浅谈.NET Core中的异步编程

什么是异步编程

同步程序中的代码是运行在单一线程上的。

异步操作：从其它线程发起后，在一个新的单独线程上运行的操作。发起异步操作的线程不必等待异步操作完成。

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异步编程的好处

对于需要长时间运行的操作，异步编程可以更大限度更有效地利用设备资源。

1. 通过异步执行 CPU 或 I/O 绑定操作，提高 UI 程序的响应性
2. 并行计算

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.NET内置的异步方法

// 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;  
}