.net - SemaphoreSlim.Wait( CancellationToken ) 对OperationCancelledException 进行正确的try/finally 吗？

Question

当使用带有取消标记的 SemaphorSlim 时，我应该如何构造 try/finally 以便正确处理 OperationCancelledException？在选项A中，取消 token 源会抛出OperationCancelledException，但不会调用Release()。在选项B中，取消 token 源会抛出OperationCancelledException并且确实会调用Release()。

// option A:
_semaphorSlim.Wait( _cancellationTokenSource.Token );
try
{
     // do work here
}
finally
{
     _semaphorSlim.Release();
}


// option B:
try
{
     _semaphorSlim.Wait( _cancellationTokenSource.Token );
     // do work here
}
finally
{
     _semaphorSlim.Release();
}

Answer 1

这里选项A更正确。取消时无需释放SemaphoreSlim，因为您从未真正获取并增加其计数。因此，除非 Wait 调用实际成功，否则您不想释放。

从此MSDN Page on using Semaphore and SemaphoreSlim :

It is the programmer's responsibility to ensure that a thread does not release the semaphore too many times. For example, suppose a semaphore has a maximum count of two, and that thread A and thread B both enter the semaphore. If a programming error in thread B causes it to call Release twice, both calls succeed. The count on the semaphore is full, and when thread A eventually calls Release, a SemaphoreFullException is thrown.