我什么时候应该调用 CancellationToken.ThrowIfCancellationRequested？

Question

我开发了一个基于 C# 的 Windows 服务，它在几个不同的任务中运行其所有逻辑。为了允许服务在停止时正常关闭，我使用 CancellationToken ，它被传递给任何接受一个的函数（主要来自我正在使用的第三方库），以便在完成之前中止处理。

我注意到，OperationCanceledException当调用函数时请求取消时，这些函数都不会抛出异常，因此我的应用程序只是继续执行，直到我ThrowIfCancellationRequested()稍后在代码中调用其他地方。我是否应该ThrowIfCancellationRequested()在调用每个函数后手动调用以确保任务尽快停止，或者我到底应该何时调用ThrowIfCancellationRequested()自己的代码？

Answer 1

是的，您应该ThrowIfCancellationRequested()在代码中的适当位置手动调用（适当的位置由您作为程序员确定）。

考虑以下简单作业处理函数的示例，该函数从队列中读取作业并对其进行处理。这些评论说明了开发人员在决定是否检查取消时可能会经历的思考。

另请注意，您是对的 - 接受令牌的标准框架函数不会抛出取消异常 - 它们只会提前返回，因此您必须自己检查取消情况。

public async Task DoWork(CancellationToken token)
{
    while(true)
    {
        // It is safe to check the token here, as we have not started any work
        token.ThrowIfCancellationRequested();

        var nextJob = GetNextJob();

        // We can check the token here, because we have not 
        // made any changes to the system.
        token.ThrowIfCancellationRequested();

        var jobInfo = httpClient.Get($"job/info/{nextJob.Id}", token); 
        // We can check the token here, because we have not 
        // made any changes to the system. 
        // Note that HttpClient won't throw an exception
        // if the token is cancelled - it will just return early, 
        // so we must check for cancellation ourselves.
        token.ThrowIfCancellationRequested();

        // The following code is a critical section - we are going to start
        // modifying various databases and things, so don't check for 
        // cancellation until we have done it all.
        ModifySystem1(nextJob);
        ModifySystem2(nextJob);
        ModifySystem3(nextJob);

        // We *could* check for cancellation here as it is safe, but since
        // we have already done all the required work *and* marking a job 
        // as complete is very fast, there is not a lot of point.
        MarkJobAsCompleted(nextJob);
    }
}

最后，您可能不想从代码中泄漏取消异常，因为它们不是“真正的”异常 - 只要有人停止您的服务，它们就会发生。

您可以使用异常过滤器捕获异常，如下所示：

public async Task DoWork(CancellationToken token)
{
    try
    {
        while(true)
        { 
            // Do job processing
        }
    }
    catch (OperationCanceledException e) when (e.CancellationToken == token)
    {
        Log.Info("Operation cancelled because service is shutting down.");
    }
    catch (Exception e)
    {
        Log.Error(e, "Ok - this is actually a real exception. Oh dear.");
    }
}