5秒后取消長時間運行的任務

Question

我創建了一個創建XML字符串的任務。 該任務可以持續幾秒鍾。 如果5秒鍾后任務仍未完成，我想“迅速”取消任務，然后繼續編寫其余的XML。 因此，我在任務中內置了取消功能。 但是，盡管我在日志中看到以下消息：

• ProcessInformationTask超時

我也在日志中看到這一行

添加過程信息花費了10001毫秒

我想知道為什么會發生這種情況，因為我想在5秒鍾后取消任務（如果尚未完成）。 因此，我希望任務最多持續5秒。 我該如何解決？ 取消操作可能設置不正確？

我調用任務的代碼

string additionalInformation = null;
var contextInfo = new StringBuilder();

var xmlWriterSettings = new XmlWriterSettings()
{
    OmitXmlDeclaration = true,
    ConformanceLevel = ConformanceLevel.Fragment
};

var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;

using (XmlWriter xmlWriter = XmlWriter.Create(contextInfo, xmlWriterSettings))
{
    try
    {
        xmlWriter.WriteStartElement("AdditionalInformation");

        //Write xml (not long running)

        var watch = System.Diagnostics.Stopwatch.StartNew();
        string processInformation = AddProcessesInformation(xmlWriterSettings);
        watch.Stop();
        var elapsedMs = watch.ElapsedMilliseconds;
        Log.Info("Adding the process information took : " + elapsedMs + " ms");

        if (!string.IsNullOrEmpty(processInformation))
        {
            xmlWriter.WriteRaw(processInformation);
        }

        //Write xml (not long running)

        xmlWriter.WriteEndElement();

        additionalInformation = contextInfo.ToString();
    }

    catch (Exception e)
    {
        Log.Info("An exception occured during writing the additional information: " + e.Message);
        return false;
    }

    return true;
} 

任務方法

private static string AddProcessesInformation(XmlWriterSettings xmlWriterSettings)
    {
        var tokenSource = new CancellationTokenSource();
        var token = tokenSource.Token;
        var contextInfo = new StringBuilder();

        var processInformationTask = Task<string>.Factory.StartNew(() =>
        {
            if (token.IsCancellationRequested)
            {
                Log.Info("Cancellation request for the ProcessInformationTask");
            }

            Thread.Sleep(10000);

            return "Ran without problems'";

        }, token);

        if (!processInformationTask.Wait(5000, token))
        {
            Log.Info("ProcessInformationTask timed out");
            tokenSource.Cancel();
        }

        return processInformationTask?.Result;
    }

Answer 1

我認為您應該在方法的每個步驟之后檢查是否多次要求取消。 這是使用for循環的示例：

    private static string AddProcessesInformation(XmlWriterSettings xmlWriterSettings)
    {
        var tokenSource = new CancellationTokenSource();
        var token = tokenSource.Token;
        var contextInfo = new StringBuilder();

        var processInformationTask = Task<string>.Factory.StartNew(() =>
        {               
            for(int i = 0; i < 10; i++)
            {
                if (token.IsCancellationRequested)
                {
                    Console.WriteLine("Cancellation request for the ProcessInformationTask");
                    return string.Empty;
                }
                Thread.Sleep(1000);
            }

            return "Ran without problems'";

        }, token);

        if (!processInformationTask.Wait(5000, token))
        {
            Console.WriteLine("ProcessInformationTask timed out");
            tokenSource.Cancel();
        }

        return processInformationTask?.Result;
    }

如果在任務方法內部調用其他方法，則可以將取消令牌傳遞給它們，並在內部使用方法token.ThrowIfCancellationRequested() 。

        var processInformationTask = Task<string>.Factory.StartNew(() =>
        {
            try
            {
                Method1(token);
                Thread.Sleep(1000);
                Method2(token);
                Thread.Sleep(1000);
            }
            catch(OperationCanceledException ex)
            {
                return string.Empty;
            }
            return "Ran without problems'";

        }, token);


    private static void Method1(CancellationToken token)
    {
        token.ThrowIfCancellationRequested();
        // do more work
    }