How to correctly unregister an event handler
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:
Fire("Hello 1");
produced two messages, as expected. Fire("Hello 2");
produced one message! This convinced me that unregistering
new
delegates works! 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.
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 { };
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");
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