Panduan: Mendengarkan Permintaan Pembatalan yang Memiliki Handel Tunggu

Jika metode diblokir saat menunggu peristiwa diberikan sinyal, metode tidak dapat memeriksa nilai token pembatalan dan merespons secara tepat waktu. Contoh pertama menunjukkan cara menyelesaikan masalah ini saat Anda bekerja dengan peristiwa seperti System.Threading.ManualResetEvent yang tidak secara asli mendukung kerangka kerja pembatalan terpadu. Contoh kedua menunjukkan pendekatan yang lebih efisien yang menggunakan System.Threading.ManualResetEventSlim, yang mendukung pembatalan terpadu.

Catatan

Saat "Hanya Kode Saya" diaktifkan, Visual Studio dalam beberapa kasus akan terputus pada baris yang melemparkan pengecualian dan menampilkan pesan kesalahan yang mengatakan "pengecualian tidak ditangani oleh kode pengguna." Kesalahan ini tidak berbahaya. Anda dapat menekan F5 untuk melanjutkannya, dan melihat perilaku penanganan pengecualian yang ditunjukkan dalam contoh di bawah ini. Untuk mencegah Visual Studio melanggar kesalahan pertama, cukup hapus centang pada kotak centang "Hanya Kode Saya" di bawah Alat, Opsi, Penelusuran Kesalahan, Umum.

Contoh 1

Contoh berikut menggunakan ManualResetEvent untuk menunjukkan cara membuka blokir handel tunggu yang tidak mendukung pembatalan terpadu.

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

Contoh 2

Contoh berikut menggunakan ManualResetEventSlim untuk menunjukkan cara membuka blokir primitif koordinasi yang mendukung pembatalan terpadu. Pendekatan yang sama dapat digunakan dengan primitif koordinasi ringan lainnya, seperti SemaphoreSlim dan 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

Lihat juga

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