当我使用ContinueWith链接一个方法并且如果该方法使用异步Action时,为什么在执行Task.WhenAll时为什么链接的方法没有等待完成?
例如以下代码
var str1 = "str1";
var str2 = "str2";
var task0 = Task.Run( async ()=> {
await Task.Delay(1000);
return "changed";
});
var task1 = task0.ContinueWith(async prevTask => {
await Task.Delay(5000);
str1 = prevTask.Result;
});
var task2 = task0.ContinueWith(prevTask => {
Task.Delay(1000).Wait();
str2 = prevTask.Result;
});
await Task.WhenAll(task0, task1, task2);
Console.WriteLine(str1);
Console.WriteLine(str2);
产生输出
str1
changed
但是,当我尝试通过异步操作对另一个任务执行相同操作但阻止对Task.Delay的调用时,则该方法可以正常工作。例如,将任务3添加为
....
var str3 = "str3";
var task3 = task0.ContinueWith(async prevTask => {
Task.Delay(2500).Wait();
str3 = prevTask.Result;
});
await Task.WhenAll(task0, task1, task2, task3);
Console.WriteLine(str1);
Console.WriteLine(str2);
Console.WriteLine(str3);
产生以下输出
str1
changed
changed
在调试时,我还看到task1的状态为“ Ran to complete”,我不明白为什么。似乎在ContinueWith中等待正在导致任务假定任务已完成,但是我不明白为什么?
链接到功能代码:http://rextester.com/WFRTKY13796
答案 0 :(得分:0)
task1
的类型为Task<Task>
,因此您需要等待任务以获取结果集。这很容易通过在行中添加await
来完成,而第二个示例使用阻塞调用。这是修改后的rexster代码:
//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
namespace Rextester
{
public class Program
{
public static void Main(string[] args)
{
Test().Wait();
}
public static async Task Test(){
var str1 = "str1_beforeChange";
var str2 = "str2_beforeChange";
var str3 = "str3_beforeChange";
var task0 = Task.Run(async ()=>{
await Task.Delay(2500);
return "afterChange";
});
//await the async continuation
var task1 = await task0.ContinueWith(async prevTask => {
await Task.Delay(5000);
str1 = prevTask.Result;
});
var task2 = task0.ContinueWith(prevTask =>{
Task.Delay(2500).Wait();
str2 = prevTask.Result;
});
var task3 = task0.ContinueWith(prevTask =>{
Task.Delay(1000).Wait();
str3 = prevTask.Result;
});
await Task.WhenAll(task0, task1, task2, task3);
Console.WriteLine(str1);
Console.WriteLine(str2);
Console.WriteLine(str3);
}
}
}