为什么 OperationCancelledException 传送的 CancellationToken 与来自 CTSource 的不同？

Question

为什么异常中保存的CancellationToken和CancellationTokenSource提供的token不一样？

[Test]
public static async Task SqlCommand_should_recognise_which_CT_triggered_its_cancellation()
{

    var timeout = TimeSpan.FromSeconds(1);
    var cts = new CancellationTokenSource(timeout);

    try
    {
        var connection = new SqlConnection(_config.ConnectionString);
        await connection.OpenAsync(cts.Token);
        var sqlQuery = new SqlCommand("select 1", connection);

        await Task.Delay(timeout + TimeSpan.FromSeconds(1));
        await sqlQuery.ExecuteScalarAsync(cts.Token);
    }
    catch (OperationCanceledException cancelledEx)
    {
        //Shouldn't they be the same?
        Assert.AreEqual(cancelledEx.CancellationToken, cts.Token);
        // The below fails as well
        // Assert.IsTrue(cancelledEx.CancellationToken == cts.Token);


    }
}

Answer 1

Why is the CancellationToken stored in the exception different to token provided by CancellationTokenSource?

这是一个实现细节。我没有查看源代码，但我怀疑正在发生的是提供给 ExecuteScalarAsync 的 CancellationToken 与一些内部 CancellationToken 这意味着 "we lost the connection" 或类似的东西。这些 linked CancellationTokens 不等同于它们的来源 CancellationToken。

这通常是 CancellationToken 的使用问题。对于链接令牌，并不总是能够确定哪个取消令牌导致了取消。出于这个原因，我建议通过 catch (OperationCanceledException ex) when (cts.IsCancellationRequested):

检查您的 自己的 取消令牌副本

static async Task Main(string[] args)
{
  var timeout = TimeSpan.FromSeconds(1);
  var cts = new CancellationTokenSource(timeout);

  try
  {
    await IndirectDelay(10, cts.Token);

    await Task.Delay(timeout + TimeSpan.FromSeconds(1));
    await IndirectDelay(10, cts.Token);
  }
  catch (OperationCanceledException ex) when (cts.IsCancellationRequested)
  {
    Console.WriteLine(ex.CancellationToken == cts.Token); // false
    Console.WriteLine("Our token is canceled.");
  }
  catch (OperationCanceledException)
  {
    Console.WriteLine("Canceled for some other reason.");
  }
  catch (Exception)
  {
    Console.WriteLine("General error.");
  }
}

private static async Task IndirectDelay(int timeout, CancellationToken token)
{
  using (var internalCts = new CancellationTokenSource())
  using (var cts = CancellationTokenSource.CreateLinkedTokenSource(token, internalCts.Token))
    await Task.Delay(timeout, cts.Token);
}

可能会出现竞争情况，您的代码会认为取消是由于超时而发生的，但实际上是因为连接断开（或任何内部逻辑正在发生） on), 但在大多数情况下这无关紧要。

为什么 OperationCancelledException 传送的 CancellationToken 与来自 CTSource 的不同？

Why is OperationCancelledException conveying different CancellationToken than the one from CTSource?

c#

ado.net

async-await