C#中的非同步多線程1-同步和非同步對比

同步版本示例：

namespace SyncSample
{
    class MyDownloadString
    {
        Stopwatch sw = new Stopwatch();
        public void DoRun()
        {
            const int LargeNumber = 6000000;
            sw.Start();
            //網路任務
            int t1 = CountCharacters(1, "https://www.baidu.com");
            int t2 = CountCharacters(2, "https://www.sogou.com/");
            //本機任務
            CountToALargeNumber(1, LargeNumber);
            CountToALargeNumber(2, LargeNumber);
            CountToALargeNumber(3, LargeNumber);
            CountToALargeNumber(4, LargeNumber);
            Console.WriteLine("Chars in Baidu:{0}", t1);
            Console.WriteLine("Chars in Sougou:{0}", t2);
        }
        private int CountCharacters(int id, string url)
        {
            WebClient wc1 = new WebClient();
            Console.WriteLine("Starting call {0}:{1,4:N0} ms", id, sw.Elapsed.TotalMilliseconds);
            string result = wc1.DownloadString(new Uri(url));
            Console.WriteLine("call {0} completed:{1,4: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 counting {0}:{1,4:N0} ms", id, sw.Elapsed.TotalMilliseconds);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            MyDownloadString ds = new MyDownloadString();
            ds.DoRun();
            Console.ReadLine();
            //同步時，會按照如下順序執行 t1-t2-count1-count2-count3-count4，最耗時的任務發生在等待網站數據（starting call-call completed）的過程
            //但如果能夠在等待的過程中就執行count1-4就會極大節省時間
        }
    }
}

耗時為：

Starting call 1: 0 ms
call 1 completed: 222 ms
Starting call 2: 222 ms
call 2 completed: 435 ms
End counting 1: 448 ms
End counting 2: 460 ms
End counting 3: 473 ms
End counting 4: 486 ms
Chars in Baidu:14462
Chars in Sougou:19466

對應非同步示例：

    class MyDownLoadString
    {
        Stopwatch sw = new Stopwatch();
        public void DoRun()
        {
            const int LargeNumber = 600000;
            sw.Start();
            Task<int> t1 = CountCharactersAsync(1, "https://www.baidu.com");
            Task<int> t2 = CountCharactersAsync(2, "http://www.sougou.com");
            CountToALargeNumber(1, LargeNumber);
            CountToALargeNumber(2, LargeNumber);
            CountToALargeNumber(3, LargeNumber);
            CountToALargeNumber(4, LargeNumber);
            Console.WriteLine("Chars in Baidu:{0}", t1.Result);
            Console.WriteLine("Chars in Sougou:{0}", t2.Result);
        }
        private async Task<int> CountCharactersAsync(int id, string site)
        {
            WebClient wc = new WebClient();
            Console.WriteLine("Starting call {0}:{1,4:N0} ms", id, sw.Elapsed.TotalMilliseconds);
            string result = await wc.DownloadStringTaskAsync(new Uri(site));
            Console.WriteLine("Call {0} completed:{1,4: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 counting {0}:{1,4:N0} ms", id, sw.Elapsed.TotalMilliseconds);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            MyDownLoadString ds = new MyDownLoadString();
            ds.DoRun();
            Console.ReadLine();
            //對於耗時的網路任務，修改成了非同步執行的方式，於是任務執行演變成了t1 start,t2 start-count1-4-t1 completed/t2 completed
            //在等待網路任務的同時，完成了本機任務
            //而這些任務，都是在主線程完成，並沒有創建其他線程
        }
    }

耗時為：

Starting call 1: 1 ms
Starting call 2: 58 ms
End counting 1: 62 ms
End counting 2: 63 ms
End counting 3: 64 ms
End counting 4: 65 ms
Call 2 completed: 199 ms
Call 1 completed: 243 ms
Chars in Baidu:14462
Chars in Sougou:16597

可見有效的壓縮了時間，這是因為4次CountToALargeNumber都是在等待網站響應的過程中完成的，而這些工作都是在主線程完成的，並沒有創建額外的線程。