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CancellationToken

概述

从思路上来说 CancellationToken 是 .NET 提供的一种类型，这种类型的变量被用来从一个任务的外部终止任务。

之所以能够实现“终止”的功能，是因为在任务的实现中有类似：

while (true) {
    if (token.IsCancellationRequested) {
        // ...
        break;
    }
    // ...
}

这样的逻辑，使得在任务外部能够控制任务内部的停止。

CancellationTokenSource 是 CancellationToken 的管理类，可以通过它来执行取消操作。

Token

/// <summary>Gets the <see cref="CancellationToken"/> associated with this <see cref="CancellationTokenSource"/>.</summary>
/// <value>The <see cref="CancellationToken"/> associated with this <see cref="CancellationTokenSource"/>.</value>
/// <exception cref="ObjectDisposedException">The token source has been disposed.</exception>
public CancellationToken Token
{
    get
    {
        ThrowIfDisposed();
        return new CancellationToken(this);
    }
}

Token 是一个计算属性，每次访问都会得到一个新的 CancellationToken 示例。

CancellationToken 是值类型，它不需要 Dispose。

TransitionToCancellationRequested()

这个私有方法在 Cancel() 等方法内部调用：

/// <summary>Transitions from <see cref="States.NotCanceledState"/> to <see cref="States.NotifyingState"/>.</summary>
/// <returns>true if it successfully transitioned; otherwise, false.</returns>
/// <remarks>If it successfully transitions, it will also have disposed of <see cref="_timer"/> and set <see cref="_kernelEvent"/>.</remarks>
private bool TransitionToCancellationRequested()
{
    if (!IsCancellationRequested &&
        Interlocked.CompareExchange(ref _state, States.NotifyingState, States.NotCanceledState) == States.NotCanceledState)
    {
        // Dispose of the timer, if any.  Dispose may be running concurrently here, but ITimer.Dispose is thread-safe.
        ITimer? timer = _timer;
        if (timer != null)
        {
            _timer = null;
            timer.Dispose();
        }

        // Set the event if it's been lazily initialized and hasn't yet been disposed of.  Dispose may
        // be running concurrently, in which case either it'll have set m_kernelEvent back to null and
        // we won't see it here, or it'll see that we've transitioned to NOTIFYING and will skip disposing it,
        // leaving cleanup to finalization.
        _kernelEvent?.Set();

        return true;
    }

    return false;
}
  • _state 是一个 volatile 变量 volatile 不保证原子性，因此在写 _state 时使用了原子操作作为锁；
  • CompareExchange 的含义是，如果当前引用等于第三个变量的值，那么将其改为第二个值，并返回修改前的状态；即，如果当前是“NotCanceledState”状态，将其改为“NotifyingState”状态；
  • 释放了内部持有的资源，timer 及 WaitHandle；

Ref

CancellationTokenSource Class