Async and Await

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Most people have already heard about the new “async” and “await” functionality coming in Visual Studio 11. This is Yet Another Introductory Post.

First, the punchline: Async will fundamentally change the way most code is written.

Yup, I believe async/await will have a bigger impact than LINQ. Understanding async will be a basic necessity just a few short years from now.

Introducing the Keywords

Let’s dive right in. I’ll use some concepts that I’ll expound on later on - just hold on for this first part.

Asynchronous methods look something like this:

public async Task DoSomethingAsync()
{
  // In the Real World, we would actually do something...
  // For this example, we‘re just going to (asynchronously) wait 100ms.
  await Task.Delay(100);
}

The “async” keyword enables the “await” keyword in that method and changes how method results are handled. That’s all the async keyword does! It does not run this method on a thread pool thread, or do any other kind of magic. The async keyword only enables the await keyword (and manages the method results).

The beginning of an async method is executed just like any other method. That is, it runs synchronously until it hits an “await” (or throws an exception).

The “await” keyword is where things can get asynchronous. Await is like a unary operator: it takes a single argument, an awaitable (an “awaitable” is an asynchronous operation). Await examines that awaitable to see if it has already completed; if the awaitable has already completed, then the method just continues running (synchronously, just like a regular method).

If “await” sees that the awaitable has not completed, then it acts asynchronously. It tells the awaitable to run the remainder of the method when it completes, and then returns from the async method.

Later on, when the awaitable completes, it will execute the remainder of the async method. If you’re awaiting a built-in awaitable (such as a task), then the remainder of the async method will execute on a “context” that was captured before the “await” returned.

I like to think of “await” as an “asynchronous wait”. That is to say, the async method pauses until the awaitable is complete (so it waits), but the actual thread is not blocked (so it’s asynchronous).

Awaitables

As I mentioned, “await” takes a single argument - an “awaitable” - which is an asynchronous operation. There are two awaitable types already common in the .NET framework: Task<T> and Task.

There are also other awaitable types: special methods such as “Task.Yield” return awaitables that are not Tasks, and the WinRT runtime (coming in Windows 8) has an unmanaged awaitable type. You can also create your own awaitable (usually for performance reasons), or use extension methods to make a non-awaitable type awaitable.

That’s all I’m going to say about making your own awaitables. I’ve only had to write a couple of awaitables in the entire time I’ve used async/await. If you want to know more about writing your own awaitables, see the Parallel Team Blog or Jon Skeet’s Blog.

One important point about awaitables is this: it is the type that is awaitable, not the method returning the type. In other words, you can await the result of an async method that returns Task … because the method returns Task, not because it’s async. So you can also await the result of a non-async method that returns Task:

public async Task NewStuffAsync()
{
  // Use await and have fun with the new stuff.
  await ...
}

public Task MyOldTaskParallelLibraryCode()
{
  // Note that this is not an async method, so we can‘t use await in here.
  ...
}

public async Task ComposeAsync()
{
  // We can await Tasks, regardless of where they come from.
  await NewStuffAsync();
  await MyOldTaskParallelLibraryCode();
}

Return Types

Async methods can return Task<T>, Task, or void. In almost all cases, you want to return Task<T> or Task, and return void only when you have to.

Why return Task<T> or Task? Because they’re awaitable, and void is not. So if you have an async method returning Task<T> or Task, then you can pass the result to await. With a void method, you don’t have anything to pass to await.

You have to return void when you have async event handlers.

Returning Values

Async methods returning Task or void do not have a return value. Async methods returning Task<T> must return a value of type T:

public async Task<int> CalculateAnswer()
{
  await Task.Delay(100); // (Probably should be longer...)

  // Return a type of "int", not "Task<int>"
  return 42;
}

This is a bit odd to get used to, but there are good reasons behind this design.

Context

In the overview, I mentioned that when you await a built-in awaitable, then the awaitable will capture the current “context” and later apply it to the remainder of the async method. What exactly is that “context”?

Simple answer:

1. If you’re on a UI thread, then it’s a UI context.
2. If you’re responding to an ASP.NET request, then it’s an ASP.NET request context.
3. Otherwise, it’s usually a thread pool context.

Complex answer:

1. If SynchronizationContext.Current is not null, then it’s the current SynchronizationContext. (UI and ASP.NET request contexts are SynchronizationContext contexts).
2. Otherwise, it’s the current TaskScheduler (TaskScheduler.Default is the thread pool context).

What does this mean in the real world? For one thing, capturing (and restoring) the UI/ASP.NET context is done transparently:

// WinForms example (it works exactly the same for WPF).
private async void DownloadFileButton_Click(object sender, EventArgs e)
{
  // Since we asynchronously wait, the UI thread is not blocked by the file download.
  await DownloadFileAsync(fileNameTextBox.Text);

  // Since we resume on the UI context, we can directly access UI elements.
  resultTextBox.Text = "File downloaded!";
}

// ASP.NET example
protected async void MyButton_Click(object sender, EventArgs e)
{
  // Since we asynchronously wait, the ASP.NET thread is not blocked by the file download.
  // This allows the thread to handle other requests while we‘re waiting.
  await DownloadFileAsync(...);

  // Since we resume on the ASP.NET context, we can access the current request.
  // We may actually be on another *thread*, but we have the same ASP.NET request context.
  Response.Write("File downloaded!");
}

This is great for event handlers, but it turns out to not be what you want for most other code (which is, really, most of the async code you’ll be writing).

Avoiding Context

Most of the time, you don’t need to sync back to the “main” context. Most async methods will be designed with composition in mind: they await other operations, and each one represents an asynchronous operation itself (which can be composed by others). In this case, you want to tell the awaiter to not capture the current context by calling ConfigureAwait and passing false, e.g.:

private async Task DownloadFileAsync(string fileName)
{
  // Use HttpClient or whatever to download the file contents.
  var fileContents = await DownloadFileContentsAsync(fileName).ConfigureAwait(false);

  // Note that because of the ConfigureAwait(false), we are not on the original context here.
  // Instead, we‘re running on the thread pool.

  // Write the file contents out to a disk file.
  await WriteToDiskAsync(fileName, fileContents).ConfigureAwait(false);

  // The second call to ConfigureAwait(false) is not *required*, but it is Good Practice.
}

// WinForms example (it works exactly the same for WPF).
private async void DownloadFileButton_Click(object sender, EventArgs e)
{
  // Since we asynchronously wait, the UI thread is not blocked by the file download.
  await DownloadFileAsync(fileNameTextBox.Text);

  // Since we resume on the UI context, we can directly access UI elements.
  resultTextBox.Text = "File downloaded!";
}

The important thing to note with this example is that each “level” of async method calls has its own context. DownloadFileButton_Click started in the UI context, and called DownloadFileAsync. DownloadFileAsync also started in the UI context, but then stepped out of its context by calling ConfigureAwait(false). The rest of DownloadFileAsync runs in the thread pool context. However, when DownloadFileAsync completes and DownloadFileButton_Click resumes, it does resume in the UI context.

A good rule of thumb is to use ConfigureAwait(false) unless you know you do need the context.

Async Composition

So far, we’ve only considered serial composition: an async method waits for one operation at a time. It’s also possible to start several operations and await for one (or all) of them to complete. You can do this by starting the operations but not awaiting them until later:

public async Task DoOperationsConcurrentlyAsync()
{
  Task[] tasks = new Task[3];
  tasks[0] = DoOperation0Async();
  tasks[1] = DoOperation1Async();
  tasks[2] = DoOperation2Async();

  // At this point, all three tasks are running at the same time.

  // Now, we await them all.
  await Task.WhenAll(tasks);
}

public async Task<int> GetFirstToRespondAsync()
{
  // Call two web services; take the first response.
  Task<int>[] tasks = new[] { WebService1Async(), WebService2Async() };

  // Await for the first one to respond.
  Task<int> firstTask = await Task.WhenAny(tasks);

  // Return the result.
  return await firstTask;
}

By using concurrent composition (Task.WhenAll or Task.WhenAny), you can perform simple concurrent operations. You can also use these methods along with Task.Run to do simple parallel computation. However, this is not a substitute for the Task Parallel Library - any advanced CPU-intensive parallel operations should be done with the TPL.

Guidelines

Read the Task-based Asynchronous Pattern (TAP) document. It is extremely well-written, and includes guidance on API design and the proper use of async/await (including cancellation and progress reporting).

There are many new await-friendly techniques that should be used instead of the old blocking techniques. If you have any of these Old examples in your new async code, you’re Doing It Wrong(TM):

Next Steps

I have published an MSDN article Best Practices in Asynchronous Programming, which further explains the “avoid async void”, “async all the way” and “configure context” guidelines.

The official MSDN documentation is quite good; they include an online version of the Task-based Asynchronous Pattern document which is excellent, covering the designs of asynchronous methods.

The async team has published an async/await FAQ that is a great place to continue learning about async. They have pointers to the best blog posts and videos on there. Also, pretty much any blog post by Stephen Toub is instructive!

Of course, another resource is my own blog.

This is a problem that is brought up repeatedly on the forums and Stack Overflow. I think it’s the most-asked question by async newcomers once they’ve learned the basics.

UI Example

Consider the example below. A button click will initiate a REST call and display the results in a text box (this sample is for Windows Forms, but the same principles apply to any UI application).

// My "library" method.
public static async Task<JObject> GetJsonAsync(Uri uri)
{
  // (real-world code shouldn‘t use HttpClient in a using block; this is just example code)
  using (var client = new HttpClient())
  {
    var jsonString = await client.GetStringAsync(uri);
    return JObject.Parse(jsonString);
  }
}

// My "top-level" method.
public void Button1_Click(...)
{
  var jsonTask = GetJsonAsync(...);
  textBox1.Text = jsonTask.Result;
}

The “GetJson” helper method takes care of making the actual REST call and parsing it as JSON. The button click handler waits for the helper method to complete and then displays its results.

This code will deadlock.

ASP.NET Example

This example is very similar; we have a library method that performs a REST call, only this time it’s used in an ASP.NET context (Web API in this case, but the same principles apply to any ASP.NET application):

// My "library" method.
public static async Task<JObject> GetJsonAsync(Uri uri)
{
  // (real-world code shouldn‘t use HttpClient in a using block; this is just example code)
  using (var client = new HttpClient())
  {
    var jsonString = await client.GetStringAsync(uri);
    return JObject.Parse(jsonString);
  }
}

// My "top-level" method.
public class MyController : ApiController
{
  public string Get()
  {
    var jsonTask = GetJsonAsync(...);
    return jsonTask.Result.ToString();
  }
}

This code will also deadlock. For the same reason.

What Causes the Deadlock

Here’s the situation: remember from my intro post that after you await a Task, when the method continues it will continue in a context.

In the first case, this context is a UI context (which applies to any UI except Console applications). In the second case, this context is an ASP.NET request context.

One other important point: an ASP.NET request context is not tied to a specific thread (like the UI context is), but it does only allow one thread in at a time. This interesting aspect is not officially documented anywhere AFAIK, but it is mentioned in my MSDN article about SynchronizationContext.

So this is what happens, starting with the top-level method (Button1_Click for UI / MyController.Get for ASP.NET):

1. The top-level method calls GetJsonAsync (within the UI/ASP.NET context).
2. GetJsonAsync starts the REST request by calling HttpClient.GetStringAsync (still within the context).
3. GetStringAsync returns an uncompleted Task, indicating the REST request is not complete.
4. GetJsonAsync awaits the Task returned by GetStringAsync. The context is captured and will be used to continue running the GetJsonAsync method later. GetJsonAsync returns an uncompleted Task, indicating that the GetJsonAsync method is not complete.
5. The top-level method synchronously blocks on the Task returned by GetJsonAsync. This blocks the context thread.
6. … Eventually, the REST request will complete. This completes the Task that was returned by GetStringAsync.
7. The continuation for GetJsonAsync is now ready to run, and it waits for the context to be available so it can execute in the context.
8. Deadlock. The top-level method is blocking the context thread, waiting for GetJsonAsync to complete, and GetJsonAsync is waiting for the context to be free so it can complete.

For the UI example, the “context” is the UI context; for the ASP.NET example, the “context” is the ASP.NET request context. This type of deadlock can be caused for either “context”.

Preventing the Deadlock

There are two best practices (both covered in my intro post) that avoid this situation:

1. In your “library” async methods, use ConfigureAwait(false) wherever possible.
2. Don’t block on Tasks; use async all the way down.

Consider the first best practice. The new “library” method looks like this:

public static async Task<JObject> GetJsonAsync(Uri uri)
{
  // (real-world code shouldn‘t use HttpClient in a using block; this is just example code)
  using (var client = new HttpClient())
  {
    var jsonString = await client.GetStringAsync(uri).ConfigureAwait(false);
    return JObject.Parse(jsonString);
  }
}

This changes the continuation behavior of GetJsonAsync so that it does not resume on the context. Instead, GetJsonAsync will resume on a thread pool thread. This enables GetJsonAsync to complete the Task it returned without having to re-enter the context. The top-level methods, meanwhile, do require the context, so they cannot use ConfigureAwait(false).

Consider the second best practice. The new “top-level” methods look like this:

public async void Button1_Click(...)
{
  var json = await GetJsonAsync(...);
  textBox1.Text = json;
}

public class MyController : ApiController
{
  public async Task<string> Get()
  {
    var json = await GetJsonAsync(...);
    return json.ToString();
  }
}

This changes the blocking behavior of the top-level methods so that the context is never actually blocked; all “waits” are “asynchronous waits”.

Note: It is best to apply both best practices. Either one will prevent the deadlock, but both must be applied to achieve maximum performance and responsiveness.

Resources

• My introduction to async/await is a good starting point.
• Stephen Toub’s blog post Await, and UI, and deadlocks! Oh, my! covers this exact type of deadlock (in January of 2011, no less!).
• If you prefer videos, Stephen Toub demoed this deadlock live (39:40 - 42:50, but the whole presentation is great!). Lucian Wischik also demoed this deadlock using VB (17:10 - 19:15).
• The Async/Await FAQ goes into detail on exactly when contexts are captured and used for continuations.

This kind of deadlock is always the result of mixing synchronous with asynchronous code. Usually this is because people are just trying out async with one small piece of code and use synchronous code everywhere else. Unfortunately, partially-asynchronous code is much more complex and tricky than just making everything asynchronous.

If you do need to maintain a partially-asynchronous code base, then be sure to check out two more of Stephen Toub’s blog posts: Asynchronous Wrappers for Synchronous Methods and Synchronous Wrappers for Asynchronous Methods, as well as my AsyncEx library.

Answered Questions

There are scores of answered questions out there that are all caused by the same deadlock problem. It has shown up on WinRT, WPF, Windows Forms, Windows Phone, MonoDroid, Monogame, and ASP.NET.

Async and Await

标签：ide   web   sas   loop   string   world   help   www   info   

原文地址：https://www.cnblogs.com/mqingqing123/p/14398054.html