取消引发的任务未设置对象引用 [英] Cancel a task raised Object reference not set

问题描述

我有一段代码。

I have a piece of code.

class Program
    {
        static void Main(string[] args)
        {
            CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
            CancellationToken token = cancellationTokenSource.Token;
            Task task = Task.Run(() =>
            {
                while (!token.IsCancellationRequested)
                {
                    Console.Write('*');
                    Thread.Sleep(1000);
                }
                token.ThrowIfCancellationRequested();
            }, token).ContinueWith((t)=>
                {
                    t.Exception.Handle((e)=>true);
                    Console.WriteLine("You have canceled the task");
                }, TaskContinuationOptions.OnlyOnCanceled); ;
            try
            {
                Console.WriteLine("Press enter to stop the task");
                Console.ReadLine();
                cancellationTokenSource.Cancel();
                task.Wait();
            }
            catch (AggregateException e)
            {
                Console.WriteLine(e.InnerExceptions[0].Message);
            }
            Console.WriteLine("Press enter to end the application");
            Console.ReadLine();
        }
    }

当输入密钥时，我得到一个异常对象引用没有设置为对象的实例。我知道这是一个非常常见的简单和愚蠢的错误。



如果我删除task.Wait（）那么错误就会消失但是无法弄清楚为什么。

I got an exception "Object reference not set to an instance of an object" when hit enter key. I know it is a very common simple and stupid error.

If I remove task.Wait() then the error will go away but just can't figure out why.

推荐答案

好的，弄清楚。

移动

Okay, figure it out.
Move

token.ThrowIfCancellationRequested();

到正确的位置。

to a proper position.

Task task = Task.Run(() =>
           {
               while (!token.IsCancellationRequested)
               {
                   Console.Write('*');
                   Thread.Sleep(1000);
               }

           }, token).ContinueWith((t)=>
               {
                   t.Exception.Handle((e)=>true);
                   Console.WriteLine("You have canceled the task");
               }, TaskContinuationOptions.OnlyOnCanceled);
           token.ThrowIfCancellationRequested();
           try
           {
               Console.WriteLine("Press enter to stop the task");
               Console.ReadLine();
               cancellationTokenSource.Cancel();
               //task.Wait();
           }
           catch (AggregateException e)
           {
               Console.WriteLine(e.InnerExceptions[0].Message);
           }
           Console.WriteLine("Press enter to end the application");
           Console.ReadLine();

class Program
    {
        static void Main(string[] args)
        {
            CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
            CancellationToken token = cancellationTokenSource.Token;
            Task task = Task.Run(() =>
            {
                while (true)
                {
                    token.ThrowIfCancellationRequested();
                    Console.Write('*');
                    Thread.Sleep(1000);
                }
               
            }, token).
            ContinueWith((t) =>
            {
                Console.WriteLine("You have canceled the task");
            }, TaskContinuationOptions.OnlyOnCanceled);
           
            try
            {
                Console.WriteLine("Press enter to stop the task");
                Console.ReadLine();
                cancellationTokenSource.Cancel();
            }
            catch (AggregateException e)
            {
                Console.WriteLine(e.InnerExceptions[0].Message);
            }
            task.Wait();
            Console.WriteLine("Press enter to end the application");
            Console.ReadLine();
        }
    }

这篇关于取消引发的任务未设置对象引用的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！