C# Task 源代码阅读（2）

上篇已经讲到Task 的默认的TaskScheduler 为ThreadPoolTaskScheduler.

这时我们回到原来的task 的start方法，在代码最后，调用了 ScheduleAndStart(true) 这个方法。接着看这个方法

 [SecuritySafeCritical] // Needed for QueueTask
        internal void ScheduleAndStart(bool needsProtection)
        {
            Contract.Assert(m_taskScheduler != null, "expected a task scheduler to have been selected");
            Contract.Assert((m_stateFlags & TASK_STATE_STARTED) == 0, "task has already started");

            // Set the TASK_STATE_STARTED bit
            if (needsProtection)
            {
                if (!MarkStarted())
                {
                    // A cancel has snuck in before we could get started.  Quietly exit.
                    return;
                }
            }
            else
            {
                m_stateFlags |= TASK_STATE_STARTED;
            }

            if (s_asyncDebuggingEnabled)
            {
                AddToActiveTasks(this);
            }

            if (AsyncCausalityTracer.LoggingOn && (Options & (TaskCreationOptions)InternalTaskOptions.ContinuationTask) == 0)
            {
                //For all other task than TaskContinuations we want to log. TaskContinuations log in their constructor
                AsyncCausalityTracer.TraceOperationCreation(CausalityTraceLevel.Required, this.Id, "Task: "+((Delegate)m_action).Method.Name, 0);
            }


            try
            {
                // Queue to the indicated scheduler.
                m_taskScheduler.InternalQueueTask(this);
            }
            catch (ThreadAbortException tae)
            {
                AddException(tae);
                FinishThreadAbortedTask(true, false);
            }
            catch (Exception e)
            {
                // The scheduler had a problem queueing this task.  Record the exception, leaving this task in
                // a Faulted state.
                TaskSchedulerException tse = new TaskSchedulerException(e);
                AddException(tse);
                Finish(false);

                // Now we need to mark ourselves as "handled" to avoid crashing the finalizer thread if we are called from StartNew()
                // or from the self replicating logic, because in both cases the exception is either propagated outside directly, or added
                // to an enclosing parent. However we won't do this for continuation tasks, because in that case we internally eat the exception
                // and therefore we need to make sure the user does later observe it explicitly or see it on the finalizer.

                if ((Options & (TaskCreationOptions)InternalTaskOptions.ContinuationTask) == 0)
                {
                    // m_contingentProperties.m_exceptionsHolder *should* already exist after AddException()
                    Contract.Assert(
                        (m_contingentProperties != null) &&
                        (m_contingentProperties.m_exceptionsHolder != null) &&
                        (m_contingentProperties.m_exceptionsHolder.ContainsFaultList),
                            "Task.ScheduleAndStart(): Expected m_contingentProperties.m_exceptionsHolder to exist " +
                            "and to have faults recorded.");

                    m_contingentProperties.m_exceptionsHolder.MarkAsHandled(false);
                }
                // re-throw the exception wrapped as a TaskSchedulerException.
                throw tse;
            }
        }

开始先做契约参数认证，接着保护数值判断。我们要看的是AddToActiveTasks(this)这个方法，注意在他之前有个判断，在s_asyncDebuggingEnabled 为true 的情况才会执行，当然默认的是false。

  // This dictonary relates the task id, from an operation id located in the Async Causality log to the actual
        // task. This is to be used by the debugger ONLY. Task in this dictionary represent current active tasks.
        private static readonly Dictionary<int, Task> s_currentActiveTasks = new Dictionary<int, Task>();
 [FriendAccessAllowed]
        internal static bool AddToActiveTasks(Task task)
        {
            Contract.Requires(task != null, "Null Task objects can't be added to the ActiveTasks collection");
            lock (s_activeTasksLock)
            {
                s_currentActiveTasks[task.Id] = task;
            }
            //always return true to keep signature as bool for backwards compatibility
            return true;
        }

这个就是僵我们要执行task 对象放入一个字典中，放入的目的是做什么呢？当然就是为何方便查询和管理。这个方法在正常流程是不会执行的。这里觉得有些奇怪的写法，Task 类里面有个静态静态字典，用于存放自己执行的类集合。当然说到管理和查询，断然我是不会放在这个类，令起新类也好。

这里的代码方法参数验证都是采用契约验证，其实我个人并不是很赞同这东西，虽然C++也有这个。我倒更希望是原本的异常抛出，或者日志记录，或者其他自定义方式。

接着看核心方法 m_taskScheduler.InternalQueueTask(this); 前面我们已经看到默认的m_taskScheduler为ThreadPoolTaskScheduler。接着看代码

 [SecurityCritical]
        internal void InternalQueueTask(Task task)
        {
            Contract.Requires(task != null);

            task.FireTaskScheduledIfNeeded(this);

            this.QueueTask(task);
        }
 [SecurityCritical]
        protected internal override void QueueTask(Task task)
        {
            if ((task.Options & TaskCreationOptions.LongRunning) != 0)
            {
                // Run LongRunning tasks on their own dedicated thread.
                Thread thread = new Thread(s_longRunningThreadWork);
                thread.IsBackground = true; // Keep this thread from blocking process shutdown
                thread.Start(task);
            }
            else
            {
                // Normal handling for non-LongRunning tasks.
                bool forceToGlobalQueue = ((task.Options & TaskCreationOptions.PreferFairness) != 0);
                ThreadPool.UnsafeQueueCustomWorkItem(task, forceToGlobalQueue);
            }
        }

现在为止就开始清晰明朗了，看到QueueTask 方法，我已经可以看到task 对象已经传到Threadpool 里面了。至此，可以说到task 一般都是在ThreadPool 里面运行。接着我们再看ThreadpoolTaskScheduler让几个重要的方法

  [SecurityCritical]
        protected internal override bool TryDequeue(Task task)
        {
            // just delegate to TP
            return ThreadPool.TryPopCustomWorkItem(task);
        }

        [SecurityCritical]
        protected override IEnumerable<Task> GetScheduledTasks()
        {
            return FilterTasksFromWorkItems(ThreadPool.GetQueuedWorkItems());
        }

        /// <summary>
        /// This internal function will do this:
        ///   (1) If the task had previously been queued, attempt to pop it and return false if that fails.
        ///   (2) Propagate the return value from Task.ExecuteEntry() back to the caller.
        /// 
        /// IMPORTANT NOTE: TryExecuteTaskInline will NOT throw task exceptions itself. Any wait code path using this function needs
        /// to account for exceptions that need to be propagated, and throw themselves accordingly.
        /// </summary>
        [SecurityCritical]
        protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
        {

        -----------------------
        }

这里有GetScheduledTasks()方法，这个方法就是用来获得当前的Task的，对于去珍断task 的运行状态非常有帮助。至此。我们一步一步看到Task 是如何运行的，当然到达Theadpool可以继续看下去。注意了ThreadPoolTaskScheduler 的访问修饰符是internal sealed，所以在用task 的时候无法用到他，还有里面的方法访问修饰符都是protected 的。到此，我们正常来运行task，还是没法获得到task的本身运行状态。很多人在代码中为了实现某个功能都会大量的使用task，每个人的写法有不一样，task 运行是否成功，是否发生异常 对于整个项目的运行至关重要。那么如何管理，如何查看task 的运行状态呢，在C# code 我们如果想把task 的异常接管到主线程种，必须task wait，但是很多task 都是无需直到返回结果，但是实际上我们还是要关心他的运行状态，那么如何来做，如何来看呢。
1.常规做法，鉴于很多人喜欢TaskFactory.StartNew() 这个写法，所以想把所有的task的加入到一个队列中比较麻烦，因为启动task 的写法很多。所以各自的task的里面自己处理异常，写好日志。
2.使用TaskScheduler，看代码的目的除了了解运行过程，更加了解如和使用这个类，我们只需要写上自己的TaskScheduler，当然继承这个类，是需要实现某些必须方法的，不管是task的start还是TaskFactory的StartNew方法,我们都可以注入自己的TaskScheduler，这样正如TaskScheduler设计初衷一样，所有的task 运行都会交给他来管理，默认的ThreadPoolTaskScheduler是没法使用的（访问修饰符），除非采用一些其他手段，这里不多介绍。所以只能自己重新去实现这个类的相关细节。