Thursday, 30 July 2009

Enhanced Weak Events Part Two - Immutable Event Entry List

Posts in this series

- Part One: DynamicMethod vs. Expression Tree Compilation
- Part Two: Immutable Event Entry List (this post)
- Part Three: Synchronized Events (to be written)
- Part Four: Custom Events and unsolved problems (to be written)
- Dynamic Code Generation With Lambda Expressions (to be written)

C# Events - Basics

Just for a start, let me reinforce some preliminary definitions in an very factual way.

  • Delegates are tuples composed by a reference (A) to a method, and a reference (B) to a target object.
    - All delegates are made immutable;
    - The reference (B) will be null if the method is static;
    - When a delegate is invoked, the referenced method is called. If the method is an instance method, the target object reference is used;
  • Multicast Delegates encapsulate a collection of delegates.
    - When invoked, all of its delegates are invoked in an unspecified order;
    - Multicast delegates can only be created by composition or decomposition. Composition is the merging of two delegates into a new multicast delegate. Decomposition is a subtraction of one delegate from another;
    - There's no such thing as an empty multicastdelegate. If a decomposition would create one, the result is null;
    - For all purposes, a multicast delegate is a delegate;
  • C# Events are nothing more than a special type of property that have add and remove accessors instead of get and set.
    - Only events of a delegate type can be declared;
    - Due to the special accessors, only the class that owns the event can reset its value or invoke its delegate, if it's not null;
    - When add is called, the current delegate is composed with the delegate that was provided;
    - Conversely, remove does a decomposition;

C# Events - Not So Basics

Having said that, let's dive deeper. When it comes to c# events, there are two interesting problems that can be easily overseen. The simpler one is that an event is always initialized with null, and they can always be decomposed to null later on. In other words, an event can be null at any time. Since it's not possible to invoke a null delegate (without causing a NullReferenceException), any class that provides an event has be to sure that the event is not null before invoking it.

The other problem appears in multithreaded applications. A thread might be adding or removeing an event handler while the event is in the middle of an invocation. This is a race situation, and has to be correctly handled. If you want to check a complete explanation on this subject, I recommend you to read Eric's post on events and races.The generally recommended solution to both problems is the following pattern:
var handler = this.MyEvent;
if (handler != null) handler(this, eventArgs);

Now, let's move to main subject of this post.

Mutable Event Entry List

As you may already have noticed, my custom events were born from Daniel Grunwald's WeakEvents. Internally, to represent each event entry* he chose to use a List<EventEntry>. Using regular a List<T> seems very convenient. It will make add and remove operations simple and quick to execute, but there is a downside. List<T>'s are mutable.

One of the major disadvantages of mutable objects appear on multithreaded applications. Consider the problems that I've introduced above. They apply to custom events too. Alas, that invocation pattern can't be used.

One goo point is that even an empty WeakEvent will never be null, so we don't event-is-null problem. The concurrency problem, though, can't be avoided. As the event entry list is mutable, any access to it should be synchronized if we want to make the custom event thread-safe.

Looking at Grunwald's code that makes the invocation of a WeakEvent, I noticed that the entry list is copied to an array before invoking the entries. No locks are made. Is that enough? Unfortunately, the answer is no. Copying the list to an array is not an atomic operation. If the list is modified in the middle of the copy, things will probably get messy. Even if he had enclosed the copy in a lock block, there would still be the performance problem caused by the copy itself.

* An event entry is an abstraction of an event handler delegate. Weak event entries, for example, serve the same purpose of a regular delegate. The sole difference being that it does not keep a strong reference to the target object.

Immutable Event Entry List

Naturally, my suggestion is to use an immutable list to store event entries. By the way, as I mentioned in the beginning, all delegates are immutable. And regular c# events use immutable multicast delegates internally. There's probably a good reason they did it that way, and I don't see why custom events should differ. Let's consider some aspects of this decision.

  • Lower invocation overhead: There's no need to copy event entries before invoking them because the list does never change. All that's needed before the invoking is to read the current list object and store a reference to it in a local variable. If the event is changed by other thread, a new list will be created, but the invocation will be working on an untouched list object.

    The only possible problem may happen when an handler is removed, but an invocation is being made. The result is that the handler is invoked even after it has been removed from the event. Actually, this is a third interesting problem. The post from Eric Lippert (events and races) that I've suggested earlier talks about this stuff too. If you read it, you'll see that my custom events are actually imitating the exactly same behavior that regular c# events have.

    Unfortunately, there is also a trade-off involved. Add and remove operations will be inevitably slower. Each time they happen, a new list has to be created, and a partial copy of the current entries will be made. Personally, invoking performance has higher priority than adding or removing performance, so I think this trade-off is acceptable. If anyone disagrees, I'll be very happy to hear your thoughts.

  • Thread-safety: Invoking an event when its entry list is immutable completely thread-safe without any need for locks. That's a great win.

    Add and remove, on the other hand, are not thread safe. I can say as an excuse that the mutable-list-approach is not thread-safe too. At least, I did not created a problem that was not there before.

    Looking at regular events, we can see that even they are not thread-safe when it comes to add and remove. Would the C# language team let so big a hole in the language? Of course not. The solution is hidden from the programmer's eyes. Whenever you declare a regular c# event, the compiler generates the two accessors (add and remove). Both of them are them decorated with this custom attribute:[System.Runtime.CompilerServices.MethodImpl(MethodImplOptions.Synchronized)]. You can make a test assembly and decompile it to see. Or you can read this old MSDN article too.

    So, in my opinion, locking should be avoided unless there's no alternative. In a reusable component (like custom events), it's preferable to let it be not thread-safe and free of locks than forcing an overhead that is not always required. A developer that uses your component should be able to use any synchronization mechanism he wants, and only if needed. Inside the .NET Framework, almost all collection implementations are not synchronized for this reason. Following the same pattern, I won't put any locks on my Custom Events. User developers should be aware of that, and put locks around add/remove it they need it.


In a world where multi-core processors are everywhere, multithreading and immutability are increasingly important. If you want to read more about immutability in C#, you might want to check this out.

If you read this far, you probably are interested in events. A very extensive article you might want to read is on codeproject.

Finally, as usual, here is the download link for the latest version (v1.1) the my CustomEvents' source code. See ya!


Anonymous said...

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jpbochi said...

Thanks for the kind words. I really appreciate that.

In fact, you let me pretty curious now. Do I know you? May I know who you are?