.NET 内置的异步方法

async 和 await 关键字

async 和 await 是 C# 异步编程的核心，由它们定义的方法称为异步方法。

异步方法示例：

// Three things to note in the signature:  
//  - The method has an async modifier.   
//  - The return type is Task or Task<T>.
//  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;  
}

什么是异步方法

方法签名包括 async 修饰符
约定方法名以 Async 为后缀
返回类型为以下几种之一
1. Task
2. Task
3. void
方法内部至少包含一句 await 表达式：
1. string urlContents = await getStringTask;
2. await 表达式的意思是：方法在此处需要等待异步操作完成后才能继续向下执行。同时方法将被挂起，控制流返回给方法的调用者

异步编程的核心就是 Task 和 Task 对象，它们模拟了异步操作。

在 C# 中一个 Task 对象可以看作一个需要继续执行的操作，一个 Task 对象可以看作一个在将来会返回 T 类型值的操作。这就像承诺将在操作完成后返回 T 类型的值。

I/O 绑定异步操作示例：从 Web Service 下载数据

private readonly HttpClient _httpClient = new HttpClient();
downloadButton.Clicked += async (o, e) =>
{
    // This line will yield control to the UI as the request
    // from the web service is happening.
    //
    // The UI thread is now free to perform other work.
    var stringData = await _httpClient.GetStringAsync(URL);
    DoSomethingWithData(stringData);
};

CPU 绑定异步操作示例：执行计算

使用 Task.Run 来启动一个后台线程用于执行 CPU 绑定的操作。

private DamageResult CalculateDamageDone()
{
    // Code omitted:
    //
    // Does an expensive calculation and returns
    // the result of that calculation.
}
calculateButton.Clicked += async (o, e) =>
{
    // This line will yield control to the UI while CalculateDamageDone()
    // performs its work.  The UI thread is free to perform other work.
    var damageResult = await Task.Run(() => CalculateDamageDone());
    DisplayDamage(damageResult);
};

.NET 类库内置的异步方法

通过 Async 后缀和返回类型为 Task（Task）来识别内置的异步方法。

网络操作相关的：

HttpClient：发送 HTTP 请求，接收 HTTP 响应
WebClient：收发数据

I/O 操作相关的：

StreamWrite
StreamReader
XmlReader

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