寻找指导，以了解与异步如何异步编程并等待作品 [英] Looking for guidance to understand how asynchronous Programming with Async and Await works

问题描述

我经过一个MSDN示例code，其中被点击时键，当程序被调用，则使用等待关键字和功能使用异步关键字函数被调用。

 专用异步无效StartButton_Click（对象发件人，RoutedEventArgs E）
        {            INT CONTENTLENGTH =等待AccessTheWebAsync（）;            resultsTextBox.Text + =
                的String.Format（\\ r \\ nLength下载的字符串：{0} \\ r \\ n，CONTENTLENGTH）;
        }异步任务＆LT; INT＆GT; AccessTheWebAsync（）
        {
            HttpClient的客户端=新的HttpClient（）;
            任务＆LT;串GT; getStringTask = client.GetStringAsync（http://msdn.microsoft.com）;
            DoIndependentWork（）;
            字符串urlContents =等待getStringTask;            返回urlContents.Length;
        }
        无效DoIndependentWork（）
        {
            resultsTextBox.Text + =工作\\ r \\ n。。。。
        }
 

1. 当 AccessTheWebAsync 被称为然后等待使用关键字，这是什么意思？


2. 当此功能AccessTheWebAsync（）将被执行，然后 DoIndependentWork（）函数被调用，我在这里想控制将等待，直到此功能 DoIndependentWork （）结束。我说得对不对？

再有一个叫另一个语句

 字符串urlContents =等待getStringTask;
 

他们为什么在这里使用等待。如果我们不使用那么这里等待会发生什么事？

请指引我理解code是它是如何工作的。

解决方案

下面是关于异步/ await方法的简要说明。

异步方法：

• 来电不一定阻塞异步的全部执行
  方法


• 可能的返回类型
  
  
  
  • 无效：发射后不管
  
  
  • 任务：允许等待异步方法终止
  
  
  • 任务＆LT; T＆GT; ：允许等待终止并获得T型结果
  
  


• 没有ref或out参数为异步方法


• 必须再次包含的await =>否则编译警告

指日可待任务

• 等待下TPL任务终止


• 返回结果的任务（如使用任务
  结果类型）


• 必须只出现在异步方法=>否则编译出错

这是异步方法部分是同步和异步部分

• 来电同步执行，直至阻塞的await
方法

• 此后，异步执行的方法休息
  异步和放大器;等待机制
  高效

 公共异步任务＆LT; INT＆GT; GetSiteLengthAsync（字符串URL）
{
    HttpClient的客户端=新的HttpClient（）; ＆LT; =同步
    任务＆LT;串GT;下载1 = client.GetStringAsync（URL）; ＆LT; =同步
    串站点1 =等待下载1; ＆LT; =异步（另一个线程）
    返回site1.Length; ＆LT; =异步（另一个线程）
}
 

i go through a msdn sample code where a function is called when button is clicked and when routine is called then Await keyword is used and function has async keyword used.

private async void StartButton_Click(object sender, RoutedEventArgs e)
        {

            int contentLength = await AccessTheWebAsync();

            resultsTextBox.Text +=
                String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
        }

async Task<int> AccessTheWebAsync()
        { 
            HttpClient client = new HttpClient();
            Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");
            DoIndependentWork();
            string urlContents = await getStringTask;

            return urlContents.Length;
        }


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

1. When AccessTheWebAsync is called then await keyword is used, what does it mean?
2. When this function AccessTheWebAsync() will be executing then DoIndependentWork() function is called and i guess here control will be waiting until this function DoIndependentWork() is finished. Am I right?

again there is another statement called

string urlContents = await getStringTask;

why they use here await. if we do not use await here then what would happen?

Please guide me to understand the code that how it is working.

解决方案

Here's a brief description about async/await methods.

Async Methods:

• Caller is not necessarily blocked for the full execution of async method
• Possible return types
  • void: "fire-and-forget"
  • Task: allows to await termination of async method
  • Task<T>: allows to await termination and get result of type T
• No ref or out parameter for async methods
• Must again contain an await => Otherwise compile warning

await for Tasks

• Await termination of a TPL task
• Return result of task (if task with result type)
• Must only occur in async methods => Otherwise compile error

An async method is partly synchronous and partly asynchronous

• Caller synchronously executes the method until a blocking await
• Thereafter, the method rest is asynchronously executed async & await Mechanism Efficient

public async Task<int> GetSiteLengthAsync(string url) 
{  
    HttpClient client = new HttpClient();               <= Sync  
    Task<string> download1 = client.GetStringAsync(url); <= Sync  
    string site1 = await download1;   <= Async (Another thread)
    return site1.Length;              <= Async (Another thread)
}

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