C# Events and Thread Safety

The JIT isn't allowed to perform the optimization you're talking about in the first part, because of the condition. I know this was raised as a spectre a while ago, but it's not valid. (I checked it with either Joe Duffy or Vance Morrison a while ago; I can't remember which.)

Without the volatile modifier it's possible that the local copy taken will be out of date, but that's all. It won't cause a NullReferenceException.

And yes, there's certainly a race condition - but there always will be. Suppose we just change the code to:

TheEvent(this, EventArgs.Empty);

Now suppose that the invocation list for that delegate has 1000 entries. It's perfectly possible that the action at the start of the list will have executed before another thread unsubscribes a handler near the end of the list. However, that handler will still be executed because it'll be a new list. (Delegates are immutable.) As far as I can see this is unavoidable.

Using an empty delegate certainly avoids the nullity check, but doesn't fix the race condition. It also doesn't guarantee that you always "see" the latest value of the variable.

I see a lot of people going toward the extension method of doing this ...

public static class Extensions   
{   
  public static void Raise<T>(this EventHandler<T> handler, 
    object sender, T args) where T : EventArgs   
  {   
    if (handler != null) handler(sender, args);   
  }   
}

That gives you nicer syntax to raise the event ...

MyEvent.Raise( this, new MyEventArgs() );

And also does away with the local copy since it is captured at method call time.

"Why is explicit-null-check the 'standard pattern'?"

I suspect the reason for this might be that the null-check is more performant.

If you always subscribe an empty delegate to your events when they are created, there will be some overheads:

Cost of constructing the empty delegate.

Cost of constructing a delegate chain to contain it.

Cost of invoking the pointless delegate every single time the event is raised.

(Note that UI controls often have a large number of events, most of which are never subscribed to. Having to create a dummy subscriber to each event and then invoke it would likely be a significant performance hit.)

I did some cursory performance testing to see the impact of the subscribe-empty-delegate approach, and here are my results:

Executing 50000000 iterations . . .
OnNonThreadSafeEvent took:      432ms
OnClassicNullCheckedEvent took: 490ms
OnPreInitializedEvent took:     614ms <--
Subscribing an empty delegate to each event . . .
Executing 50000000 iterations . . .
OnNonThreadSafeEvent took:      674ms
OnClassicNullCheckedEvent took: 674ms
OnPreInitializedEvent took:     2041ms <--
Subscribing another empty delegate to each event . . .
Executing 50000000 iterations . . .
OnNonThreadSafeEvent took:      2011ms
OnClassicNullCheckedEvent took: 2061ms
OnPreInitializedEvent took:     2246ms <--
Done

Note that for the case of zero or one subscribers (common for UI controls, where events are plentiful), the event pre-initialised with an empty delegate is notably slower (over 50 million iterations...)

For more information and source code, visit this blog post on .NET Event invocation thread safety that I published just the day before this question was asked (!)

(My test set-up may be flawed so feel free to download the source code and inspect it yourself. Any feedback is much appreciated.)

I truly enjoyed this read - not! Even though I need it to work with the C# feature called events!

Why not fix this in the compiler? I know there are MS people who read these posts, so please don't flame this!

1 - the Null issue) Why not make events be .Empty instead of null in the first place? How many lines of code would be saved for null check or having to stick a = delegate {} onto the declaration? Let the compiler handle the Empty case, IE do nothing! If it all matters to the creator of the event, they can check for .Empty and do whatever they care with it! Otherwise all the null checks / delegate adds are hacks around the problem!

Honestly I'm tired of having to do this with every event - aka boilerplate code!

public event Action<thisClass, string> Some;
protected virtual void DoSomeEvent(string someValue)
{
  var e = Some; // avoid race condition here! 
  if(null != e) // avoid null condition here! 
     e(this, someValue);
}

2 - the race condition issue) I read Eric's blog post, I agree that the H (handler) should handle when it dereferences itself, but cannot the event be made immutable/thread safe? IE, set a lock flag on its creation, so that whenever it is called, it locks all subscribing and un-subscribing to it while its executing?

Conclusion,

Are not modern day languages supposed to solve problems like these for us?

With C# 6 and above, code could be simplified using new ?. operator as in:

TheEvent?.Invoke(this, EventArgs.Empty);

Here is the MSDN documentation.

According to Jeffrey Richter in the book CLR via C#, the correct method is:

// Copy a reference to the delegate field now into a temporary field for thread safety
EventHandler<EventArgs> temp =
Interlocked.CompareExchange(ref NewMail, null, null);
// If any methods registered interest with our event, notify them
if (temp != null) temp(this, e);

Because it forces a reference copy. For more information, see his Event section in the book.

I've been using this design pattern to ensure that event handlers aren't executed after they're unsubscribed. It's working pretty well so far, although I haven't tried any performance profiling.

private readonly object eventMutex = new object();

private event EventHandler _onEvent = null;

public event EventHandler OnEvent
{
  add
  {
    lock(eventMutex)
    {
      _onEvent += value;
    }
  }

  remove
  {
    lock(eventMutex)
    {
      _onEvent -= value;
    }
  }

}

private void HandleEvent(EventArgs args)
{
  lock(eventMutex)
  {
    if (_onEvent != null)
      _onEvent(args);
  }
}

I'm mostly working with Mono for Android these days, and Android doesn't seem to like it when you try to update a View after its Activity has been sent to the background.

This practice is not about enforcing a certain order of operations. It's actually about avoiding a null reference exception.

The reasoning behind people caring about the null reference exception and not the race condition would require some deep psychological research. I think it has something to do with the fact that fixing the null reference problem is much easier. Once that is fixed, they hang a big "Mission Accomplished" banner on their code and unzip their flight suit.

Note: fixing the race condition probably involves using a synchronous flag track whether the handler should run

So I'm a little late to the party here. :)

As for the use of null rather than the null object pattern to represent events with no subscribers, consider this scenario. You need to invoke an event, but constructing the object (EventArgs) is non-trivial, and in the common case your event has no subscribers. It would be beneficial to you if you could optimize your code to check to see if you had any subscribers at all before you committed processing effort to constructing the arguments and invoking the event.

With this in mind, a solution is to say "well, zero subscribers is represented by null." Then simply perform the null check before performing your expensive operation. I suppose another way of doing this would have been to have a Count property on the Delegate type, so you'd only perform the expensive operation if myDelegate.Count > 0. Using a Count property is a somewhat nice pattern that solves the original problem of allowing optimization, and it also has the nice property of being able to be invoked without causing a NullReferenceException.

Keep in mind, though, that since delegates are reference types, they are allowed to be null. Perhaps there was simply no good way of hiding this fact under the covers and supporting only the null object pattern for events, so the alternative may have been forcing developers to check both for null and for zero subscribers. That would be even uglier than the current situation.

Note: This is pure speculation. I'm not involved with the .NET languages or CLR.

for single threaded applicaitons, you are correc this is not an issue.

However, if you are making a component that exposes events, there is no guarantee that a consumer of your component is not going to go multithreading, in which case you need to prepare for the worst.

Using the empty delegate does solve the problem, but also causes a performance hit on every call to the event, and could possibly have GC implications.

You are right that the consumer trie dto unsubscribe in order for this to happen, but if they made it past the temp copy, then consider the message already in transit.

If you don't use the temporary variable, and don't use the empty delegate, and someone unsubscribes, you get a null reference exception, which is fatal, so I think the cost is worth it.

I've never really considered this to be much of an issue because I generally only protect against this sort of potential threading badness in static methods (etc) on my reusable components, and I don't make static events.

Am I doing it wrong?

Wire all your events at construction and leave them alone. The design of the Delegate class cannot possibly handle any other usage correctly, as I will explain in the final paragraph of this post.

First of all, there's no point in trying to intercept an event notification when your event handlers must already make a synchronized decision about whether/how to respond to the notification.

Anything that may be notified, should be notified. If your event handlers are properly handling the notifications (i.e. they have access to an authoritative application state and respond only when appropriate), then it will be fine to notify them at any time and trust they will respond properly.

The only time a handler shouldn't be notified that an event has occurred, is if the event in fact hasn't occurred! So if you don't want a handler to be notified, stop generating the events (i.e. disable the control or whatever is responsible for detecting and bringing the event into existence in the first place).

Honestly, I think the Delegate class is unsalvageable. The merger/transition to a MulticastDelegate was a huge mistake, because it effectively changed the (useful) definition of an event from something that happens at a single instant in time, to something that happens over a timespan. Such a change requires a synchronization mechanism that can logically collapse it back into a single instant, but the MulticastDelegate lacks any such mechanism. Synchronization should encompass the entire timespan or instant the event takes place, so that once an application makes the synchronized decision to begin handling an event, it finishes handling it completely (transactionally). With the black box that is the MulticastDelegate/Delegate hybrid class, this is near impossible, so adhere to using a single-subscriber and/or implement your own kind of MulticastDelegate that has a synchronization handle that can be taken out while the handler chain is being used/modified. I'm recommending this, because the alternative would be to implement synchronization/transactional-integrity redundantly in all your handlers, which would be ridiculously/unnecessarily complex.

Please take a look here: http://www.danielfortunov.com/software/%24daniel_fortunovs_adventures_in_software_development/2009/04/23/net_event_invocation_thread_safety This is the correct solution and should always be used instead of all other workarounds.

“You can ensure that the internal invocation list always has at least one member by initializing it with a do-nothing anonymous method. Because no external party can have a reference to the anonymous method, no external party can remove the method, so the delegate will never be null” — Programming .NET Components, 2nd Edition, by Juval Löwy

public static event EventHandler<EventArgs> PreInitializedEvent = delegate { };  

public static void OnPreInitializedEvent(EventArgs e)  
{  
    // No check required - event will never be null because  
    // we have subscribed an empty anonymous delegate which  
    // can never be unsubscribed. (But causes some overhead.)  
    PreInitializedEvent(null, e);  
}  

I don't believe the question is constrained to the c# "event" type. Removing that restriction, why not re-invent the wheel a bit and do something along these lines?

Raise event thread safely - best practice

Ability to sub/unsubscribe from any thread while within a raise (race condition removed)

Operator overloads for += and -= at the class level.

Generic caller-defined delegate

Thanks for a useful discussion. I was working on this problem recently and made the following class which is a bit slower, but allows to avoid callings to disposed objects.

The main point here is that invocation list can be modified even event is raised.

/// <summary>
/// Thread safe event invoker
/// </summary>
public sealed class ThreadSafeEventInvoker
{
    /// <summary>
    /// Dictionary of delegates
    /// </summary>
    readonly ConcurrentDictionary<Delegate, DelegateHolder> delegates = new ConcurrentDictionary<Delegate, DelegateHolder>();

    /// <summary>
    /// List of delegates to be called, we need it because it is relatevely easy to implement a loop with list
    /// modification inside of it
    /// </summary>
    readonly LinkedList<DelegateHolder> delegatesList = new LinkedList<DelegateHolder>();

    /// <summary>
    /// locker for delegates list
    /// </summary>
    private readonly ReaderWriterLockSlim listLocker = new ReaderWriterLockSlim();

    /// <summary>
    /// Add delegate to list
    /// </summary>
    /// <param name="value"></param>
    public void Add(Delegate value)
    {
        var holder = new DelegateHolder(value);
        if (!delegates.TryAdd(value, holder)) return;

        listLocker.EnterWriteLock();
        delegatesList.AddLast(holder);
        listLocker.ExitWriteLock();
    }

    /// <summary>
    /// Remove delegate from list
    /// </summary>
    /// <param name="value"></param>
    public void Remove(Delegate value)
    {
        DelegateHolder holder;
        if (!delegates.TryRemove(value, out holder)) return;

        Monitor.Enter(holder);
        holder.IsDeleted = true;
        Monitor.Exit(holder);
    }

    /// <summary>
    /// Raise an event
    /// </summary>
    /// <param name="args"></param>
    public void Raise(params object[] args)
    {
        DelegateHolder holder = null;

        try
        {
            // get root element
            listLocker.EnterReadLock();
            var cursor = delegatesList.First;
            listLocker.ExitReadLock();

            while (cursor != null)
            {
                // get its value and a next node
                listLocker.EnterReadLock();
                holder = cursor.Value;
                var next = cursor.Next;
                listLocker.ExitReadLock();

                // lock holder and invoke if it is not removed
                Monitor.Enter(holder);
                if (!holder.IsDeleted)
                    holder.Action.DynamicInvoke(args);
                else if (!holder.IsDeletedFromList)
                {
                    listLocker.EnterWriteLock();
                    delegatesList.Remove(cursor);
                    holder.IsDeletedFromList = true;
                    listLocker.ExitWriteLock();
                }
                Monitor.Exit(holder);

                cursor = next;
            }
        }
        catch
        {
            // clean up
            if (listLocker.IsReadLockHeld)
                listLocker.ExitReadLock();
            if (listLocker.IsWriteLockHeld)
                listLocker.ExitWriteLock();
            if (holder != null && Monitor.IsEntered(holder))
                Monitor.Exit(holder);

            throw;
        }
    }

    /// <summary>
    /// helper class
    /// </summary>
    class DelegateHolder
    {
        /// <summary>
        /// delegate to call
        /// </summary>
        public Delegate Action { get; private set; }

        /// <summary>
        /// flag shows if this delegate removed from list of calls
        /// </summary>
        public bool IsDeleted { get; set; }

        /// <summary>
        /// flag shows if this instance was removed from all lists
        /// </summary>
        public bool IsDeletedFromList { get; set; }

        /// <summary>
        /// Constuctor
        /// </summary>
        /// <param name="d"></param>
        public DelegateHolder(Delegate d)
        {
            Action = d;
        }
    }
}

And the usage is:

    private readonly ThreadSafeEventInvoker someEventWrapper = new ThreadSafeEventInvoker();
    public event Action SomeEvent
    {
        add { someEventWrapper.Add(value); }
        remove { someEventWrapper.Remove(value); }
    }

    public void RaiseSomeEvent()
    {
        someEventWrapper.Raise();
    }

Test

I tested it in the following manner. I have a thread which creates and destroys objects like this:

var objects = Enumerable.Range(0, 1000).Select(x => new Bar(foo)).ToList();
Thread.Sleep(10);
objects.ForEach(x => x.Dispose());

In a Bar (a listener object) constructor I subscribe to SomeEvent (which is implemented as shown above) and unsubscribe in Dispose:

    public Bar(Foo foo)
    {
        this.foo = foo;
        foo.SomeEvent += Handler;
    }

    public void Handler()
    {
        if (disposed)
            Console.WriteLine("Handler is called after object was disposed!");
    }

    public void Dispose()
    {
        foo.SomeEvent -= Handler;
        disposed = true;
    }

Also I have couple of threads which raise event in a loop.

All these actions are performed simultaneously: many listeners are created and destroyed and event is being fired at the same time.

If there were a race conditions I should see a message in a console, but it is empty. But if I use clr events as usual I see it full of warning messages. So, I can conclude that it is possible to implement a thread safe events in c#.

What do you think?