示範C#裡的async和await的使用

寫了段小代碼，給同事示範一下這2個文法糖的代碼執行順序：

class Program    {        static void Main()        {            Msg("Begin");            DisplayValue();            Msg("End");            Console.Read();        }        #region async 測試        public static async Task<double> GetValueAsync(double num1, double num2)        {            Msg("Into  GetValueAsync");            var ret = await Task.Run(() =>            {                Msg("Into  GetValueAsync Task");                for (int i = 0; i < 1000000; i++)                    num1 = num1 / num2;                Msg("out  GetValueAsync Task");                return num1;            });            Msg("out  GetValueAsync");            return ret;        }        public static async void DisplayValue()        {            Msg("Into  DisplayValue");            var result = GetValueAsync(1234.5, 1.01);            Msg("Middle  DisplayValue");            Msg("Value is : " + await result);            Msg("Out  DisplayValue");        }        #endregion        private static int idx;        static void Msg(string msg)        {            Interlocked.Increment(ref idx);            var id = Thread.CurrentThread.ManagedThreadId.ToString();            Console.WriteLine(idx.ToString() + ". 線程:" + id + "  " + msg);            Thread.Sleep(100); // 避免cpu不定，導致順序打亂        }    }

代碼執行結果：

1. 線程:1  Begin2. 線程:1  Into  DisplayValue3. 線程:1  Into  GetValueAsync4. 線程:1  Middle  DisplayValue5. 線程:3  Into  GetValueAsync Task6. 線程:1  End7. 線程:3  out  GetValueAsync Task8. 線程:3  out  GetValueAsync9. 線程:3  Value is : 2.47032822920623E-32210. 線程:3  Out  DisplayValue

把GetValueAsync方法的async和await 移除

public static Task<double> GetValueAsync(double num1, double num2){    Msg("Into  GetValueAsync");    var ret = Task.Run(() =>    {        Msg("Into  GetValueAsync Task");        for (int i = 0; i < 1000000; i++)            num1 = num1 / num2;        Msg("out  GetValueAsync Task");        return num1;    });    Msg("out  GetValueAsync");    return ret;}

執行順序就變了，結果如下：

1. 線程:1  Begin2. 線程:1  Into  DisplayValue3. 線程:1  Into  GetValueAsync4. 線程:1  out  GetValueAsync5. 線程:3  Into  GetValueAsync Task6. 線程:1  Middle  DisplayValue7. 線程:1  End8. 線程:3  out  GetValueAsync Task9. 線程:3  Value is : 2.47032822920623E-32210. 線程:3  Out  DisplayValue

你看明白了嗎。如果不明白，把上面的測試代碼複製到你的項目裡，改一改，運行一下吧，
簡單總結一下：
- await指令調起線程執行非同步方法呼叫和方法內後面的代碼
- await指令會阻塞當前方法後面的代碼，但是不會阻塞外部調用
- await必須搭配async使用，而async不一定需要有await