最近在写程序的时候，经常遇到大量需要异步访问的情况，但是对于async和await到底怎么写，还不是非常明确。
1.普通的程序怎么写？
class Program
{
static void Main(string[] args)
{
MyDownLoadString ds = new MyDownLoadString();
ds.DoRun();
Console.ReadKey();
}
class MyDownLoadString
{
Stopwatch sw = new Stopwatch();
public void DoRun()
{
const int LargeNumber = 6000000;
sw.Start();
int t1 = CountCharacters(1, "http://www.microsoft.com");
int t2 = CountCharacters(2, "http://www.illustratedcsharp.com");
CountToALargeNumber(1, LargeNumber);
CountToALargeNumber(2, LargeNumber);
CountToALargeNumber(3, LargeNumber);
CountToALargeNumber(4, LargeNumber);
Console.WriteLine("Chars in Call1:{0}",t1);
Console.WriteLine("Chars in Call1:{0}",t2);
}
private int CountCharacters(int id, string uriString)
{
WebClient wc1 = new WebClient();
Console.WriteLine("Call {0} start: {1:N0}ms ", id, sw.Elapsed.TotalMilliseconds);
string result = wc1.DownloadString(new Uri(uriString));
Console.WriteLine("Call {0} completed: {1:N0}ms", id, sw.Elapsed.TotalMilliseconds);
return result.Length;
}
private void CountToALargeNumber(int id, int value)
{
for (long i = 0; i < value; i++) ;
Console.WriteLine("End CountToALargeNumber {0} : {1:N0}ms", id, sw.Elapsed.TotalMilliseconds);
}
}
}
结果：
Call 1 start: 1ms
Call 1 completed: 903ms
Call 2 start: 903ms
Call 2 completed: 1,355ms
End CountToALargeNumber 1 : 1,375ms
End CountToALargeNumber 2 : 1,399ms
End CountToALargeNumber 3 : 1,417ms
End CountToALargeNumber 4 : 1,435ms
Chars in Call1:161702
Chars in Call1:5164
从运行结果可以看到，同步执行的时间主要花在了两次请求外部地址上，计算长度并不费时，用图来表示就像下面
2.使用async和await怎么写？
修改上面代码，如下
class MyDownLoadString
{
Stopwatch sw = new Stopwatch();
public void DoRun()
{
const int LargeNumber = 6000000;
sw.Start();
// Task 保存结果对象，后面t1.Result则是获取结果
Task t1 = CountCharactersAsync(1, "http://www.microsoft.com");
Task t2 = CountCharactersAsync(2, "http://www.illustratedcsharp.com");
//无需等待CountCharactersAsync执行完成
CountToALargeNumber(1, LargeNumber);
CountToALargeNumber(2, LargeNumber);
CountToALargeNumber(3, LargeNumber);
CountToALargeNumber(4, LargeNumber);
//t1.Result获取结果
Console.WriteLine("Chars in Call1:{0}",t1.Result);
Console.WriteLine("Chars in Call1:{0}",t2.Result);
}
private async Task CountCharactersAsync(int id, string uriString)
{
WebClient wc = new WebClient();
Console.WriteLine("Call {0} start: {1:N0}ms ", id, sw.Elapsed.TotalMilliseconds);
string result = await wc.DownloadStringTaskAsync(new Uri(uriString));
Trace.TraceInformation("Taceing Async Call {0} @time:{1:N0}ms", id, sw.Elapsed.TotalMilliseconds);
Console.WriteLine("Call {0} completed: {1:N0}ms", id, sw.Elapsed.TotalMilliseconds);
return result.Length;
}
private void CountToALargeNumber(int id, int value)
{
for (long i = 0; i < value; i++) ;
Console.WriteLine("End CountToALargeNumber {0}: {1:N0}ms", id, sw.Elapsed.TotalMilliseconds);
}
}
运行结果：
Call 1 start: 2ms
Call 2 start: 253ms
End CountToALargeNumber 1: 288ms
End CountToALargeNumber 2: 359ms
End CountToALargeNumber 3: 560ms
Call 1 completed: 770ms
End CountToALargeNumber 4: 844ms
Call 2 completed: 887ms
Chars in Call1:162262
Chars in Call2:5164
修改如上面的代码之后，我们就可以无需等待两次CountCharactersAsync返回结果，而是直接调用了下面的CountToALargeNumber，在CountCharactersAsync请求返回的时候再获取结果。
3.async和await的细节
async和await可以创建和使用异步方法，这个特性的由三个部分组成：
- ①调用方法(calling method)：该方法调用异步方法，然后在异步方法（可能使用同一个线程也可能不在一个线程）执行其任务的时候继续执行
- ②异步方法（async）: 该方法异步执行其工作，然后立即方法到调用方法
- ③await表达式：用于异步方法内部，指明需要异步执行的惹怒我。一个异步方法可以包含任意多个await表达式，如果一个都不包含编译器会发出警告