Menyelesaikan tugas Anda

Saat Anda mengekspos tugas dari TaskCompletionSource<TResult>, Anda mengelola masa pakai tugas. Selesaikan tugas itu di setiap jalur. Jika ada jalur yang melewatkan proses penyelesaian, pemanggil akan menunggu tanpa batas waktu.

Menyelesaikan setiap jalur kode

Selalu selesaikan tugas dalam jalur keberhasilan dan kegagalan. Gunakan blok catch untuk logika pembersihan ketika tugas gagal. Gunakan blok finally untuk logika pembersihan yang harus selalu dijalankan. Blok kode berikut menunjukkan penambahan penanganan untuk jalur kesalahan.

public sealed class MissingSetExceptionFix
{
    public Task<string> StartAsync(bool fail)
    {
        var tcs = new TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously);

        try
        {
            if (fail)
            {
                throw new InvalidOperationException("Simulated failure");
            }

            tcs.TrySetResult("success");
        }
        catch (Exception ex)
        {
            tcs.TrySetException(ex);
        }

        return tcs.Task;
    }
}

Public NotInheritable Class MissingSetExceptionFix
    Public Function StartAsync(fail As Boolean) As Task(Of String)
        Dim tcs = New TaskCompletionSource(Of String)(TaskCreationOptions.RunContinuationsAsynchronously)

        Try
            If fail Then
                Throw New InvalidOperationException("Simulated failure")
            End If

            tcs.TrySetResult("success")
        Catch ex As Exception
            tcs.TrySetException(ex)
        End Try

        Return tcs.Task
    End Function
End Class

Kode berikut menangkap pengecualian, mencatatnya, dan lupa untuk memanggil SetException atau TrySetException. Bug ini sering muncul dan menyebabkan penelepon menunggu selamanya. Untuk detail selengkapnya tentang penanganan pengecualian dengan tugas, lihat Penanganan pengecualian tugas.

// ⚠️ DON'T copy this snippet. It demonstrates a problem that causes hangs.
public sealed class MissingSetExceptionBug
{
    public Task<string> StartAsync(bool fail)
    {
        var tcs = new TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously);

        try
        {
            if (fail)
            {
                throw new InvalidOperationException("Simulated failure");
            }

            tcs.SetResult("success");
        }
        catch (Exception)
        {
            // BUG: forgot SetException or TrySetException.
        }

        return tcs.Task;
    }
}

' ⚠️ DON'T copy this snippet. It demonstrates a problem that causes hangs.
Public NotInheritable Class MissingSetExceptionBug
    Public Function StartAsync(fail As Boolean) As Task(Of String)
        Dim tcs = New TaskCompletionSource(Of String)(TaskCreationOptions.RunContinuationsAsynchronously)

        Try
            If fail Then
                Throw New InvalidOperationException("Simulated failure")
            End If

            tcs.SetResult("success")
        Catch ex As Exception
            ' BUG: forgot SetException or TrySetException.
        End Try

        Return tcs.Task
    End Function
End Class

Lebih suka TrySet* dalam perlombaan penyelesaian

Jalur bersamaan sering berlomba untuk menyelesaikan tujuan yang sama TaskCompletionSource. SetResult, SetException, dan SetCanceled lempar jika tugas sudah selesai. Dalam kode rawan ras, gunakan TrySetResult, TrySetException, dan TrySetCanceled. Untuk pola-pola lain yang perlu dihindari dalam skenario bersamaan, lihat Bug umum pada async/await.

public static class TrySetRaceExample
{
    public static void ShowRaceSafeCompletion()
    {
        var tcs = new TaskCompletionSource<int>(TaskCreationOptions.RunContinuationsAsynchronously);

        bool first = tcs.TrySetResult(42);
        bool second = tcs.TrySetException(new TimeoutException("Too late"));

        Console.WriteLine($"First completion won: {first}");
        Console.WriteLine($"Second completion accepted: {second}");
        Console.WriteLine($"Result: {tcs.Task.Result}");
    }
}

Public Module TrySetRaceExample
    Public Sub ShowRaceSafeCompletion()
        Dim tcs = New TaskCompletionSource(Of Integer)(TaskCreationOptions.RunContinuationsAsynchronously)

        Dim first As Boolean = tcs.TrySetResult(42)
        Dim second As Boolean = tcs.TrySetException(New TimeoutException("Too late"))

        Console.WriteLine($"First completion won: {first}")
        Console.WriteLine($"Second completion accepted: {second}")
        Console.WriteLine($"Result: {tcs.Task.Result}")
    End Sub
End Module

Jangan hilangkan referensi selama reset

Bug umum muncul dalam primitif asinkron yang dapat di-reset. Perbaiki jalur reset dengan menukar referensi secara atomik dan menyelesaikan tugas sebelumnya (misalnya, dengan pembatalan):

public sealed class ResetFix
{
    private TaskCompletionSource<bool> _signal = NewSignal();

    public Task WaitAsync() => _signal.Task;

    public void Reset()
    {
        TaskCompletionSource<bool> previous = Interlocked.Exchange(ref _signal, NewSignal());
        previous.TrySetCanceled();
    }

    public void Pulse()
    {
        _signal.TrySetResult(true);
    }

    private static TaskCompletionSource<bool> NewSignal() =>
        new(TaskCreationOptions.RunContinuationsAsynchronously);
}

Public NotInheritable Class ResetFix
    Private _signal As TaskCompletionSource(Of Boolean) = NewSignal()

    Public Function WaitAsync() As Task
        Return _signal.Task
    End Function

    Public Sub Reset()
        Dim previous As TaskCompletionSource(Of Boolean) = Interlocked.Exchange(_signal, NewSignal())
        previous.TrySetCanceled()
    End Sub

    Public Sub Pulse()
        _signal.TrySetResult(True)
    End Sub

    Private Shared Function NewSignal() As TaskCompletionSource(Of Boolean)
        Return New TaskCompletionSource(Of Boolean)(TaskCreationOptions.RunContinuationsAsynchronously)
    End Function
End Class

Jangan lakukan ini: Jika Anda mengganti TaskCompletionSource instans sebelum menyelesaikan instans sebelumnya, pelayan yang memegang tugas lama mungkin tidak pernah selesai.

// ⚠️ DON'T copy this snippet. It demonstrates a problem where old waiters never complete.
public sealed class ResetBug
{
    private TaskCompletionSource<bool> _signal = NewSignal();

    public Task WaitAsync() => _signal.Task;

    public void Reset()
    {
        // BUG: waiters on the old task might never complete.
        _signal = NewSignal();
    }

    public void Pulse()
    {
        _signal.TrySetResult(true);
    }

    private static TaskCompletionSource<bool> NewSignal() =>
        new(TaskCreationOptions.RunContinuationsAsynchronously);
}

' ⚠️ DON'T copy this snippet. It demonstrates a problem where old waiters never complete.
Public NotInheritable Class ResetBug
    Private _signal As TaskCompletionSource(Of Boolean) = NewSignal()

    Public Function WaitAsync() As Task
        Return _signal.Task
    End Function

    Public Sub Reset()
        ' BUG: waiters on the old task might never complete.
        _signal = NewSignal()
    End Sub

    Public Sub Pulse()
        _signal.TrySetResult(True)
    End Sub

    Private Shared Function NewSignal() As TaskCompletionSource(Of Boolean)
        Return New TaskCompletionSource(Of Boolean)(TaskCreationOptions.RunContinuationsAsynchronously)
    End Function
End Class

Checklist

• Selesaikan setiap tugas yang diekspos TaskCompletionSource pada jalur keberhasilan, kegagalan, dan pembatalan.
• Gunakan TrySet* API di jalur yang mungkin mengalami kondisi balapan.
• Selama reset, selesaikan atau batalkan tugas lama sebelum Anda menghilangkan referensinya.
• Tambahkan pengujian berbasis batas waktu sehingga macet gagal dengan cepat di CI.

Baca juga

• Penanganan pengecualian tugas
• Menerapkan TAP
• Bug umum pada async/await