Cancel a task raised Object reference not set

Question

1.00/5 (1 vote)

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I have a piece of code.

C#

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();
        }
    }

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.

Posted 6-Jan-15 5:00am

zhshqzyc

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Comments

[no name] 6-Jan-15 11:09am

Because you trhow this exception by yourself after the first "whle".

Sergey Alexandrovich Kryukov 6-Jan-15 17:25pm

This "throw" is done for a good reason, really. It only should be handled properly...
—SA

[no name] 7-Jan-15 3:44am

Thank you for the hint. I try to understand the code...until now without big success :-)
Bruno

Sergey Alexandrovich Kryukov 6-Jan-15 17:26pm

Is Sleep(1000) only for debugging purposes? For other purposes, it would be pointless...
Anyway, you should explain more, and start with what you want to achieve, exactly. The idea looks good, but I can only guess...
—SA

[no name] 7-Jan-15 3:42am

Sorry I lead you to a wrong direction. Please read the comments of Sergey in your question.

2 solutions

Solution 1

Okay, figure it out.
Move

C#

token.ThrowIfCancellationRequested();

to a proper position.

C#

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();

Posted 6-Jan-15 5:19am

zhshqzyc

Comments

[no name] 6-Jan-15 11:26am

Now you can uncomment //Task.Wait(); :-)

[no name] 6-Jan-15 11:35am

But I just found "You have canceled the task" is still not displayed. Something is still wrong.

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zhshqzyc · Accepted Answer · 2015-01-07T02:42:00

C#

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();
        }
    }