Procedure: Luisteren naar annuleringsaanvragen met wachtgrepen

Als een methode wordt geblokkeerd terwijl wordt gewacht tot een gebeurtenis wordt gesignaleerd, kan deze de waarde van het annuleringstoken niet controleren en tijdig reageren. In het eerste voorbeeld ziet u hoe u dit probleem kunt oplossen wanneer u met gebeurtenissen werkt, zoals System.Threading.ManualResetEvent die geen systeemeigen ondersteuning bieden voor het geïntegreerde annuleringsframework. In het tweede voorbeeld ziet u een meer gestroomlijnde benadering die gebruikmaakt van System.Threading.ManualResetEventSlim, wat wel ondersteuning biedt voor uniforme annulering.

Notitie

Wanneer Just My Code is ingeschakeld, wordt Visual Studio in sommige gevallen verbroken op de regel die de uitzondering genereert en wordt er een foutbericht weergegeven met de tekst 'Uitzondering die niet wordt verwerkt door gebruikerscode'. Deze fout is goedaardig. U kunt op F5 drukken om verder te gaan en het gedrag voor uitzonderingsafhandeling te bekijken dat in de onderstaande voorbeelden wordt gedemonstreerd. Als u wilt voorkomen dat Visual Studio de eerste fout veroorzaakt, schakelt u het selectievakje Just My Code uit onder Extra, Opties, Foutopsporing, Algemeen.

Voorbeeld 1

In het volgende voorbeeld wordt een ManualResetEvent voorbeeld gebruikt om te laten zien hoe u wachtgrepen kunt deblokkeren die geen ondersteuning bieden voor uniforme annulering.

using System;
using System.Threading;
using System.Threading.Tasks;

class CancelOldStyleEvents
{
    // Old-style MRE that doesn't support unified cancellation.
    static ManualResetEvent mre = new ManualResetEvent(false);

    static void Main()
    {
        var cts = new CancellationTokenSource();

        // Pass the same token source to the delegate and to the task instance.
        Task.Run(() => DoWork(cts.Token), cts.Token);
        Console.WriteLine("Press s to start/restart, p to pause, or c to cancel.");
        Console.WriteLine("Or any other key to exit.");

        // Old-style UI thread.
        bool goAgain = true;
        while (goAgain)
        {
            char ch = Console.ReadKey(true).KeyChar;

            switch (ch)
            {
                case 'c':
                    cts.Cancel();
                    break;
                case 'p':
                    mre.Reset();
                    break;
                case 's':
                    mre.Set();
                    break;
                default:
                    goAgain = false;
                    break;
            }

            Thread.Sleep(100);
        }
        cts.Dispose();
    }

    static void DoWork(CancellationToken token)
    {
        while (true)
        {
            // Wait on the event if it is not signaled.
            int eventThatSignaledIndex =
                   WaitHandle.WaitAny(new WaitHandle[] { mre, token.WaitHandle },
                                      new TimeSpan(0, 0, 20));

            // Were we canceled while waiting?
            if (eventThatSignaledIndex == 1)
            {
                Console.WriteLine("The wait operation was canceled.");
                throw new OperationCanceledException(token);
            }
            // Were we canceled while running?
            else if (token.IsCancellationRequested)
            {
                Console.WriteLine("I was canceled while running.");
                token.ThrowIfCancellationRequested();
            }
            // Did we time out?
            else if (eventThatSignaledIndex == WaitHandle.WaitTimeout)
            {
                Console.WriteLine("I timed out.");
                break;
            }
            else
            {
                Console.Write("Working... ");
                // Simulating work.
                Thread.SpinWait(5000000);
            }
        }
    }
}

Imports System.Threading
Imports System.Threading.Tasks

Class CancelOldStyleEvents
    ' Old-style MRE that doesn't support unified cancellation.
    Shared mre As New ManualResetEvent(False)

    Shared Sub Main()
        Dim cts As New CancellationTokenSource()

        ' Pass the same token source to the delegate and to the task instance.
        Task.Run(Sub() DoWork(cts.Token), cts.Token)
        Console.WriteLine("Press c to cancel, p to pause, or s to start/restart.")
        Console.WriteLine("Or any other key to exit.")

        ' Old-style UI thread.
        Dim goAgain As Boolean = True
        While goAgain
            Dim ch As Char = Console.ReadKey(True).KeyChar
            Select Case ch
                Case "c"c
                    cts.Cancel()
                Case "p"c
                    mre.Reset()
                Case "s"c
                    mre.Set()
                Case Else
                    goAgain = False
            End Select

            Thread.Sleep(100)
        End While
        cts.Dispose()
    End Sub

    Shared Sub DoWork(ByVal token As CancellationToken)
        While True
            ' Wait on the event if it is not signaled.
            Dim waitHandles() As WaitHandle = {mre, token.WaitHandle}
            Dim eventThatSignaledIndex =
                WaitHandle.WaitAny(waitHandles, _
                                   New TimeSpan(0, 0, 20))

            ' Were we canceled while waiting?
            ' The first If statement is equivalent to
            ' token.ThrowIfCancellationRequested()
            If eventThatSignaledIndex = 1 Then
                Console.WriteLine("The wait operation was canceled.")
                Throw New OperationCanceledException(token)
                ' Were we canceled while running?
            ElseIf token.IsCancellationRequested = True Then
                Console.WriteLine("Cancelling per user request.")
                token.ThrowIfCancellationRequested()
                ' Did we time out?
            ElseIf eventThatSignaledIndex = WaitHandle.WaitTimeout Then
                Console.WriteLine("The wait operation timed out.")
                Exit While
            Else
                ' Simulating work.
                Console.Write("Working... ")
                Thread.SpinWait(5000000)
            End If
        End While
    End Sub
End Class

Voorbeeld 2

In het volgende voorbeeld wordt een ManualResetEventSlim voorbeeld gebruikt om te laten zien hoe u coördinatieprimitieven kunt deblokkeren die wel ondersteuning bieden voor uniforme annulering. Dezelfde benadering kan worden gebruikt met andere lichtgewicht coördinatieprimitief, zoals SemaphoreSlim en CountdownEvent.

using System;
using System.Threading;
using System.Threading.Tasks;

class CancelNewStyleEvents
{
   // New-style MRESlim that supports unified cancellation
   // in its Wait methods.
   static ManualResetEventSlim mres = new ManualResetEventSlim(false);

   static void Main()
   {
      var cts = new CancellationTokenSource();

      // Pass the same token source to the delegate and to the task instance.
      Task.Run(() => DoWork(cts.Token), cts.Token);
      Console.WriteLine("Press c to cancel, p to pause, or s to start/restart,");
      Console.WriteLine("or any other key to exit.");

      // New-style UI thread.
         bool goAgain = true;
         while (goAgain)
         {
             char ch = Console.ReadKey(true).KeyChar;

             switch (ch)
             {
                 case 'c':
                     // Token can only be canceled once.
                     cts.Cancel();
                     break;
                 case 'p':
                     mres.Reset();
                     break;
                 case 's':
                     mres.Set();
                     break;
                 default:
                     goAgain = false;
                     break;
             }

             Thread.Sleep(100);
         }
         cts.Dispose();
     }

     static void DoWork(CancellationToken token)
     {

         while (true)
         {
             if (token.IsCancellationRequested)
             {
                 Console.WriteLine("Canceled while running.");
                 token.ThrowIfCancellationRequested();
             }

             // Wait on the event to be signaled
             // or the token to be canceled,
             // whichever comes first. The token
             // will throw an exception if it is canceled
             // while the thread is waiting on the event.
             try
             {
                 // mres is a ManualResetEventSlim
                 mres.Wait(token);
             }
             catch (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("The wait operation was canceled.");
                 throw;
             }

             Console.Write("Working...");
             // Simulating work.
             Thread.SpinWait(500000);
         }
     }
 }

Imports System.Threading
Imports System.Threading.Tasks

Class CancelNewStyleEvents

    ' New-style MRESlim that supports unified cancellation
    ' in its Wait methods.
    Shared mres As ManualResetEventSlim = New ManualResetEventSlim(False)

    Shared Sub Main()

        Dim cts As New CancellationTokenSource()

        ' Pass the same token source to the delegate and to the task instance.
        Task.Run(Sub() DoWork(cts.Token), cts.Token)
        Console.WriteLine("Press c to cancel, p to pause, or s to start/restart,")
        Console.WriteLine("or any other key to exit.")

        ' New-style UI thread.
        Dim goAgain As Boolean = True
        While goAgain = True

            Dim ch As Char = Console.ReadKey(True).KeyChar

            Select Case ch
                Case "c"c
                    ' Token can only be canceled once.
                    cts.Cancel()
                Case "p"c
                    mres.Reset()
                Case "s"c
                    mres.Set()
                Case Else
                    goAgain = False
            End Select

            Thread.Sleep(100)
        End While
        cts.Dispose()
    End Sub

    Shared Sub DoWork(ByVal token As CancellationToken)
        While True
            If token.IsCancellationRequested Then
                Console.WriteLine("Canceled while running.")
                token.ThrowIfCancellationRequested()
            End If

            ' Wait on the event to be signaled
            ' or the token to be canceled,
            ' whichever comes first. The token
            ' will throw an exception if it is canceled
            ' while the thread is waiting on the event.
            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)
        End While
    End Sub
End Class

Zie ook

• Annulering in beheerde threads