Snapshot'ing the event handler value as method argument in C#

Question

From this question (and others) there's this example of snapshoting the event handler value before invoking:

var tmp = _myEventHandler;
if(tmp != null) {
    tmp(sender, args);
}

However, if I pass in the event handler and args into a function does this do the same thing?

protected void Invoke(MyEventHandler handler, MyEventArgs args)
{
    if (handler != null)
        handler(this, args);
}

I would say yes, but after thinking about this I don't know if it is the same - like could an optimization process inline this function and remove the snapshot variable?

score 5 · Accepted Answer · answered Jan 22 '12 at 16:00

5

It indeed does the same thing. The code is simply checking that the event handler is non-null and hence available for raising. This code is no different for a field / parameter / local. The C# compiler or JITer cannot inline this in such a way that the temporary is removed because it would change the semantics of the program.

answered Jan 22 '12 at 16:00

JaredPar

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This is a good explanation. A parameter is just like a local variable. It has the same semantics. – usr Jan 22 '12 at 16:41

score 0 · Answer 2 · edited May 23 '17 at 12:28

The race condition is explained in your other links, such as Marc Gravell's answer.

As @JaredPar points out, your code effectively does the same thing. However, it violates the standard event conventions for .NET, which recommend the following:

protected virtual void OnAlarmRaised(AlarmRaisedEventArgs e)

You are free to violate the pattern, but doing so goes against the grain for minimal (if any) benefit.

Snapshot'ing the event handler value as method argument in C#

2 Answers2