The .NET Framework version 4 introduces a new unified model for cooperative cancellation of asynchronous or long-running synchronous operations. This model is based on a lightweight object called a cancellation token. The object that invokes a cancelable operation, for example by creating a new thread or task, passes the token to the operation. That operation can in turn pass copies of the token to other operations. At some later time, the object that created the token can use it to request that the operation stop what it is doing. Only the requesting object can issue the cancellation request, and each listener is responsible for noticing the request and responding to it in a timely manner. The following illustration shows the relationship between a token source and all the copies of its token.
The new cancellation model makes it easier to create cancellation-aware applications and libraries, and it supports the following features:
Cancellation is cooperative and is not forced on the listener. The listener determines how to gracefully terminate in response to a cancellation request.
Requesting is distinct from listening. An object that invokes a cancelable operation can control when (if ever) cancellation is requested.
The requesting object issues the cancellation request to all copies of the token by using just one method call.
A listener can listen to multiple tokens simultaneously by joining them into one linked token.
User code can notice and respond to cancellation requests from library code, and library code can notice and respond to cancellation requests from user code.
Listeners can be notified of cancellation requests by polling, callback registration, or waiting on wait handles.
The new cancellation framework is implemented as a set of related types, which are listed in the following table.
Object that creates a cancellation token, and also issues the cancellation request for all copies of that token.
Lightweight value type passed to one or more listeners, typically as a method parameter. Listeners monitor the value of the IsCancellationRequested property of the token by polling, callback, or wait handle.
New overloads of this exception accept a CancellationToken as an input parameter. Listeners can optionally throw this exception to verify the source of the cancellation and notify others that it has responded to a cancellation request.
The new cancellation model is integrated into the .NET Framework in several types. The most important ones are System.Threading.Tasks.Parallel, System.Threading.Tasks.Task, System.Threading.Tasks.Task(Of TResult) and System.Linq.ParallelEnumerable. We recommend that you use this new cancellation model for all new library and application code.
In the following example, the requesting object creates a CancellationTokenSource object, and then passes its Token property to the cancelable operation. The operation that receives the request monitors the value of the IsCancellationRequested property of the token by polling. When the value becomes true, the listener can terminate in whatever manner is appropriate. In this example, the method just exits, which is all that is required in many cases.
The example uses the QueueUserWorkItem method to demonstrate that the new cancellation framework is compatible with legacy APIs. For an example that uses the new, preferred System.Threading.Tasks.Task type, see How to: Cancel a Task and Its Children.
Shared Sub CancelWithThreadPoolMiniSnippet() 'Thread 1: The Requestor ' Create the token source. Dim cts As New CancellationTokenSource() ' Pass the token to the cancelable operation. ThreadPool.QueueUserWorkItem(New WaitCallback(AddressOf DoSomeWork), cts.Token) ' Request cancellation by setting a flag on the token. cts.Cancel() ' end block End Sub 'Thread 2: The Listener Shared Sub DoSomeWork(ByVal obj As Object) Dim token As CancellationToken = CType(obj, CancellationToken) For i As Integer = 0 To 1000000 ' Simulating work. Thread.SpinWait(5000000) If token.IsCancellationRequested Then ' Perform cleanup if necessary. '... ' Terminate the operation. Exit For End If Next End Sub
In the new cancellation framework, cancellation refers to operations, not objects. The cancellation request means that the operation should stop as soon as possible after any required cleanup is performed. One cancellation token should refer to one "cancelable operation," however that operation may be implemented in your program. After the IsCancellationRequested property of the token has been set to true, it cannot be reset to false. Therefore, cancellation tokens cannot be reused after they have been canceled.
If you require an object cancellation mechanism, you can base it on the operation cancellation mechanism, as shown in the following example.
Dim cts As New CancellationTokenSource() Dim token As CancellationToken = cts.Token ' User defined Class with its own method for cancellation Dim obj1 As New MyCancelableObject() Dim obj2 As New MyCancelableObject() Dim obj3 As New MyCancelableObject() ' Register the object's cancel method with the token's ' cancellation request. token.Register(Sub() obj1.Cancel()) token.Register(Sub() obj2.Cancel()) token.Register(Sub() obj3.Cancel()) ' Request cancellation on the token. cts.Cancel()
If an object supports more than one concurrent cancelable operation, pass a separate token as input to each distinct cancelable operation. That way, one operation can be cancelled without affecting the others.
The implementer of a cancelable operation determines, in the user delegate, how to terminate the operation in response to a cancellation request. In many cases, the user delegate can just perform any required cleanup and then return immediately.
However, in more complex cases, it might be necessary for the user delegate to notify library code that cancellation has occurred. In such cases, the correct way to terminate the operation is for the delegate to call , which will cause a OperationCanceledException to be thrown. New overloads of this exception in the .NET Framework version 4 take a CancellationToken as an argument. Library code can catch this exception on the user delegate thread, and examine the exception's token to determine whether the exception indicates cooperative cancellation, or some other exceptional situation.
Listening by Polling
For long-running computations that loop or recurse, you can listen for a cancellation request by periodically polling the value of the CancellationToken.IsCancellationRequested property. If the value is true, then the method should clean up and terminate as quickly as possible. The optimal frequency of polling depends on the type of application. It is up to the developer to determine the best polling frequency for any given program. Polling itself does not significantly impact performance. The following example shows one possible way to poll.
Shared Sub NestedLoops(ByVal rect As Rectangle, ByVal token As CancellationToken) For x As Integer = 0 To rect.columns For y As Integer = 0 To rect.rows ' Simulating work. Thread.SpinWait(5000) Console.Write("0' end block,1' end block ", x, y) Next ' Assume that we know that the inner loop is very fast. ' Therefore, checking once per row is sufficient. If token.IsCancellationRequested = True Then ' Cleanup or undo here if necessary... Console.WriteLine("\r\nCancelling after row 0' end block.", x) Console.WriteLine("Press any key to exit.") ' then... Exit For ' ...or, if using Task: ' token.ThrowIfCancellationRequested() End If Next End Sub
For a more complete example, see How to: Listen for Cancellation Requests by Polling.
Listening by Registering a Callback
Some operations can become blocked in such a way that they cannot check the value of the cancellation token in a timely manner. For these cases, you can register a callback method that unblocks the method when a cancellation request is received.
The Register method returns a CancellationTokenRegistration object that is used specifically for this purpose. The following example shows how to use the Register method to cancel an asynchronous Web request.
Dim cts As New CancellationTokenSource() Dim token As CancellationToken = cts.Token Dim wc As New WebClient() ' To request cancellation on the token ' will call CancelAsync on the WebClient. token.Register(Sub() wc.CancelAsync()) Console.WriteLine("Starting request") wc.DownloadStringAsync(New Uri("http://www.contoso.com"))
The CancellationTokenRegistration object manages thread synchronization and ensures that the callback will stop executing at a precise point in time.
In order to ensure system responsiveness and to avoid deadlocks, the following guidelines must be followed when registering callbacks:
The callback method should be fast because it is called synchronously and therefore the call to Cancel does not return until the callback returns.
If you call Dispose while the callback is running, and you hold a lock that the callback is waiting on, your program can deadlock. After Dispose returns, you can free any resources required by the callback.
Callbacks should not perform any manual thread or SynchronizationContext usage in a callback. If a callback must run on a particular thread, use the System.Threading.CancellationTokenRegistration constructor that enables you to specify that the target syncContext is the active SynchronizationContext.Current. Performing manual threading in a callback can cause deadlock.
For a more complete example, see How to: Register Callbacks for Cancellation Requests.
Listening by Using a Wait Handle
When a cancelable operation can block while it waits on a synchronization primitive such as a System.Threading.ManualResetEvent or System.Threading.Semaphore, you can use the CancellationToken.WaitHandle property to enable the operation to wait on both the event and the cancellation request. The wait handle of the cancellation token will become signaled in response to a cancellation request, and the method can use the return value of the WaitAny method to determine whether it was the cancellation token that signaled. The operation can then just exit, or throw a OperationCanceledException, as appropriate.
In new code that targets the .NET Framework version 4, System.Threading.ManualResetEventSlim and System.Threading.SemaphoreSlim both support the new cancellation framework in their Wait methods. You can pass the CancellationToken to the method, and when the cancellation is requested, the event wakes up and throws an OperationCanceledException.
Try ' mres is a ManualResetEventSlim mres.Wait(token) Catch e As OperationCanceledException ' Throw immediately to be responsive. The ' alternative is to do one more item of work, ' and throw on next iteration, because ' IsCancellationRequested will be true. Console.WriteLine("Canceled while waiting.") Throw End Try ' Simulating work. Console.Write("Working...") Thread.SpinWait(500000)
For a more complete example, see How to: Listen for Cancellation Requests That Have Wait Handles.
Listening to Multiple Tokens Simultaneously
In some cases, a listener may have to listen to multiple cancellation tokens simultaneously. For example, a cancelable operation may have to monitor an internal cancellation token in addition to a token passed in externally as an argument to a method parameter. To accomplish this, create a linked token source that can join two or more tokens into one token, as shown in the following example.
Public Sub DoWork(ByVal externalToken As CancellationToken) ' Create a new token that combines the internal and external tokens. Dim internalToken As CancellationToken = internalTokenSource.Token Dim linkedCts As CancellationTokenSource = CancellationTokenSource.CreateLinkedTokenSource(internalToken, externalToken) Using (linkedCts) Try DoWorkInternal(linkedCts.Token) Catch e As OperationCanceledException If e.CancellationToken = internalToken Then Console.WriteLine("Operation timed out.") ElseIf e.CancellationToken = externalToken Then Console.WriteLine("Canceled by external token.") externalToken.ThrowIfCancellationRequested() End If End Try End Using End Sub
Notice that you must call Dispose on the linked token source when you are done with it. For a more complete example, see How to: Listen for Multiple Cancellation Requests.
The unified cancellation framework makes it possible for library code to cancel user code, and for user code to cancel library code in a cooperative manner. Smooth cooperation depends on each side following these guidelines:
If library code provides cancelable operations, it should also provide public methods that accept an external cancellation token so that user code can request cancellation.
If library code calls into user code, the library code should interpret an OperationCanceledException(externalToken) as cooperative cancellation, and not necessarily as a failure exception.
User-delegates should attempt to respond to cancellation requests from library code in a timely manner.