处理在已取消的任务中引发的异常

Question

I'm working through the examples in the Exam Ref 70-483: Programming in C# book and have run into a problem with the following code from listing 1-44. In this example, the author is trying to demonstrate that a continuation task has access to unhandled exceptions thrown in the antecedent task, and can handle them if it is appropriate to do so.

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);
        }
        throw new OperationCanceledException();
    }, token).ContinueWith((t) =>
    {
        t.Exception.Handle((e) => true);
        Console.WriteLine("You have canceled the task.");
    }, TaskContinuationOptions.OnlyOnCanceled);

    Console.WriteLine("Press enter to stop the task.");
    Console.ReadLine();

    cancellationTokenSource.Cancel();
    task.Wait();

    Console.WriteLine("Press enter to end the application.");
    Console.ReadLine();
}

Unfortunately, this line of code in the continuation

t.Exception.Handle((e) => true);

throws an exception because t.Exception is null .

Setting a breakpoint at that line, I can see that t.Status is Canceled , not Faulted . Is this why the exception is not available? What is the correct way to handle the exception thrown in the antecedent task?

Answer 1

Use

task.ContinueWith(HandleError, TaskContinuationOptions.OnlyOnFaulted);

Cancelled tasks are not exceptional... And therefore will not include an exception. Faulted ones will.

Answer 2

Change the code for Listing 1-44 to:

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);