How to correctly unregister an event handler

问题:

In a code review, I stumbled over this (simplified) code fragment to unregister an event handler:

 Fire -= new MyDelegate(OnFire);

I thought that this does not unregister the event handler because it creates a new delegate which had never been registered before. But searching MSDN I found several code samples which use this idiom.

So I started an experiment:

internal class Program
{
    public delegate void MyDelegate(string msg);
    public static event MyDelegate Fire;

    private static void Main(string[] args)
    {
        Fire += new MyDelegate(OnFire);
        Fire += new MyDelegate(OnFire);
        Fire("Hello 1");
        Fire -= new MyDelegate(OnFire);
        Fire("Hello 2");
        Fire -= new MyDelegate(OnFire);
        Fire("Hello 3");
    }

    private static void OnFire(string msg)
    {
        Console.WriteLine("OnFire: {0}", msg);
    }

}

To my surprise, the following happened:

1. Fire("Hello 1"); produced two messages, as expected.
2. Fire("Hello 2"); produced one message!
   This convinced me that unregistering new delegates works!
3. Fire("Hello 3"); threw a NullReferenceException.
   Debugging the code showed that Fire is null after unregistering the event.

I know that for event handlers and delegate, the compiler generates a lot of code behind the scene. But I still don't understand why my reasoning is wrong.

What am I missing?

Additional question: from the fact that Fire is null when there are no events registered, I conclude that everywhere an event is fired, a check against null is required.

回答1:

The C# compiler's default implementation of adding an event handler calls Delegate.Combine, while removing an event handler calls Delegate.Remove:

Fire = (MyDelegate) Delegate.Remove(Fire, new MyDelegate(Program.OnFire));

The Framework's implementation of Delegate.Remove doesn't look at the MyDelegate object itself, but at the method the delegate refers to (Program.OnFire). Thus, it's perfectly safe to create a new MyDelegate object when unsubscribing an existing event handler. Because of this, the C# compiler allows you to use a shorthand syntax (that generates exactly the same code behind the scenes) when adding/removing event handlers: you can omit the new MyDelegate part:

Fire += OnFire;
Fire -= OnFire;

When the last delegate is removed from the event handler, Delegate.Remove returns null. As you have found out, it's essential to check the event against null before raising it:

MyDelegate handler = Fire;
if (handler != null)
    handler("Hello 3");

It's assigned to a temporary local variable to defend against a possible race condition with unsubscribing event handlers on other threads. (See my blog post for details on the thread safety of assigning the event handler to a local variable.) Another way to defend against this problem is to create an empty delegate that is always subscribed; while this uses a little more memory, the event handler can never be null (and the code can be simpler):

public static event MyDelegate Fire = delegate { };

回答2:

You should always check whether a delegate has no targets (its value is null) before firing it. As said before, one way of doing this is to subscribe with a do-nothing anonymous method which won't be removed.

public event MyDelegate Fire = delegate {};

However, this is just a hack to avoid NullReferenceExceptions.

Just simply cheking whether a delegate is null before invoking is not threadsafe as an other thread can deregister after the null-check and making it null when invoking. There is an other solution is to copy the delegate into a temporary variable:

public event MyDelegate Fire;
public void FireEvent(string msg)
{
    MyDelegate temp = Fire;
    if (temp != null)
        temp(msg);
}

Unfortunately, the JIT compiler may optimize the code, eliminate the temporary variable, and use the original delegate. (as per Juval Lowy - Programming .NET Components)

So to avoid this problem, you could use method which accepts a delegate as parameter:

[MethodImpl(MethodImplOptions.NoInlining)]
public void FireEvent(MyDelegate fire, string msg)
{
    if (fire != null)
        fire(msg);
}

Note that without the MethodImpl(NoInlining) attribute the JIT compiler could inline the method making it worthless. Since delegates are immutable this implementation is threadsafe. You could use this method as:

FireEvent(Fire,"Hello 3");