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) == , "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) == )
{
//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, );
} 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) == )
{
// 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) != )
{
// 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) != );
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。