C# Task ContinueWith的实现

看了上一篇C# Task 是什么？返回值如何实现? Wait如何实现 我们提到FinishContinuations方法中会调用TaskContinuation实例，那么我们的ContinueWith就应该非常简单，只需要把TASK放到TaskContinuation结合中就可以了，ContinueWith可以是 Action<Task<TResult>>也可以是 Func<Task<TResult>,TNewResult> ，其中Task<TResult>的实现如下：

 public class Task<TResult> : Task{
    //Creates a continuation that executes when the target Task{TResult}" completes
    public Task ContinueWith(Action<Task<TResult>> continuationAction)
    {
        StackCrawlMark stackMark = StackCrawlMark.LookForMyCaller;
        return ContinueWith(continuationAction, TaskScheduler.Current, default(CancellationToken), TaskContinuationOptions.None, ref stackMark);
    }
    internal Task ContinueWith(Action<Task<TResult>> continuationAction, TaskScheduler scheduler, CancellationToken cancellationToken,TaskContinuationOptions continuationOptions, ref StackCrawlMark stackMark)
    {
        if (continuationAction == null)
        {
            throw new ArgumentNullException("continuationAction");
        }
        if (scheduler == null)
        {
            throw new ArgumentNullException("scheduler");
        }
        TaskCreationOptions creationOptions;
        InternalTaskOptions internalOptions;
        CreationOptionsFromContinuationOptions(continuationOptions,out creationOptions,out internalOptions);
        
        Task continuationTask = new ContinuationTaskFromResultTask<TResult>(this, continuationAction, null,    creationOptions, internalOptions,ref stackMark);
        ContinueWithCore(continuationTask, scheduler, cancellationToken, continuationOptions);
        return continuationTask;
    }
    
    public Task<TNewResult> ContinueWith<TNewResult>(Func<Task<TResult>, Object, TNewResult> continuationFunction, Object state)
    {
        StackCrawlMark stackMark = StackCrawlMark.LookForMyCaller;
        return ContinueWith<TNewResult>(continuationFunction, state, TaskScheduler.Current, default(CancellationToken), TaskContinuationOptions.None, ref stackMark);
    }

    // Same as the above overload, just with a stack mark.
    internal Task<TNewResult> ContinueWith<TNewResult>(Func<Task<TResult>, Object, TNewResult> continuationFunction, Object state,TaskScheduler scheduler, CancellationToken cancellationToken, TaskContinuationOptions continuationOptions, ref StackCrawlMark stackMark)
    {
        if (continuationFunction == null)
        {
            throw new ArgumentNullException("continuationFunction");
        }
        if (scheduler == null)
        {
            throw new ArgumentNullException("scheduler");
        }

        TaskCreationOptions creationOptions;
        InternalTaskOptions internalOptions;
        CreationOptionsFromContinuationOptions(continuationOptions,out creationOptions,out internalOptions);

        Task<TNewResult> continuationFuture = new ContinuationResultTaskFromResultTask<TResult,TNewResult>(this, continuationFunction, state,creationOptions, internalOptions,ref stackMark);
        ContinueWithCore(continuationFuture, scheduler, cancellationToken, continuationOptions);
        return continuationFuture;
    }
 }

ContinueWith的核心是调用Task的ContinueWithCore方法，这里把我们的Action或Fun包装成子的Task，比如这里的ContinuationResultTaskFromResultTask实现【很是标准】如下：

 internal sealed class ContinuationResultTaskFromResultTask<TAntecedentResult, TResult> : Task<TResult>
    {
        private Task<TAntecedentResult> m_antecedent;
        public ContinuationResultTaskFromResultTask(
            Task<TAntecedentResult> antecedent, Delegate function, object state, TaskCreationOptions creationOptions, InternalTaskOptions internalOptions, ref StackCrawlMark stackMark) :
            base(function, state, Task.InternalCurrentIfAttached(creationOptions), default(CancellationToken), creationOptions, internalOptions, null)
        {
            Contract.Requires(function is Func<Task<TAntecedentResult>, TResult> || function is Func<Task<TAntecedentResult>, object, TResult>, "Invalid delegate type in ContinuationResultTaskFromResultTask");
            m_antecedent = antecedent;
            PossiblyCaptureContext(ref stackMark);
        }

        internal override void InnerInvoke()
        {
            var antecedent = m_antecedent;
            Contract.Assert(antecedent != null,  "No antecedent was set for the ContinuationResultTaskFromResultTask.");
            m_antecedent = null;
            antecedent.NotifyDebuggerOfWaitCompletionIfNecessary();

            // Invoke the delegate
            Contract.Assert(m_action != null);
            var func = m_action as Func<Task<TAntecedentResult>, TResult>;
            if (func != null)
            {
                m_result = func(antecedent);
                return;
            }
            var funcWithState = m_action as Func<Task<TAntecedentResult>, object, TResult>;
            if (funcWithState != null)
            {
                m_result = funcWithState(antecedent, m_stateObject);
                return;
            }
            Contract.Assert(false, "Invalid m_action in ContinuationResultTaskFromResultTask");
        }
    }

ContinuationResultTaskFromResultTask<TAntecedentResult, TResult> 就重写基类Task的InnerInvoke方法，现在回到Task的ContinueWithCore方法：

public class Task : IThreadPoolWorkItem, IAsyncResult, IDisposable
{
    /// Registers the continuation and possibly runs it (if the task is already finished).
    internal void ContinueWithCore(Task continuationTask, TaskScheduler scheduler,CancellationToken cancellationToken, TaskContinuationOptions options)
    {
        Contract.Requires(continuationTask != null, "Task.ContinueWithCore(): null continuationTask");
        Contract.Requires(scheduler != null, "Task.ContinueWithCore(): null scheduler");
        Contract.Requires(!continuationTask.IsCompleted, "Did not expect continuationTask to be completed");

        // Create a TaskContinuation
        TaskContinuation continuation = new StandardTaskContinuation(continuationTask, options, scheduler);

        // If cancellationToken is cancellable, then assign it.  
        if (cancellationToken.CanBeCanceled)
        {
            if (IsCompleted || cancellationToken.IsCancellationRequested)
            {
                continuationTask.AssignCancellationToken(cancellationToken, null, null);
            }
            else
            {
                continuationTask.AssignCancellationToken(cancellationToken, this, continuation);
            }
        }

        // In the case of a pre-canceled token, continuationTask will have been completed
        // in a Canceled state by now.  If such is the case, there is no need to go through
        // the motions of queuing up the continuation for eventual execution.
        if (!continuationTask.IsCompleted)
        {
            if ((this.Options & (TaskCreationOptions)InternalTaskOptions.PromiseTask) != 0 && !(this is ITaskCompletionAction))
            {
                var etwLog = TplEtwProvider.Log;
                if (etwLog.IsEnabled())
                {
                    etwLog.AwaitTaskContinuationScheduled(TaskScheduler.Current.Id, Task.CurrentId ?? 0, continuationTask.Id);
                }
            }

            // Attempt to enqueue the continuation
            bool continuationQueued = AddTaskContinuation(continuation, addBeforeOthers: false);

            // If the continuation was not queued (because the task completed), then run it now.
            if (!continuationQueued) continuation.Run(this, bCanInlineContinuationTask: true);
        }
    }
    
    private bool AddTaskContinuation(object tc, bool addBeforeOthers)
    {
        Contract.Requires(tc != null);
        if (IsCompleted) return false;

        // Try to just jam tc into m_continuationObject
        if ((m_continuationObject != null) || (Interlocked.CompareExchange(ref m_continuationObject, tc, null) != null))
        {            
            return AddTaskContinuationComplex(tc, addBeforeOthers);
        }
        else return true;
    }
    
     private bool AddTaskContinuationComplex(object tc, bool addBeforeOthers)
    {
        Contract.Requires(tc != null, "Expected non-null tc object in AddTaskContinuationComplex");
        object oldValue = m_continuationObject;
        // Logic for the case where we were previously storing a single continuation
        if ((oldValue != s_taskCompletionSentinel) && (!(oldValue is List<object>)))
        {
            List<object> newList = new List<object>();
            newList.Add(oldValue);
            Interlocked.CompareExchange(ref m_continuationObject, newList, oldValue);
        }

        // m_continuationObject is guaranteed at this point to be either a List or
        // s_taskCompletionSentinel.
        List<object> list = m_continuationObject as List<object>;
        Contract.Assert((list != null) || (m_continuationObject == s_taskCompletionSentinel),"Expected m_continuationObject to be list or sentinel");
        if (list != null)
        {
            lock (list)
            {
                if (m_continuationObject != s_taskCompletionSentinel)
                {
                    // Before growing the list we remove possible null entries that are the
                    // result from RemoveContinuations()
                    if (list.Count == list.Capacity)
                    {
                        list.RemoveAll(s_IsTaskContinuationNullPredicate);
                    }

                    if (addBeforeOthers)
                        list.Insert(0, tc);
                    else
                        list.Add(tc);

                    return true; // continuation successfully queued, so return true.
                }
            }
        }
        // We didn't succeed in queuing the continuation, so return false.
        return false;
    }
        
    /// Handles everything needed for associating a CancellationToken with a task which is being constructed.
    /// This method is meant to be be called either from the TaskConstructorCore or from ContinueWithCore
    private void AssignCancellationToken(CancellationToken cancellationToken, Task antecedent, TaskContinuation continuation)
    {
        ContingentProperties props = EnsureContingentPropertiesInitialized(needsProtection: false);
        props.m_cancellationToken = cancellationToken;

        try
        {
            if (AppContextSwitches.ThrowExceptionIfDisposedCancellationTokenSource)
            {
                cancellationToken.ThrowIfSourceDisposed();
            }                    
            if ((((InternalTaskOptions)Options & (InternalTaskOptions.QueuedByRuntime | InternalTaskOptions.PromiseTask | InternalTaskOptions.LazyCancellation)) == 0))
            {
                if (cancellationToken.IsCancellationRequested)
                {
                    // Fast path for an already-canceled cancellationToken
                    this.InternalCancel(false);
                }
                else
                {
                    // Regular path for an uncanceled cancellationToken
                    CancellationTokenRegistration ctr;
                    if (antecedent == null)
                    {
                        ctr = cancellationToken.InternalRegisterWithoutEC(s_taskCancelCallback, this);
                    }
                    else
                    {
                        ctr = cancellationToken.InternalRegisterWithoutEC(s_taskCancelCallback,new Tuple<Task, Task, TaskContinuation>(this, antecedent, continuation));
                    }
                    props.m_cancellationRegistration = new Shared<CancellationTokenRegistration>(ctr);
                }
            }
        }
        catch
        {
            if ((m_parent != null) &&((Options & TaskCreationOptions.AttachedToParent) != 0)&& ((m_parent.Options & TaskCreationOptions.DenyChildAttach) == 0))
            {
                m_parent.DisregardChild();
            }
            throw;
        }
    }    
    
    private readonly static Action<Object> s_taskCancelCallback = new Action<Object>(TaskCancelCallback);
    private static void TaskCancelCallback(Object o)
    {
        var targetTask = o as Task;
        if (targetTask == null)
        {
            var tuple = o as Tuple<Task, Task, TaskContinuation>;
            if (tuple != null)
            {
                targetTask = tuple.Item1;

                Task antecedentTask = tuple.Item2;
                TaskContinuation continuation = tuple.Item3;
                antecedentTask.RemoveContinuation(continuation);
            }
        }
        Contract.Assert(targetTask != null,"targetTask should have been non-null, with the supplied argument being a task or a tuple containing one");
        targetTask.InternalCancel(false);
    }
}

ContinueWithCore实现也比较简单，首先把当前的continuationTask转换为StandardTaskContinuation，然后把CancellationToken赋给continuationTask，如果continuationTask没有完成， 那么调用AddTaskContinuation把continuationTask加到等待对象中，如果AddTaskContinuation添加失败，就直接调用continuationTask。 让我妈来看看StandardTaskContinuation的实现：

 internal abstract class TaskContinuation
    {
        internal abstract void Run(Task completedTask, bool bCanInlineContinuationTask);

        /// <summary>Tries to run the task on the current thread, if possible; otherwise, schedules it.</summary>
        protected static void InlineIfPossibleOrElseQueue(Task task, bool needsProtection)
        {
            Contract.Requires(task != null);
            Contract.Assert(task.m_taskScheduler != null);
            if (needsProtection)
            {
                if (!task.MarkStarted())
                    return; // task has been previously started or canceled.  Stop processing.
            }
            else
            {
                task.m_stateFlags |= Task.TASK_STATE_STARTED;
            }

            // Try to inline it but queue if we can't
            try
            {
                if (!task.m_taskScheduler.TryRunInline(task, taskWasPreviouslyQueued: false))
                {
                    task.m_taskScheduler.InternalQueueTask(task);
                }
            }
            catch (Exception e)
            {
               
                if (!(e is ThreadAbortException && (task.m_stateFlags & Task.TASK_STATE_THREAD_WAS_ABORTED) != 0))    // this ensures TAEs from QueueTask will be wrapped in TSE
                {
                    TaskSchedulerException tse = new TaskSchedulerException(e);
                    task.AddException(tse);
                    task.Finish(false);
                }
            }
        }
        internal abstract Delegate[] GetDelegateContinuationsForDebugger();
    }
    
    /// <summary>Provides the standard implementation of a task continuation.</summary>
    internal class StandardTaskContinuation : TaskContinuation
    {
        internal readonly Task m_task;       
        internal readonly TaskContinuationOptions m_options;
        private readonly TaskScheduler m_taskScheduler;
        internal StandardTaskContinuation(Task task, TaskContinuationOptions options, TaskScheduler scheduler)
        {
            Contract.Requires(task != null, "TaskContinuation ctor: task is null");
            Contract.Requires(scheduler != null, "TaskContinuation ctor: scheduler is null");
            m_task = task;
            m_options = options;
            m_taskScheduler = scheduler;
            if (AsyncCausalityTracer.LoggingOn)
                AsyncCausalityTracer.TraceOperationCreation(CausalityTraceLevel.Required, m_task.Id, "Task.ContinueWith: " + ((Delegate)task.m_action).Method.Name, 0);

            if (Task.s_asyncDebuggingEnabled)
            {
                Task.AddToActiveTasks(m_task);
            }
        }

        /// <summary>Invokes the continuation for the target completion task.</summary>
        /// <param name="completedTask">The completed task.</param>
        /// <param name="bCanInlineContinuationTask">Whether the continuation can be inlined.</param>
        internal override void Run(Task completedTask, bool bCanInlineContinuationTask)
        {
            Contract.Assert(completedTask != null);
            Contract.Assert(completedTask.IsCompleted, "ContinuationTask.Run(): completedTask not completed");

            // Check if the completion status of the task works with the desired 
            // activation criteria of the TaskContinuationOptions.
            TaskContinuationOptions options = m_options;
            bool isRightKind =
                completedTask.IsRanToCompletion ?
                    (options & TaskContinuationOptions.NotOnRanToCompletion) == 0 :
                    (completedTask.IsCanceled ?
                        (options & TaskContinuationOptions.NotOnCanceled) == 0 :
                        (options & TaskContinuationOptions.NotOnFaulted) == 0);

            // If the completion status is allowed, run the continuation.
            Task continuationTask = m_task;
            if (isRightKind)
            {
                if (!continuationTask.IsCanceled && AsyncCausalityTracer.LoggingOn)
                {
                    // Log now that we are sure that this continuation is being ran
                    AsyncCausalityTracer.TraceOperationRelation(CausalityTraceLevel.Important, continuationTask.Id, CausalityRelation.AssignDelegate);
                }
                continuationTask.m_taskScheduler = m_taskScheduler;
              
                if (bCanInlineContinuationTask && // inlining is allowed by the caller
                    (options & TaskContinuationOptions.ExecuteSynchronously) != 0) // synchronous execution was requested by the continuation's creator
                {
                    InlineIfPossibleOrElseQueue(continuationTask, needsProtection: true);
                }
                else
                {
                    try { continuationTask.ScheduleAndStart(needsProtection: true); }
                    catch (TaskSchedulerException)
                    {
                        // No further action is necessary -- ScheduleAndStart() already transitioned the 
                        // task to faulted.  But we want to make sure that no exception is thrown from here.
                    }
                }
            }
            // Otherwise, the final state of this task does not match the desired
            // continuation activation criteria; cancel it to denote this.
            else continuationTask.InternalCancel(false);
        }
        internal override Delegate[] GetDelegateContinuationsForDebugger()
        {
            if (m_task.m_action == null)
            {
                return m_task.GetDelegateContinuationsForDebugger();
            }
            return new Delegate[] { m_task.m_action as Delegate };
        }
    }

StandardTaskContinuation的实现非常简单，而Task的AssignCancellationToken方法也没什么可以说的，只是需要注意下一下回调s_taskCancelCallback。Task的AddTaskContinuation方法首先检查当前Task是否结束，结束了就不用再调用AddTaskContinuationComplex方法了，直接调用continuation.Run方法，AddTaskContinuationComplex方法会把task添加到m_continuationObject中，最后FinishContinuations在调用m_continuationObject中的TaskContinuation.Run方法。

总结一下：ContinueWith方法主要调用ContinueWithCore方法，ContinueWithCore方法主要是调用AddTaskContinuation，AddTaskContinuation方法把Task加到m_continuationObject，【如果主的Task已经完成，那么这里AddTaskContinuation返回false，则直接调用TaskContinuation.Run】，当主的Task完成时会调用FinishContinuations方法，FinishContinuations方法会检测m_continuationObject中TaskContinuation对象，一次调用它们的Run方法。