Async方法Await，Task不同的使用方式与执行方式

缺乏、安全感 2022-10-17 05:23 150阅读 0赞

### 目录 ###

*   *  简介
     *  没有返回值，没有Await(NoReturnNoAwait)
     *  没有返回值(NoReturn)
     *  有await单返回Task(NoReturnTask)
     *  有async并返回有参数Task并在外部使用task等待( SumFactoryTask)
     *  没有有async并返回有参数Task并在外部使用task等待(SumFactoryNoAsyncTask)
     *  结论

## 简介 ##

本篇讲述异步方法在配合await及task时，在不同场景下的不同执行方式。

## 没有返回值，没有Await(NoReturnNoAwait) ##

private async static Task Test()
    { 
        Console.WriteLine($"当前主线程id={ Thread.CurrentThread.ManagedThreadId.ToString("00")}");
        { 
            NoReturnNoAwait();
        }
        Console.WriteLine($"Main Thread Task ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
        Console.Read();
    }
    /// <summary>
    /// 只有async没有await
    /// 跟普通方法没有区别
    /// </summary>
    private static async void NoReturnNoAwait()
    { 
        //主线程执行
        Task task = Task.Run(() =>//启动新线程完成任务
        { 
            Console.WriteLine($"NoReturnNoAwait Sleep3000 before,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
            Thread.Sleep(1000);
            Console.WriteLine($"NoReturnNoAwait Sleep3000 after,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
        });
        
        //主线程执行
        Console.WriteLine($"NoReturnNoAwait Sleep after Task,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
    }

执行顺序如下：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3R4MTcyMTExMDI0MA_size_16_color_FFFFFF_t_70]

## 没有返回值(NoReturn) ##

private async static Task Test()
    { 
         Console.WriteLine($"当前主线程id={ Thread.CurrentThread.ManagedThreadId.ToString("00")}");
         { 
             NoReturn();
             for (int i = 0; i < 10; i++)
             { 
                 Thread.Sleep(300);
                 Console.WriteLine($"Main Thread Task ManagedThreadId={ Thread.CurrentThread.ManagedThreadId.ToString("00")} i={ i}");
             }
         }
         
         Console.WriteLine($"Main Thread Task ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
         Console.Read();
     }

private static async void NoReturn()
    { 
        //主线程执行
        Console.WriteLine($"NoReturn Sleep before await,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
        TaskFactory taskFactory = new TaskFactory();
        Task task = taskFactory.StartNew(() =>
        { 
            Console.WriteLine($"NoReturn Sleep3000 before,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
            Thread.Sleep(3000);
            Console.WriteLine($"NoReturn Sleep3000 after,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
        });
        //task.ContinueWith(t => //这是一个回调
        //{ 
        // Console.WriteLine($"NoReturn Sleep after await,ThreadId={Thread.CurrentThread.ManagedThreadId}");
        //});
    
        await task;
        //主线程到await这里就返回了，执行主线程任务
        //同时task的子线程就开始工作，直到Task完成，然后继续后续任务（后续任务的线程ID不一定是这个子线程，可以是子线程，也可以是其他线程，还可以是主线程） 
        //像什么？ 效果上等价于continuewith
        //task.ContinueWith(t =>
        //{ 
        // Console.WriteLine($"NoReturn Sleep after await,ThreadId={Thread.CurrentThread.ManagedThreadId}");
        //});
    
        Console.WriteLine($"NoReturn Sleep after await,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3R4MTcyMTExMDI0MA_size_16_color_FFFFFF_t_70 1]

## 有await单返回Task(NoReturnTask) ##

private async static Task Test()
    { 
        Console.WriteLine($"当前主线程id={ Thread.CurrentThread.ManagedThreadId.ToString("00")}");
        { 
            Task t = NoReturnTask();
            Console.WriteLine($"Main Thread Task ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
            //t.Wait();//主线程等待Task的完成 阻塞的
            //await t;//await后的代码会由线程池的线程执行 非阻塞
        }
        Console.WriteLine($"Main Thread Task ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
        Console.Read();
    }

/// <summary>
    /// 无返回值 async Task == async void
    /// Task和Task<T>能够使用await, Task.WhenAny, Task.WhenAll等方式组合使用。Async Void 不行
    /// </summary>
    /// <returns></returns>
    private static async Task NoReturnTask() //在async/await方法里面如果没有返回值，默认返回一个Task
    { 
        //这里还是主线程的id
        Console.WriteLine($"NoReturnTask Sleep before await,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
    
        Task task = Task.Run(() =>
        { 
            Console.WriteLine($"NoReturnTask Sleep3000 before,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
            Thread.Sleep(1000);
            Console.WriteLine($"NoReturnTask Sleep3000 after,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
        });
        await task;
        Console.WriteLine($"NoReturnTask Sleep after await,ThreadId={ Thread.CurrentThread.ManagedThreadId}");
     
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3R4MTcyMTExMDI0MA_size_16_color_FFFFFF_t_70 2]

## 有async并返回有参数Task并在外部使用task等待( SumFactoryTask) ##

private async static Task Test()
    { 
        Console.WriteLine($"当前主线程id={ Thread.CurrentThread.ManagedThreadId.ToString("00")}");
        
        { 
            Task<long> t = SumAsync();
            Console.WriteLine($"Main Thread Task ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
            long lResult = t.Result;//访问result,阻塞式 主线程等待所有的任务挖成 //如果访问Result，就相当于是同步方法！
            t.Wait();//等价于上一行,阻塞式--同步
            //await t;//非阻塞，
        }
       
        Console.WriteLine($"Main Thread Task ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
        Console.Read();
    }

/// <summary>
    /// 带返回值的Task 
    /// 要使用返回值就一定要等子线程计算完毕
    /// </summary>
    /// <returns>async 就只返回long</returns>
    private static async Task<long> SumAsync()
    { 
        Console.WriteLine($"SumAsync 111 start ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
        long result = 0;
    
        await Task.Run(() =>
        { 
            for (int k = 0; k < 10; k++)
            { 
                Console.WriteLine($"SumAsync { k} await Task.Run ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(1000);
            }
            for (long i = 0; i < 999_999_999; i++)
            { 
                result += i;
            }
        });
    
        Console.WriteLine($"SumFactory 111 end ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");    
        return result;
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3R4MTcyMTExMDI0MA_size_16_color_FFFFFF_t_70 3]

## 没有有async并返回有参数Task并在外部使用task等待(SumFactoryNoAsyncTask) ##

private async static Task Test()
    { 
        Console.WriteLine($"当前主线程id={ Thread.CurrentThread.ManagedThreadId.ToString("00")}");
        
        { 
             Task<long> t = SumAsync();
             Console.WriteLine($"Main Thread Task ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
             long lResult = t.Result;//没有await和async 普通的task
             t.Wait();
        }
       
        Console.WriteLine($"Main Thread Task ManagedThreadId={ Thread.CurrentThread.ManagedThreadId}");
        Console.Read();
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3R4MTcyMTExMDI0MA_size_16_color_FFFFFF_t_70 4]

## 结论 ##

当主线程发现需要启用子线程时，子线程的内容会自动执行，主线程不受影响继续向下执行。  
        当主线程碰到await时，主线程会返回到上一个方法中，当前方法中未执行的代码会被打包交给线程池，并由线程池分配给子线程来执行(但不一定与await后的线程是同一个子线程)。  
        当主线程遇到Task<>.Result(或Result<>.Wait )时，会等待子线程执行过后再继续执行。

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