Control Flow in Async Programs

You can write and maintain asynchronous programs more easily by using the Async and Await keywords.

However, the results might surprise you if you don't understand how your program operates.

This topic traces追溯 the flow of control through a simple async program to show you when control moves from one method to another and what information is transferred each time.

In general, you mark methods that contain asynchronous code with the Async (Visual Basic) or async (C#) modifier.

In a method that's marked with an async modifier, you can use an Await (Visual Basic) or await (C#) operator to specify where the method pauses to wait for a called asynchronous process to complete.

For more information, see Asynchronous Programming with Async and Await (C# and Visual Basic).

The following example uses async methods to download the contents of a specified website as a string and to display the length of the string.

The example contains the following two methods.

startButton_Click, which calls AccessTheWebAsync and displays the result.
AccessTheWebAsync, which downloads the contents of a website as a string and returns the length of the string.

AccessTheWebAsync uses an asynchronous HttpClient method, GetStringAsync(String), to download the contents.

Numbered display lines appear at strategic战略上的 points throughout the program to help you understand how the program runs and to explain what happens at each point that is marked.

The display lines are labeled "ONE" through "SIX."

The labels represent the order in which the program reaches these lines of code.

The following code shows an outline of the program.

需要添加对System.Net.Http.dll的引用，

 private async void button1_Click(object sender, EventArgs e)
        {
            //One
            Task<int> getLengthTask = AccessTheWebAsync();

            //Four
            int concentLength = await getLengthTask;

            //Six
            textBox1.Text = $"Length of downloaded string is {concentLength}";
        }

        private async Task<int> AccessTheWebAsync()
        {
            //Two
            HttpClient httpClient = new HttpClient();
            Task<string> getStringTask = httpClient.GetStringAsync("http://www.cnblogs.com/chucklu/");

            //Three
            string urlContents = await getStringTask;
            Console.WriteLine(urlContents);

            //Five
            return urlContents.Length;
        }

Each of the labeled locations, "ONE" through "SIX," displays information about the current state of the program.

The following output is produced.

ONE:   Entering startButton_Click.
           Calling AccessTheWebAsync.

TWO:   Entering AccessTheWebAsync.
           Calling HttpClient.GetStringAsync.

THREE: Back in AccessTheWebAsync.
           Task getStringTask is started.
           About to await getStringTask & return a Task<int> to startButton_Click.

FOUR:  Back in startButton_Click.
           Task getLengthTask is started.
           About to await getLengthTask -- no caller to return to.

FIVE:  Back in AccessTheWebAsync.
           Task getStringTask is complete.
           Processing the return statement.
           Exiting from AccessTheWebAsync.

SIX:   Back in startButton_Click.
           Task getLengthTask is finished.
           Result from AccessTheWebAsync is stored in contentLength.
           About to display contentLength and exit.

Length of the downloaded string: 33946.