以稍后等待它。因此，您可以获得一种运行其他东西的方法，如示例所述。

Second question is I tried to change the implementation as follows to check my idea...

为了能够让代码运行，请尝试按如下方式更改方法 DoPriorWork:

private Task<string> DoPriorWork()
{
    return Task.Run(() =>
    {
        var retStr = "good";
        Console.WriteLine("running dopriorwork");
        Thread.Sleep(10000);
        return retStr;
    });
}

请注意，我正在排队并返回一个“热"任务，使用静态方法 Task.Run()。

希望这对您有所帮助!