- A+
所属分类:.NET技术
async和await详解
1.非UI线程中执行
Test()函数带有async 和await ,返回值写成Task。
1 using System; 2 using System.Threading; 3 using System.Threading.Tasks; 4 5 namespace _00_测试 6 { 7 class Program 8 { 9 static void Main(string[] args) 10 { 11 Console.WriteLine($"Main Start:{Thread.CurrentThread.ManagedThreadId}"); 12 Task task = Test(); 13 Console.WriteLine($"Main End:{Thread.CurrentThread.ManagedThreadId}"); 14 Console.ReadKey(); 15 } 16 private async static Task Test() 17 { 18 Console.WriteLine($"当前主线程ID::{Thread.CurrentThread.ManagedThreadId}"); 19 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 20 Task task1 = Task.Factory.StartNew(() => 21 { 22 Thread.Sleep(100); 23 Console.WriteLine($"task1:{Thread.CurrentThread.ManagedThreadId}"); 24 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 25 }); 26 await task1; 27 28 Console.WriteLine($"task1 结束后的线程ID:{Thread.CurrentThread.ManagedThreadId}"); 29 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 30 31 await Task.Run(() => 32 { 33 Thread.Sleep(100); 34 Console.WriteLine($"task2:{Thread.CurrentThread.ManagedThreadId}"); 35 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 36 }); 37 Console.WriteLine($"Test End:{Thread.CurrentThread.ManagedThreadId}"); 38 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 39 } 40 } 41 }
上面是控制台应用程序,主线程的ID为1,第一个await和后面的代码都是子线程完成的。第二个await和后面的代码,也是子线程完成的。在非UI线程中执行的async异步方法,await等待的异步操作和后面要执行的代码,都是从线程池获取一个线程来执行的。
2.UI线程中执行
1 using System; 2 using System.Collections.Generic; 3 using System.ComponentModel; 4 using System.Data; 5 using System.Drawing; 6 using System.Linq; 7 using System.Text; 8 using System.Threading; 9 using System.Threading.Tasks; 10 using System.Windows.Forms; 11 12 namespace _009__数据库 13 { 14 public partial class Form1 : Form 15 { 16 public Form1() 17 { 18 InitializeComponent(); 19 } 20 21 private async void button1_Click(object sender, EventArgs e) 22 { 23 Console.WriteLine($"Main Start:{Thread.CurrentThread.ManagedThreadId}"); 24 Task task = Test(); 25 await task; 26 Console.WriteLine($"Main End:{Thread.CurrentThread.ManagedThreadId}"); 27 Console.ReadKey(); 28 } 29 private async static Task Test() 30 { 31 Console.WriteLine($"当前主线程ID::{Thread.CurrentThread.ManagedThreadId}"); 32 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 33 Task task1 = Task.Factory.StartNew(() => 34 { 35 Thread.Sleep(100); 36 Console.WriteLine($"task1:{Thread.CurrentThread.ManagedThreadId}"); 37 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 38 }); 39 await task1; 40 41 Console.WriteLine($"task1 结束后的线程ID:{Thread.CurrentThread.ManagedThreadId}"); 42 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 43 44 await Task.Run(() => 45 { 46 Thread.Sleep(100); 47 Console.WriteLine($"task2:{Thread.CurrentThread.ManagedThreadId}"); 48 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 49 }); 50 Console.WriteLine($"Test End:{Thread.CurrentThread.ManagedThreadId}"); 51 Console.WriteLine($"是否是线程池线程:{Thread.CurrentThread.IsThreadPoolThread}"); 52 } 53 } 54 }
在UI线程中,async切换线程的规律和非UI线程不一样了。在UI线程中,await后面紧跟的代码,一直都是在UI线程中执行的。
注意:非UI线程中,await后面的动作都是子线程完成的;UI线程中,await后面的动作都是主线程完成的。
3.带返回值的异步方法
非UI线程
1 using System; 2 using System.Threading; 3 using System.Threading.Tasks; 4 5 namespace _00_测试 6 { 7 class Program 8 { 9 static void Main(string[] args) 10 { 11 Console.WriteLine($"Main Start:{Thread.CurrentThread.ManagedThreadId}"); 12 Task<int> task = Test(); 13 Console.WriteLine($"结果为:{task.Result}"); 14 Console.WriteLine($"Main End:{Thread.CurrentThread.ManagedThreadId}"); 15 Console.ReadKey(); 16 } 17 private async static Task<int> Test() 18 { 19 int Value = 0; 20 Task task1 = Task.Factory.StartNew(() => 21 { 22 Value++; 23 Thread.Sleep(100); 24 }); 25 await task1; 26 return Value; 27 } 28 } 29 }
执行Test()异步方法,然后获取异步方法的返回值,执行异步方法的线程会一直阻塞,直到等到要获取的返回值。但是,在UI线程中,执行异步方案的是主线程,直接就死锁了。
UI线程
1 using System; 2 using System.Collections.Generic; 3 using System.ComponentModel; 4 using System.Data; 5 using System.Drawing; 6 using System.Linq; 7 using System.Text; 8 using System.Threading; 9 using System.Threading.Tasks; 10 using System.Windows.Forms; 11 12 namespace _009__数据库 13 { 14 public partial class Form1 : Form 15 { 16 public Form1() 17 { 18 InitializeComponent(); 19 } 20 21 private void button1_Click(object sender, EventArgs e) 22 { 23 Console.WriteLine($"Main Start:{Thread.CurrentThread.ManagedThreadId}"); 24 Task<int> task = Test(); 25 Console.WriteLine($"结果为:{task.Result}"); 26 Console.WriteLine($"Main End:{Thread.CurrentThread.ManagedThreadId}"); 27 Console.ReadKey(); 28 } 29 private async static Task<int> Test() 30 { 31 int Value = 0; 32 Task task1 = Task.Factory.StartNew(() => 33 { 34 Value++; 35 Thread.Sleep(100); 36 }); 37 await task1; 38 return Value; 39 } 40 } 41 }
在winform中,点击按钮,界面直接卡死了!!!
分析, 执行Test()异步方法,然后获取异步方法的返回值,但是在UI线程中,await后面的操作是UI线程执行的。那么,首先异步方法执行了await中的异步任务,UI线程已经开始等这个执行结果了,UI线程阻塞等待中;而await后面的
return Value;这一行,需要UI线程执行啊,此时UI线程阻塞等结果呢无法执行其他操作,就这么UI等返回值,子线程等UI线程等UI线程来执行 return Value;这行代码。谁也不让谁的等待下去,这就是死锁了。