Merge pull request #3389 from lambdageek/bug-43099

[mono.git] / mcs / class / referencesource / mscorlib / system / diagnostics / eventing / activitytracker.cs

using System;
using System.Diagnostics;
using System.Threading;
#if !ES_BUILD_AGAINST_DOTNET_V35
using Contract = System.Diagnostics.Contracts.Contract;
#else
using Contract = Microsoft.Diagnostics.Contracts.Internal.Contract;
#endif

#if ES_BUILD_STANDALONE
namespace Microsoft.Diagnostics.Tracing
#else    
using System.Threading.Tasks;
namespace System.Diagnostics.Tracing
#endif
{
    /// <summary>
    /// Tracks activities.  This is meant to be a singleton (accessed by the ActivityTracer.Instance static property)
    ///  
    /// Logically this is simply holds the m_current variable that holds the async local that holds the current ActivityInfo
    /// An ActivityInfo is represents a activity (which knows its creator and thus knows its path). 
    ///
    /// Most of the magic is in the async local (it gets copied to new tasks)
    /// 
    /// On every start event call OnStart 
    /// 
    ///     Guid activityID;
    ///     Guid relatedActivityID;
    ///     if (OnStart(activityName, out activityID, out relatedActivityID, ForceStop, options))
    ///         // Log Start event with activityID and relatedActivityID
    ///     
    /// On every stop event call OnStop
    /// 
    ///     Guid activityID;
    ///     if (OnStop(activityName, ref activityID  ForceStop))
    ///         // Stop event with activityID
    ///            
    /// On any normal event log the event with activityTracker.CurrentActivityId
    /// </summary>
    internal class ActivityTracker
    {

        /// <summary>
        /// Called on work item begins.  The activity name = providerName + activityName without 'Start' suffix.
        /// It updates CurrentActivityId to track.   
        /// 
        /// It returns true if the Start should be logged, otherwise (if it is illegal recurision) it return false. 
        /// 
        /// The start event should use as its activity ID the CurrentActivityId AFTER calling this routine and its
        /// RelatedActivityID the CurrentActivityId BEFORE calling this routine (the creator).  
        /// 
        /// If activity tracing is not on, then activityId and relatedActivityId are not set
        /// </summary>
        public void OnStart(string providerName, string activityName, int task, ref Guid activityId, ref Guid relatedActivityId, EventActivityOptions options)
        {
            if (m_current == null)        // We are not enabled
            {
                // We  used to rely on the TPL provider turning us on, but that has the disadvantage that you don't get Start-Stop tracking
                // until you use Tasks for the first time (which you may never do).   Thus we change it to pull rather tan push for whether
                // we are enabled.   
                if (m_checkedForEnable)
                    return;
                m_checkedForEnable = true;
#if ES_BUILD_STANDALONE
                Enable();           // Enable it unconditionally.  
#else
                if (System.Threading.Tasks.TplEtwProvider.Log.IsEnabled(EventLevel.Informational, System.Threading.Tasks.TplEtwProvider.Keywords.TasksFlowActivityIds))
                     Enable();
#endif
                if(m_current == null)
                {
                    return;
                }
            }

            Contract.Assert((options & EventActivityOptions.Disable) == 0);

            var currentActivity = m_current.Value;
            var fullActivityName = NormalizeActivityName(providerName, activityName, task);

            var etwLog = TplEtwProvider.Log;
            if (etwLog.Debug)
            {
                etwLog.DebugFacilityMessage("OnStartEnter", fullActivityName);
                etwLog.DebugFacilityMessage("OnStartEnterActivityState", ActivityInfo.LiveActivities(currentActivity));
            }

            if (currentActivity != null)
            {
                // Stop activity tracking if we reached the maximum allowed depth 
                if (currentActivity.m_level >= MAX_ACTIVITY_DEPTH)
                {
                    activityId = Guid.Empty;
                    relatedActivityId = Guid.Empty;
                    if (etwLog.Debug)
                        etwLog.DebugFacilityMessage("OnStartRET", "Fail");
                    return;
                }
                // Check for recursion, and force-stop any activities if the activity already started.
                if ((options & EventActivityOptions.Recursive) == 0)
                {
                    ActivityInfo existingActivity = FindActiveActivity(fullActivityName, currentActivity);
                    if (existingActivity != null)
                    {
                        // 
                        OnStop(providerName, activityName, task, ref activityId);
                        currentActivity = m_current.Value;
                    }
                }
            }

            // Get a unique ID for this activity.
            long id;
            if (currentActivity == null)
                id = Interlocked.Increment(ref m_nextId);
            else
                id = Interlocked.Increment(ref currentActivity.m_lastChildID);

            // The previous ID is my 'causer' and becomes my related activity ID
            relatedActivityId = EventSource.CurrentThreadActivityId;

            // Add to the list of started but not stopped activities. 
            ActivityInfo newActivity = new ActivityInfo(fullActivityName, id, currentActivity, relatedActivityId, options);
            m_current.Value = newActivity;

            // Remember the current ID so we can log it 
            activityId = newActivity.ActivityId;

            if (etwLog.Debug)
            {
                etwLog.DebugFacilityMessage("OnStartRetActivityState", ActivityInfo.LiveActivities(newActivity));
                etwLog.DebugFacilityMessage1("OnStartRet", activityId.ToString(), relatedActivityId.ToString());
            }
        }

        /// <summary>
        /// Called when a work item stops.  The activity name = providerName + activityName without 'Stop' suffix.
        /// It updates m_current variable to track this fact.   The Stop event associated with stop should log the ActivityID associated with the event.
        ///
        /// If activity tracing is not on, then activityId and relatedActivityId are not set
        /// </summary>
        public void OnStop(string providerName, string activityName, int task, ref Guid activityId)
        {
            if (m_current == null)        // We are not enabled
                return;

            var fullActivityName = NormalizeActivityName(providerName, activityName, task);

            var etwLog = TplEtwProvider.Log;
            if (etwLog.Debug)
            {
                etwLog.DebugFacilityMessage("OnStopEnter", fullActivityName);
                etwLog.DebugFacilityMessage("OnStopEnterActivityState", ActivityInfo.LiveActivities(m_current.Value));
            }

            for (; ; ) // This is a retry loop.
            {
                ActivityInfo currentActivity = m_current.Value;
                ActivityInfo newCurrentActivity = null;               // if we have seen any live activities (orphans), at he first one we have seen.   

                // Search to find the activity to stop in one pass.   This insures that we don't let one mistake
                // (stopping something that was not started) cause all active starts to be stopped 
                // By first finding the target start to stop we are more robust.  
                ActivityInfo activityToStop = FindActiveActivity(fullActivityName, currentActivity);

                // ignore stops where we can't find a start because we may have popped them previously.
                if (activityToStop == null)
                {
                    activityId = Guid.Empty;
                    // 
                    if (etwLog.Debug)
                        etwLog.DebugFacilityMessage("OnStopRET", "Fail");
                    return;
                }

                activityId = activityToStop.ActivityId;

                // See if there are any orphans that need to be stopped.  
                ActivityInfo orphan = currentActivity;
                while (orphan != activityToStop && orphan != null)
                {
                    if (orphan.m_stopped != 0)      // Skip dead activities.
                    {
                        orphan = orphan.m_creator;
                        continue;
                    }
                    if (orphan.CanBeOrphan())
                    {
                        // We can't pop anything after we see a valid orphan, remember this for later when we update m_current.  
                        if (newCurrentActivity == null)
                            newCurrentActivity = orphan;
                    }
                    else
                    {
                        orphan.m_stopped = 1;
                        Contract.Assert(orphan.m_stopped != 0);
                    }
                    orphan = orphan.m_creator;
                }

                // try to Stop the activity atomically.  Other threads may be trying to do this as well.
                if (Interlocked.CompareExchange(ref activityToStop.m_stopped, 1, 0) == 0)
                {
                    // I succeeded stopping this activity. Now we update our m_current pointer 

                    // If I haven't yet determined the new current activity, it is my creator.
                    if (newCurrentActivity == null)
                        newCurrentActivity = activityToStop.m_creator;

                    m_current.Value = newCurrentActivity;

                    if (etwLog.Debug)
                    {
                        etwLog.DebugFacilityMessage("OnStopRetActivityState", ActivityInfo.LiveActivities(newCurrentActivity));
                        etwLog.DebugFacilityMessage("OnStopRet", activityId.ToString());
                    }
                    return;
                }
                // We failed to stop it.  We must have hit a ---- to stop it.  Just start over and try again.  
            }
        }

        /// <summary>
        /// Turns on activity tracking.    It is sticky, once on it stays on (---- issues otherwise)
        /// </summary>
        [System.Security.SecuritySafeCritical]
        public void Enable()
        {
            if (m_current == null)
            {
                m_current = new AsyncLocal<ActivityInfo>(ActivityChanging);
            }
        }

        /// <summary>
        /// An activity tracker is a singleton, this is how you get the one and only instance.
        /// </summary>
        public static ActivityTracker Instance { get { return s_activityTrackerInstance; } }


        #region private

        /// <summary>
        /// The current activity ID.  Use this to log normal events.  
        /// </summary>
        private Guid CurrentActivityId { get { return m_current.Value.ActivityId; } }

        /// <summary>
        /// Searched for a active (nonstopped) activity with the given name.  Returns null if not found.  
        /// </summary>
        private ActivityInfo FindActiveActivity(string name, ActivityInfo startLocation)
        {
            var activity = startLocation;
            while (activity != null)
            {
                if (name == activity.m_name && activity.m_stopped == 0)
                    return activity;
                activity = activity.m_creator;
            }
            return null;
        }

        /// <summary>
        /// Strip out "Start" or "End" suffix from activity name and add providerName prefix.
        /// If 'task'  it does not end in Start or Stop and Task is non-zero use that as the name of the activity
        /// </summary>
        private string NormalizeActivityName(string providerName, string activityName, int task)
        {
            if (activityName.EndsWith(EventSource.s_ActivityStartSuffix))
                activityName = activityName.Substring(0, activityName.Length - EventSource.s_ActivityStartSuffix.Length);
            else if (activityName.EndsWith(EventSource.s_ActivityStopSuffix))
                activityName = activityName.Substring(0, activityName.Length - EventSource.s_ActivityStopSuffix.Length);
            else if (task != 0)
                activityName = "task" + task.ToString();

            // We use provider name to distinguish between activities from different providers.
            return providerName + activityName;
        }

        // *******************************************************************************
        /// <summary>
        /// An ActivityInfo represents a particular activity.   It is almost read-only.   The only
        /// fields that change after creation are
        ///    m_lastChildID - used to generate unique IDs for the children activities and for the most part can be ignored.
        ///    m_stopped - indicates that this activity is dead 
        /// This read-only-ness is important because an activity's  m_creator chain forms the 
        /// 'Path of creation' for the activity (which is also its unique ID) but is also used as
        /// the 'list of live parents' which indicate of those ancestors, which are alive (if they
        /// are not marked dead they are alive).   
        /// </summary>
        private class ActivityInfo
        {
            public ActivityInfo(string name, long uniqueId, ActivityInfo creator, Guid activityIDToRestore, EventActivityOptions options)
            {
                m_name = name;
                m_eventOptions = options;
                m_creator = creator;
                m_uniqueId = uniqueId;
                m_level = creator != null ? creator.m_level + 1 : 0;
                m_activityIdToRestore = activityIDToRestore;

                // Create a nice GUID that encodes the chain of activities that started this one.
                CreateActivityPathGuid(out m_guid, out m_activityPathGuidOffset);
            }

            public Guid ActivityId
            {
                get
                {
                    return m_guid;
                }
            }

            public static string Path(ActivityInfo activityInfo)
            {
                if (activityInfo == null)
                    return ("");
                return Path(activityInfo.m_creator) + "/" + activityInfo.m_uniqueId;
            }

            public override string ToString()
            {
                string dead = "";
                if (m_stopped != 0)
                    dead = ",DEAD";
                return m_name + "(" + Path(this) + dead + ")";
            }

            public static string LiveActivities(ActivityInfo list)
            {
                if (list == null)
                    return "";
                return list.ToString() + ";" + LiveActivities(list.m_creator);
            }

            public bool CanBeOrphan()
            {
                if ((m_eventOptions & EventActivityOptions.Detachable) != 0)
                    return true;
                return false;
            }

            #region private

            #region CreateActivityPathGuid
            /// <summary>
            /// Logically every activity Path (see Path()) that describes the activities that caused this 
            /// (rooted in an activity that predates activity tracking.  
            ///
            /// We wish to encode this path in the Guid to the extent that we can.  Many of the paths have
            /// many small numbers in them and we take advantage of this in the encoding to output as long
            /// a path in the GUID as possible.   
            /// 
            /// Because of the possibility of GUID collision, we only use 96 of the 128 bits of the GUID
            /// for encoding the path.  The last 32 bits are a simple checksum (and random number) that 
            /// identifies this as using the convention defined here.   
            ///
            /// It returns both the GUID which has the path as well as the offset that points just beyond
            /// the end of the activity (so it can be appended to).  Note that if the end is in a nibble
            /// (it uses nibbles instead of bytes as the unit of encoding, then it will point at the unfinished
            /// byte (since the top nibble can't be zero you can determine if this is true by seeing if 
            /// this byte is nonZero.   This offset is needed to efficiently create the ID for child activities. 
            /// </summary>
            [System.Security.SecuritySafeCritical]
            private unsafe void CreateActivityPathGuid(out Guid idRet, out int activityPathGuidOffset)
            {
                fixed (Guid* outPtr = &idRet)
                {
                    int activityPathGuidOffsetStart = 0;
                    if (m_creator != null)
                    {
                        activityPathGuidOffsetStart = m_creator.m_activityPathGuidOffset;
                        idRet = m_creator.m_guid;
                    }
                    else
                    {
                        // 
                        int appDomainID = 0;
#if !ES_BUILD_PCL  
                        appDomainID = System.Threading.Thread.GetDomainID();
#endif
                        // We start with the appdomain number to make this unique among appdomains.
                        activityPathGuidOffsetStart = AddIdToGuid(outPtr, activityPathGuidOffsetStart, (uint)appDomainID);
                    }

                    activityPathGuidOffset = AddIdToGuid(outPtr, activityPathGuidOffsetStart, (uint)m_uniqueId);


                    // If the path does not fit, Make a GUID by incrementing rather than as a path, keeping as much of the path as possible
                    if (12 < activityPathGuidOffset)
                        CreateOverflowGuid(outPtr);
                }
            }

            /// <summary>
            /// If we can't fit the activity Path into the GUID we come here.   What we do is simply
            /// generate a 4 byte number (s_nextOverflowId).  Then look for an ancestor that has  
            /// sufficient space for this ID.   By doing this, we preserve the fact that this activity
            /// is a child (of unknown depth) from that ancestor.
            /// </summary>
            [System.Security.SecurityCritical]
            private unsafe void CreateOverflowGuid(Guid* outPtr)
            {
                // Search backwards for an ancestor that has sufficient space to put the ID.  
                for (ActivityInfo ancestor = m_creator; ancestor != null; ancestor = ancestor.m_creator)
                {
                    if (ancestor.m_activityPathGuidOffset <= 10)  // we need at least 2 bytes.  
                    {
                        uint id = unchecked((uint)Interlocked.Increment(ref ancestor.m_lastChildID));        // Get a unique ID 
                        // Try to put the ID into the GUID
                        *outPtr = ancestor.m_guid;
                        int endId = AddIdToGuid(outPtr, ancestor.m_activityPathGuidOffset, id, true);

                        // Does it fit?
                        if (endId <= 12)
                            break;
                    }
                }
            }

            /// <summary>
            /// The encoding for a list of numbers used to make Activity  GUIDs.   Basically
            /// we operate on nibbles (which are nice because they show up as hex digits).  The
            /// list is ended with a end nibble (0) and depending on the nibble value (Below)
            /// the value is either encoded into nibble itself or it can spill over into the
            /// bytes that follow.   
            /// </summary>
            enum NumberListCodes : byte
            {
                End = 0x0,             // ends the list.   No valid value has this prefix.   
                LastImmediateValue = 0xA,

                PrefixCode = 0xB,      // all the 'long' encodings go here.  If the next nibble is MultiByte1-4
                // than this is a 'overflow' id.   Unlike the hierarchical IDs these are 
                // allocated densely but don't tell you anything about nesting. we use 
                // these when we run out of space in the GUID to store the path.

                MultiByte1 = 0xC,   // 1 byte follows.  If this Nibble is in the high bits, it the high bits of the number are stored in the low nibble.   
                // commented out because the code does not explicitly reference the names (but they are logically defined).  
                // MultiByte2 = 0xD,   // 2 bytes follow (we don't bother with the nibble optimization)
                // MultiByte3 = 0xE,   // 3 bytes follow (we don't bother with the nibble optimization)
                // MultiByte4 = 0xF,   // 4 bytes follow (we don't bother with the nibble optimization)
            }

            /// Add the activity id 'id' to the output Guid 'outPtr' starting at the offset 'whereToAddId'
            /// Thus if this number is 6 that is where 'id' will be added.    This will return 13 (12
            /// is the maximum number of bytes that fit in a GUID) if the path did not fit.  
            /// If 'overflow' is true, then the number is encoded as an 'overflow number (which has a
            /// special (longer prefix) that indicates that this ID is allocated differently 
            [System.Security.SecurityCritical]
            private static unsafe int AddIdToGuid(Guid* outPtr, int whereToAddId, uint id, bool overflow = false)
            {
                byte* ptr = (byte*)outPtr;
                byte* endPtr = ptr + 12;
                ptr += whereToAddId;
                if (endPtr <= ptr)
                    return 13;                // 12 means we might exactly fit, 13 means we definately did not fit

                if (0 < id && id <= (uint)NumberListCodes.LastImmediateValue && !overflow)
                    WriteNibble(ref ptr, endPtr, id);
                else
                {
                    uint len = 4;
                    if (id <= 0xFF)
                        len = 1;
                    else if (id <= 0xFFFF)
                        len = 2;
                    else if (id <= 0xFFFFFF)
                        len = 3;

                    if (overflow)
                    {
                        if (endPtr <= ptr + 2)        // I need at least 2 bytes
                            return 13;

                        // Write out the prefix code nibble and the length nibble 
                        WriteNibble(ref ptr, endPtr, (uint)NumberListCodes.PrefixCode);
                    }
                    // The rest is the same for overflow and non-overflow case
                    WriteNibble(ref ptr, endPtr, (uint)NumberListCodes.MultiByte1 + (len - 1));

                    // Do we have an odd nibble?   If so flush it or use it for the 12 byte case.   
                    if (ptr < endPtr && *ptr != 0)
                    {
                        // If the value < 4096 we can use the nibble we are otherwise just outputting as padding. 
                        if (id < 4096)
                        {
                            // Indicate this is a 1 byte multicode with 4 high order bits in the lower nibble.  
                            *ptr = (byte)(((uint)NumberListCodes.MultiByte1 << 4) + (id >> 8));
                            id &= 0xFF;     // Now we only want the low order bits.  
                        }
                        ptr++;
                    }

                    // Write out the bytes.
                    while (0 < len)
                    {
                        if (endPtr <= ptr)
                        {
                            ptr++;        // Indicate that we have overflowed
                            break;
                        }
                        *ptr++ = (byte)id;
                        id = (id >> 8);
                        --len;
                    }
                }

                // Compute the checksum 
                uint* sumPtr = (uint*)outPtr;
                // We set the last DWORD the sum of the first 3 DWORDS in the GUID.   This 
                sumPtr[3] = sumPtr[0] + sumPtr[1] + sumPtr[2] + 0x599D99AD;                        // This last number is a random number (it identifies us as us)

                return (int)(ptr - ((byte*)outPtr));
            }

            /// <summary>
            /// Write a single Nible 'value' (must be 0-15) to the byte buffer represented by *ptr.  
            /// Will not go past 'endPtr'.  Also it assumes that we never write 0 so we can detect
            /// whether a nibble has already been written to ptr  because it will be nonzero.   
            /// Thus if it is non-zero it adds to the current byte, otherwise it advances and writes
            /// the new byte (in the high bits) of the next byte.  
            /// </summary>
            [System.Security.SecurityCritical]
            private static unsafe void WriteNibble(ref byte* ptr, byte* endPtr, uint value)
            {
                Contract.Assert(0 <= value && value < 16);
                Contract.Assert(ptr < endPtr);

                if (*ptr != 0)
                    *ptr++ |= (byte)value;
                else
                    *ptr = (byte)(value << 4);
            }

            #endregion // CreateGuidForActivityPath

            readonly internal string m_name;                        // The name used in the 'start' and 'stop' APIs to help match up
            readonly long m_uniqueId;                               // a small number that makes this activity unique among its siblings
            internal readonly Guid m_guid;                          // Activity Guid, it is basically an encoding of the Path() (see CreateActivityPathGuid)
            internal readonly int m_activityPathGuidOffset;         // Keeps track of where in m_guid the causality path stops (used to generated child GUIDs)
            internal readonly int m_level;                          // current depth of the Path() of the activity (used to keep recursion under control)
            readonly internal EventActivityOptions m_eventOptions;  // Options passed to start. 
            internal long m_lastChildID;                            // used to create a unique ID for my children activities
            internal int m_stopped;                                 // This work item has stopped
            readonly internal ActivityInfo m_creator;               // My parent (creator).  Forms the Path() for the activity.
            readonly internal Guid m_activityIdToRestore;           // The Guid to restore after a stop.   
            #endregion
        }

        // This callback is used to initialize the m_current AsyncLocal Variable.   
        // Its job is to keep the ETW Activity ID (part of thread local storage) in [....]
        // with m_current.ActivityID
        void ActivityChanging(AsyncLocalValueChangedArgs<ActivityInfo> args)
        {
            ActivityInfo cur =  args.CurrentValue;
            ActivityInfo prev = args.PreviousValue;

            // Are we popping off a value?   (we have a prev, and it creator is cur) 
            // Then check if we should use the GUID at the time of the start event
            if (prev != null && prev.m_creator == cur)
            {
                // If the saved activity ID is not the same as the creator activity
                // that takes precedence (it means someone explicitly did a SetActivityID)
                // Set it to that and get out
                if (cur == null || prev.m_activityIdToRestore != cur.ActivityId)
                {
                    EventSource.SetCurrentThreadActivityId(prev.m_activityIdToRestore);
                    return;
                }
            }

            // OK we did not have an explicit SetActivityID set.   Then we should be 
            // setting the activity to current ActivityInfo.  However that activity 
            // might be dead, in which case we should skip it, so we never set 
            // the ID to dead things.   
            while(cur != null)
            {
                // We found a live activity (typically the first time), set it to that.  
                if (cur.m_stopped == 0)
                {
                    EventSource.SetCurrentThreadActivityId(cur.ActivityId);
                    return;
                }
                cur = cur.m_creator;
            }
            // we can get here if there is no information on our activity stack (everything is dead)
            // currently we do nothing, as that seems better than setting to Guid.Emtpy.  
        }

        /// <summary>
        /// Async local variables have the properly that the are automatically copied whenever a task is created and used
        /// while that task is running.   Thus m_current 'flows' to any task that is caused by the current thread that
        /// last set it.   
        /// 
        /// This variable points a a linked list that represents all Activities that have started but have not stopped.  
        /// </summary>
        AsyncLocal<ActivityInfo> m_current;
        bool m_checkedForEnable;

        // Singleton
        private static ActivityTracker s_activityTrackerInstance = new ActivityTracker();

        // Used to create unique IDs at the top level.  Not used for nested Ids (each activity has its own id generator)
        static long m_nextId = 0;
        private const ushort MAX_ACTIVITY_DEPTH = 100;            // Limit maximum depth of activities to be tracked at 100. 
        // This will avoid leaking memory in case of activities that are never stopped.

        #endregion
    }

#if ES_BUILD_STANDALONE
    /******************************** SUPPORT *****************************/
    /// <summary>
    /// This is supplied by the framework.   It is has the semantics that the value is copied to any new Tasks that is created
    /// by the current task.   Thus all causally related code gets this value.    Note that reads and writes to this VARIABLE 
    /// (not what it points it) to this does not need to be protected by locks because it is inherently thread local (you always
    /// only get your thread local copy which means that you never have ----s.  
    /// </summary>
    /// 
    [EventSource(Name="Microsoft.Tasks.Nuget")]
    internal class TplEtwProvider : EventSource
    {
        public class Keywords
        {
            public const EventKeywords Debug = (EventKeywords) 1;
        }

        public static TplEtwProvider Log = new TplEtwProvider();
        public bool Debug { get { return IsEnabled(EventLevel.Verbose, Keywords.Debug); } }

        public void DebugFacilityMessage(string Facility, string Message) { WriteEvent(1, Facility, Message); }
        public void DebugFacilityMessage1(string Facility, string Message, string Arg) { WriteEvent(2, Facility, Message, Arg); }
        public void SetActivityId(Guid Id) { WriteEvent(3, Id); }
    }
#endif

#if ES_BUILD_AGAINST_DOTNET_V35 || ES_BUILD_PCL || NO_ASYNC_LOCAL

    internal sealed class AsyncLocalValueChangedArgs<T>
    {
        public AsyncLocalValueChangedArgs()
        {
        }

        public T PreviousValue { get { return default(T); } }
        public T CurrentValue { get { return default(T); } }

    }

    internal sealed class AsyncLocal<T>
    {
        public AsyncLocal()
        {
        }

        public AsyncLocal(Action<AsyncLocalValueChangedArgs<T>> valueChangedHandler)
        {

        }

        public T Value
        {
            get
            {
                object obj = null;  // 
                return (obj == null) ? default(T) : (T)obj;
            }
            set
            {
                // 
            }
        }
    }
#endif

}