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How to: Listen for Cancellation Requests by Polling

The following example shows one way that user code can poll a cancellation token at regular intervals to see whether cancellation has been requested from the calling thread. This example uses the System.Threading.Tasks.Task type, but the same pattern applies to asynchronous operations created directly by the System.Threading.ThreadPool type or the System.Threading.Thread type.

Polling requires some kind of loop or recursive code that can periodically read the value of the Boolean IsCancellationRequested property. If you are using the System.Threading.Tasks.Task type and you are waiting for the task to complete on the calling thread, you can use the ThrowIfCancellationRequested method to check the property and throw the exception. By using this method, you ensure that the correct exception is thrown in response to a request. If you are using a Task, then calling this method is better than manually throwing an OperationCanceledException. If you do not have to throw the exception, then you can just check the property and return from the method if the property is true.

Class CancelByPolling

    Shared Sub Main()

        Dim tokenSource As New CancellationTokenSource()
        ' Toy object for demo purposes
        Dim rect As New Rectangle()
        rect.columns = 1000
        rect.rows = 500

        ' Simple cancellation scenario #1. Calling thread does not wait
        ' on the task to complete, and the user delegate simply returns
        ' on cancellation request without throwing.
        Task.Factory.StartNew(Sub() NestedLoops(rect, tokenSource.Token), tokenSource.Token)

        ' Simple cancellation scenario #2. Calling thread does not wait
        ' on the task to complete, and the user delegate throws 
        ' OperationCanceledException to shut down task and transition its state.
        ' Task.Factory.StartNew(Sub() PollByTimeSpan(tokenSource.Token), tokenSource.Token)

        Console.WriteLine("Press 'c' to cancel")
        If Console.ReadKey().KeyChar = "c"c Then

            Console.WriteLine("Press any key to exit.")
        End If


    End Sub
    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.
                Console.Write("0' end block,1' end block ", x, y)

            ' 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
    End Sub

End Class

Calling ThrowIfCancellationRequested is extremely fast and does not introduce significant overhead in loops.

If you are calling ThrowIfCancellationRequested, you only have to explicitly check the IsCancellationRequested property if you have other work to do in response to the cancellation besides throwing the exception. In this example, you can see that the code actually accesses the property twice: once in the explicit access and again in the ThrowIfCancellationRequested method. But because the act of reading the IsCancellationRequested property involves only one volatile read instruction per access, the double access is not significant from a performance perspective. It is still preferable to call the method rather than manually throw the OperationCanceledException.