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『技术群里聊些啥』Task 不是你想 Cancel，想 Cancel 就能 Cancel
source link: https://blog.51cto.com/MyIO/5283532
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『技术群里聊些啥』Task 不是你想 Cancel，想 Cancel 就能 Cancel
原创在群里看到有人问如何取消这个 Task 的执行：
实际上这并不会取消S1eepMode1方法的执行：
这是为什么呢？
首先，让我们看看s_cts.Cancel()都做了啥：
public void Cancel() => Cancel(false);
public void Cancel(bool throwOnFirstException)
{
ThrowIfDisposed();
NotifyCancellation(throwOnFirstException);
}
private void NotifyCancellation(bool throwOnFirstException)
{
// If we're the first to signal cancellation, do the main extra work.
if (!IsCancellationRequested
&& Interlocked.CompareExchange(ref _state, NotifyingState, NotCanceledState) == NotCanceledState)
{
...
}
}
public void Cancel(bool throwOnFirstException)
{
ThrowIfDisposed();
NotifyCancellation(throwOnFirstException);
}
private void NotifyCancellation(bool throwOnFirstException)
{
// If we're the first to signal cancellation, do the main extra work.
if (!IsCancellationRequested
&& Interlocked.CompareExchange(ref _state, NotifyingState, NotCanceledState) == NotCanceledState)
{
...
}
}
实际上，Cancel方法仅仅是将变量_state的值改为NotifyingState。那Task.Run传递s_cts.Token又有什么用呢？
public static Task<TResult> Run<TResult>(Func<Task<TResult>?> function, CancellationToken cancellationToken)
{
if (function == null) ThrowHelper.ThrowArgumentNullException(ExceptionArgument.function);
// Short-circuit if we are given a pre-canceled token
if (cancellationToken.IsCancellationRequested)
return Task.FromCanceled<TResult>(cancellationToken);
...
}
{
if (function == null) ThrowHelper.ThrowArgumentNullException(ExceptionArgument.function);
// Short-circuit if we are given a pre-canceled token
if (cancellationToken.IsCancellationRequested)
return Task.FromCanceled<TResult>(cancellationToken);
...
}
原来，是在创建 Task 前先检查令牌是否已经Cancel，以便快速终止。那么，到底怎么才能Cacel已创建的Task呢？
其实，Task.Run的方法实现已经告诉我们正确的解决方案，那就是判断cancellationToken.IsCancellationRequested:
public static async Task S1eepMode1(CancellationToken cancellationToken)
{
while (true)
{
if (cancellationToken.IsCancellationRequested)
return;
...
}
}
{
while (true)
{
if (cancellationToken.IsCancellationRequested)
return;
...
}
}
另外，也可以采取抛出异常的方式：
public static async Task S1eepMode1(CancellationToken cancellationToken)
{
while (true)
{
cancellationToken.ThrowIfCancellationRequested();
...
}
}
{
while (true)
{
cancellationToken.ThrowIfCancellationRequested();
...
}
}
示例应用程序并不会捕获到这个异常，相关问题可以看我以前的文章《 如何保证执行异步方法时不会遗漏 await 关键字》
在创建 Task 时请记住，即使你执行了令牌取消操作，也并不意味着 Task 会停止运行。
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