Does PLINQ respect SynchronizationContext?

问题:

Say, I have the following code:

IPrincipal capturedPrincipal = Thread.CurrentPrincipal;
myseq.AsParallel().Select(x =>
{
    Thread.CurrenctPrincipal = capturedPrincipal;
    /*call code protected with CAS*/
});

to be sure Thread.CurrenctPrincipal will be propagated to every thread where Select's delegate will be executed on. I've realized that if I had proper SynchronizationContext set up this would happen automatically. So does PLINQ use SynchronizationContext when queuing work items on ThreadPool? And if no, why?

p.s.

I think it's important to note that the code above is executed in WCF environment hosted under IIS/WAS (no ASP.NET compatibility).

Edit: Found a similar question confirming same behavior I witness.

Edit2: Modified casperOne's test a bit and it fails saying threadid is the same:

    [Test]
    public void Test1()
    {
        var principal = new GenericPrincipal(new GenericIdentity("test"), new string[0]);
        Thread.CurrentPrincipal = principal;
        int threadID = Thread.CurrentThread.ManagedThreadId;

        Enumerable.Range(0, 4000).AsParallel()
            .WithExecutionMode(ParallelExecutionMode.ForceParallelism)
            .Select(x =>
                {
                    Assert.AreSame(Thread.CurrentPrincipal, principal);
                    Assert.AreNotEqual(Thread.CurrentThread.ManagedThreadId, threadID);
                    return x;
                })
            .ToArray();
    }

回答1:

There are two questions here. The first is whether or not Thread.CurrentPrincipal is propagated to threads in PLINQ.

The answer is yes, as this is part of the ExecutionContext and the ExecutionContext is captured from the calling thread and copied to the new/recycled thread when a new thread/task/thread pool thread is started.

The following test case (run in .NET 4.0) shows this:

[TestMethod]
public void TestMethod1()
{
    // Capture the current logged in account.
    // Could be a GenericPrincipal as well with some random value
    // set on the identity name.
    IPrincipal p = new WindowsPrincipal(WindowsIdentity.GetCurrent());

    // Set the current principal.
    Thread.CurrentPrincipal = p;

    // Set the synchronization context.
    SynchronizationContext.SetSynchronizationContext(
        new SynchronizationContext());

    // Context is not null.
    Assert.IsNotNull(SynchronizationContext.Current);

    // PLINQ.
    var plinqThreadDetails = 
        // Go parallel.  This number needs to be reasonably
        // high to force parallelization as PLINQ might
        // use this thread if the size is small.
        from i in Enumerable.Range(0, 4000).AsParallel().
            // Force parallelization.  At best, this is
            // a suggestion.
            WithExecutionMode(ParallelExecutionMode.ForceParallelism)
        select new {
            // These values are retreived on another thread.
            IdentityName = Thread.CurrentPrincipal.Identity.Name,
            Thread.CurrentThread.ManagedThreadId,
        };

    // Was there any parallelization?
    bool anyParallel = false;

    // Make assertions.
    // The managed thread id is different than the current one.
    foreach (var plinqThreadDetail in plinqThreadDetails)
    { 
        // But the principal still flowed, even though on a different
        // thread.
        Assert.AreEqual(Thread.CurrentPrincipal.Identity.Name,
            plinqThreadDetail.IdentityName);

        // Update any parallel.
        anyParallel |= (plinqThreadDetail.ManagedThreadId !=
            Thread.CurrentThread.ManagedThreadId);
    }

    // There was *some* parallelization.
    Assert.IsTrue(anyParallel);
}

Regarding whether or not SynchronizationContext is used in PLINQ, it's not, and it doesn't make sense to.

Considering that using a SynchronizationContext usually means serializing a call to a particular context (which is usually a thread, think UI applications, but not always, given the ASP.NET synchronization context), you'd kill any gains that PLINQ would gain from parallelization because every call would have to be marshaled back through the SynchronizationContext.

The benefits in PLINQ come from being able to execute these operations at the same time, not one-at-a-time.

The following test case (very much along the lines of the previous one) proves that the SynchronizationContext is not captured for PLINQ threads:

[TestMethod]
public void TestMethod2()
{
    // Set the synchronization context.
    SynchronizationContext.SetSynchronizationContext(
        new SynchronizationContext());

    // Context is not null.
    Assert.IsNotNull(SynchronizationContext.Current);

    // PLINQ.
    var plinqThreadDetails = 
        // Go parallel.  This number needs to be reasonably
        // high to force parallelization as PLINQ might
        // use this thread if the size is small.
        from i in Enumerable.Range(0, 4000).AsParallel().
            // Force parallelization.
            WithExecutionMode(ParallelExecutionMode.ForceParallelism)
        select new {
            // These values are retreived on another thread.
            SynchronizationContextIsNull =
                SynchronizationContext.Current == null,
            Thread.CurrentThread.ManagedThreadId,
        };

    // Make assertions.
    // Was there any parallelization?
    bool anyParallel = false;

    // Make assertions.
    // The synchronization context on the PLINQ thread was
    // not set, only if on a different thread.
    foreach (var plinqThreadDetail in plinqThreadDetails)
    {
        // If the thread id is different.
        if (plinqThreadDetail.ManagedThreadId !=
            Thread.CurrentThread.ManagedThreadId)
        {
            // The synchronization context was null.
            Assert.IsTrue(plinqThreadDetail.SynchronizationContextIsNull);

            // There was something on another thread.
            anyParallel = true;
        }
        else
        {
            // The synchronization context is not null.
            Assert.IsFalse(plinqThreadDetail.SynchronizationContextIsNull);
        }
    }

    // There was *some* parallelization.
    Assert.IsTrue(anyParallel);
}