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the previous chapter However, it is fully integrated into the TPL and PLINQ For this reason, it is described here In general, here is how task cancellation works A cancellation token is obtained from a cancellation token source This token is then passed to the task The task must then monitor that token for a cancellation request (This request can come only from the cancellation token source) If a cancellation request is received, the task must end Sometimes it is sufficient for the task to simply stop, taking no further action Other times, the task should call ThrowIfCancellationRequested( ) on the cancellation token This lets the canceling code know that the task was cancelled Now, we will look at the cancellation process in detail A cancellation token is an instance of CancellationToken, which is a structure defined in SystemThreading It defines several properties and methods We will use two of them The first is the read-only property IsCancellationRequested It is shown here: public bool IsCancellationRequested { get; } It returns true if cancellation has been requested on the invoking token and false otherwise The second member that we will use is the ThrowIfCancellationRequested( ) method It is shown here: public void ThrowIfCancellationRequested( ) If the cancellation token on which it is called has received a cancellation request, then this method will throw an OperationCanceledException Otherwise, it takes no action The cancelling code can watch for this exception to confirm that cancellation did, indeed, occur This is normally done by catching AggregateException and then examining the inner exception, via the InnerException or InnerExceptions properties (InnerExceptions is a collection of exceptions Collections are described in 25) A cancellation token is obtained from a cancellation source This is an object of CancellationTokenSource, which is defined in SystemThreading To obtain a token, first create a CancellationTokenSource instance (You can use its default constructor for this purpose) The cancellation token associated with that source is available through the readonly Token property, which is shown here: public CancellationToken Token { get; } This is the token that must be passed to the task that you want to be able to cancel To use cancellation, the task must receive a copy of the cancellation token and then monitor that token, watching for cancellation There are three ways to watch for cancellation: polling, using a callback method, and using a wait handle The easiest is polling, and that is the approach used here To use polling, the task will check the IsCancellationRequested property of the cancellation token, described earlier If this.

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property is true, cancellation has been requested and the task should terminate Polling can be quite efficient if it is done appropriately For example, if a task contains nested loops, then checking IsCancellationRequested in the outer loop would often be better than checking it with each iteration of the inner loop To create a task that calls ThrowIfCancellationRequested( ) when cancelled, you will often want to pass the cancellation token to both the task and the Task constructor, whether directly or indirectly through the StartNew( ) method Passing the cancellation token to the task enables a cancellation request by outside code to change the state of the task to be cancelled Here, we will use this version of StartNew( ): public Task StartNew(Action<Object> action, Object state, CancellationToken cancellationToken) In this use, the cancellation token will be passed to both state and cancellationToken This means that the cancellation token will be passed to both the delegate that implements the task and to the Task instance, itself The form of Action that supports this is shown here: public delegate void Action<in T>(T obj) In this case, T is Object Because of this, inside the task, obj must be cast to CancellationToken One other point: when you are done with the token source, you should release its resources by calling Dispose( ) There are various ways to determine if a task has been cancelled The approach used here is to test the value of IsCanceled on the Task instance If it is true, the task was cancelled The following program demonstrates cancellation It uses polling to monitor the state of the cancellation token Notice that ThrowIfCancellationRequested( ) is called on entry into MyTask( ) This enables the task to be terminated if it was cancelled before it was started Inside the loop, IsCancellationRequested is checked When this property is true (which it will be after Cancel( ) is called on the token source), a message indicating cancellation is displayed and ThrowIfCancellationRequested( ) is called to cancel the task.

// A simple example of cancellation that uses polling using System; using SystemThreading; using SystemThreadingTasks; class DemoCancelTask { // A method to be run as a task static void MyTask(Object ct) { CancellationToken cancelTok = (CancellationToken) ct; // Check if cancelled prior to starting cancelTokThrowIfCancellationRequested(); ConsoleWriteLine("MyTask() starting"); for(int count = 0; count < 10; count++) { // This example uses polling to watch for cancellation if(cancelTokIsCancellationRequested) { ConsoleWriteLine("Cancellation request received");

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Part II: