How do Tasks in the Task Parallel Library affect ActivityID?

Before using the Task Parallel Library, I have often used CorrelationManager.ActivityId to keep track of tracing/error reporting with multiple threads.

ActivityId is stored in Thread Local Storage, so each thread get's its own copy. The idea is that when you fire up a thread (activity), you assign a new ActivityId. The ActivityId will be written to the logs with any other trace information, making it possible to single out the trace information for a single 'Activity'. This is really useful with WCF as the ActivityId can be carried over to the service component.

Here is an example of what I'm talking about:

static void Main(string[] args)
    ThreadPool.QueueUserWorkItem(new WaitCallback((o) =>

static void DoWork()
        Trace.CorrelationManager.ActivityId = Guid.NewGuid();
        //The functions below contain tracing which logs the ActivityID.
    catch (Exception ex)
        Trace.Write(Trace.CorrelationManager.ActivityId + " " + ex.ToString());

Now, with the TPL, my understanding is that multiple Tasks share Threads. Does this mean that ActivityId is prone to being reinitialized mid-task (by another task)? Is there a new mechanism to deal with activity tracing?


I ran some experiments and it turns out the assumption in my question is incorrect - multiple tasks created with the TPL do not run on the same thread at the same time.

ThreadLocalStorage is safe to use with TPL in .NET 4.0, since a thread can only be used by one task at a time.

The assumption that tasks can share threads concurrently was based on an interview I heard about c# 5.0 on DotNetRocks (sorry, I can't remember which show it was) - so my question may (or may not) become relevant soon.

My experiment starts a number of tasks, and records how many tasks ran, how long they took, and how many threads were consumed. The code is below if anyone would like to repeat it.

class Program
    static void Main(string[] args)
        int totalThreads = 100;
        TaskCreationOptions taskCreationOpt = TaskCreationOptions.None;
        Task task = null;
        Stopwatch stopwatch = new Stopwatch();
        Task[] allTasks = new Task[totalThreads];
        for (int i = 0; i < totalThreads; i++)
            task = Task.Factory.StartNew(() =>
           }, taskCreationOpt);

            allTasks[i] = task;


        Console.WriteLine(String.Format("Completed {0} tasks in {1} milliseconds", totalThreads, stopwatch.ElapsedMilliseconds));
        Console.WriteLine(String.Format("Used {0} threads", threadIds.Count));

    private static List<int> threadIds = new List<int>();
    private static object locker = new object();
    private static void DoLongRunningWork()
        lock (locker)
            //Keep a record of the managed thread used.
            if (!threadIds.Contains(Thread.CurrentThread.ManagedThreadId))
        Guid g1 = Guid.NewGuid();
        Trace.CorrelationManager.ActivityId = g1;
        Guid g2 = Trace.CorrelationManager.ActivityId;

The output (of course this will depend on the machine) was:

Completed 100 tasks in 23097 milliseconds
Used 23 threads

Changing taskCreationOpt to TaskCreationOptions.LongRunning gave different results:

Completed 100 tasks in 3458 milliseconds 
Used 100 threads

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