异步并等待：为什么其签名定义了Task的返回类型的异步方法显式地返回Task？

Question

I used How to: Extend the async Walkthrough by Using Task.WhenAll (C#) to develop a program to independently submit multiple database queries asynchronously. The program works ok, but there is a nuance I don't understand and for which I can't find an explanation:

In the example, the signature for the method SumPageSizesAsync is:

private async Task SumPageSizesAsync()

The body of the method SumPageSizesAsync does not explicitly return the whenAllTask object (ie, the WhenAll roll-up Task ). Why doesn't the method SumPageSizesAsync explicitly return the whenAllTask task? How does the line of code in StartButton_Click that calls SumPageSizesAsync "know" which task is being returned?

private async void StartButton_Click(object sender, RoutedEventArgs e)
{
    .
    Task sumTask = SumPageSizesAsync();
    .
}


private async Task SumPageSizesAsync()
{
    HttpClient client = new HttpClient() { MaxResponseContentBufferSize = 1000000 };

    List<string> urlList = SetUpURLList();

    IEnumerable<Task<int>> downloadTasksQuery = from url in urlList select ProcessURLAsync(url);
    Task<int>[] downloadTasks = downloadTasksQuery.ToArray();

    Task<int[]> whenAllTask = Task.WhenAll(downloadTasks);

    int[] lengths = await whenAllTask;

    int total = lengths.Sum();

    // Display the total count for all of the web addresses.  
    resultsTextBox.Text += string.Format("\r\n\r\nTotal bytes returned:  {0}\r\n", total);

    // where's the return statement?
}

Answer 1

Why doesn't the method SumPageSizesAsync explicitly return the whenAllTask task?

That is not the Task that this method returns. The Task that is returned is one that is created by the compiler.
As soon as the await is encountered, the following happens:
- If whenAllTask is already finished, then just continue executing the rest of the method.
- Otherwise, create a Task that does the following things:
1: Wait for whenAllTask to finish.
2: and then execute the rest of the method.
This Task is then immediately returned.
I always think of the return statement as being embedded in the await . "return a Task that waits for this other Task to finish and then executes the rest of this method."

Answer 2

I completely agree on answer posted by @Dennis_E and more information can also be found in C# 6 in a Nutshell (page number 600).

According to author, while executing the async function, with task return type. If there is no explicit return of task from the method body, complier creates a task .

This task is then used for following purpose,

1. Singling completion of the method.
2. Asynchronous call chaining.

Answer 3

It is an async void function, meaning it returns Task. Since it is already awaited inside, there is nothing no return here.