C# 5.0語法糖，依託Task存在，本身不會產生執行緒，達到了通過非同步方法實現同步效果的目的（解決非同步多執行緒執行順序難控制的問題）

1.定義

async可以隨便新增，可以不用新增await

await只能出在Task前面，但是方法必須宣告async，不能單獨出現

await/async之後，原本沒有返回值，可以返回Task

　　　　　　　　原本返回T型別的,返回Task<T>　

　　　　　　　　儘量不要返回void,因為不能再await了　　　

2.執行順序

namespace AwaitAsyncTest
{
    class Program
    {
        
 
static void Main(string[] args)
        {
            AwaitAsyncClassNew awaitTest = new AwaitAsyncClassNew();
            awaitTest.Show();
            Console.ReadKey();
        }


    }

   class AwaitAsyncClassNew
    {
        public void Show()
        {
            Console.WriteLine($"This is MainThread Start{Thread.CurrentThread.ManagedThreadId}
 
");
            this.NoReturnTask();
            Console.WriteLine($"This is MainThread End {Thread.CurrentThread.ManagedThreadId}");
        }

        public async Task NoReturnTask()
        {
            Console.WriteLine($"This is NoReturnTask Start{Thread.CurrentThread.ManagedThreadId}");
            Task task 
 
= Task.Run(() =>
            {
                Console.WriteLine($"NoReturnTask Start{Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(1000);
                Console.WriteLine($"NoReturnTask End{Thread.CurrentThread.ManagedThreadId}");
            });//呼叫執行緒發起，啟動新執行緒執行內部動作
            await task;//呼叫執行緒回去忙自己的事
            Console.WriteLine($"This is NoReturnTask End{Thread.CurrentThread.ManagedThreadId}");//由子執行緒完成（如果沒有await，應該由呼叫執行緒執行）
        }
　　}

執行結果：

This is MainThread Start1
This is NoReturnTask Start1
This is MainThread End 1
NoReturnTask Start3
NoReturnTask End3
This is NoReturnTask End3 --本來屬於主執行緒的卻被子執行緒執行了

相當於await的後續程式碼包裝成了一個回撥，執行次此回撥的執行緒有多種可能性

將await開始的程式碼換成

        public async Task NoReturnTask()
        {
            Console.WriteLine($"This is NoReturnTask Start{Thread.CurrentThread.ManagedThreadId}");
            Task task = Task.Run(() =>
            {
                Console.WriteLine($"NoReturnTask Start{Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(1000);
                Console.WriteLine($"NoReturnTask End{Thread.CurrentThread.ManagedThreadId}");
            });
            //await task;
            //Console.WriteLine($"This is NoReturnTask End{Thread.CurrentThread.ManagedThreadId}");
            task.ContinueWith(t => 
                Console.WriteLine($"This is NoReturnTask End{Thread.CurrentThread.ManagedThreadId}"));
        }

結果為

This is MainThread Start1
This is NoReturnTask Start1
This is MainThread End 1
NoReturnTask Start3
NoReturnTask End3
This is NoReturnTask End4

如果再更多層的await，執行順序依次向後（更像是順序執行了），相當於用同步的形式去寫非同步

public async Task NoReturnTask()
        {
            Console.WriteLine($"This is NoReturnTask Start{Thread.CurrentThread.ManagedThreadId}");
            Task task = Task.Run(() =>
            {
                Console.WriteLine($"NoReturnTask Start{Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(1000);
                Console.WriteLine($"NoReturnTask End{Thread.CurrentThread.ManagedThreadId}");
            });
            await task;
            Console.WriteLine($"This is NoReturnTask End{Thread.CurrentThread.ManagedThreadId}");
            //task.ContinueWith(t => 
            //    Console.WriteLine($"This is NoReturnTask End{Thread.CurrentThread.ManagedThreadId}"));
            await Task.Run(() =>
            {
                Console.WriteLine($"NoReturnTask2 Start{Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(1000);
                Console.WriteLine($"NoReturnTask2 End{Thread.CurrentThread.ManagedThreadId}");
            });
            Console.WriteLine($"This is NoReturnTask2 End{Thread.CurrentThread.ManagedThreadId}");

        }

結果為

This is MainThread Start1
This is NoReturnTask Start1
NoReturnTask Start3
This is MainThread End 1 --主執行緒結束
NoReturnTask End3
This is NoReturnTask End3 --執行完task1
NoReturnTask2 Start4 ---執行Task2
NoReturnTask2 End4
This is NoReturnTask2 End4

3.執行緒ID問題

4.返回值問題