How to correctly unregister an event handler

Question

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

 Fire -= new MyDelegate(OnFire);

I though

Answer 1

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");