Why is an additional if statement to check if an event is null before invoking it?

Question

I am going through this code below:

public delegate void PersonArrivedEventHandler(string personName, byte[] personId);

    public class SCLib
    {
        public event personArrivedEventHandler personrrived;

        public SCLib()
        {
            // Simulate that the person arrived after 2000 milli-seconds.
            Task.Run(() =>
            {
                System.Threading.Thread.Sleep(2000);
                OnPersonArrived("personName", new byte[] { 0x20, 0x21, 0x22 });
            });
        }

        protected virtual void OnPersonArrived(string smartCardReaderName, byte[] smartCardId)
        {
            if (this.personArrived != null)
            {
                PersonArrived(personName, personId);
            }
        }
    }

But, I don't know what is the significance of this line, if (this.personArrived != null).

Why is this check done here? Is there any significance of the if statement here? I removed this line and ran the program and everything works as before.

Thanks.

"I just removed all the smoke detectors from the house and everything in it kept working as it did before, so I guess smoke detectors are unnecessary". No: everything *except the smoke detectors* worked as before. When you remove a safety system that safety system no longer operates. — Eric Lippert, Feb 06 '14 at 13:41

score 1 · Accepted Answer · answered Feb 06 '14 at 10:49

1

If the event is not subscribed by the consumer of the class then invoking the event would raise exception as PersonArrived is null if not subscribed.

answered Feb 06 '14 at 10:49

Adil

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thanks, and just checked without subscribing to the event and it was null as you said. But internally how does a class know somebody has or not subscribed to its event ? – now he who must not be named. Feb 06 '14 at 11:04
1

It would be null if the object on which it is going to be called does not subscribe it. – Adil Feb 06 '14 at 11:13
Thanks. Makes sense. `But internally how does a class know somebody has or not subscribed to its event ?` . Well, the answer is: through its object state. – now he who must not be named. Feb 06 '14 at 11:17

score 1 · Answer 2 · answered Feb 06 '14 at 10:50

1

If you use an event which you have not assign a handler it will generate an exception because it is null, so you need to check it before launch the event.

answered Feb 06 '14 at 10:50

BitExodus

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score 1 · Answer 3 · answered Feb 06 '14 at 10:50

1

Because it will be null if no delegates are attached to the event. If you try to invoke such a null event you will get the standard NullReferenceException.

answered Feb 06 '14 at 10:50

Grzenio

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score 1 · Answer 4 · answered Feb 06 '14 at 10:54

'Events' are subscribe by class object using add handler.

SCLibObject.personrrived += new personArrivedEventHandler(somemethod)

If class object is not subscribe the event then you will get NullReferenceException. So before calling events check if it is null or not.

score 1 · Answer 5 · edited May 23 '17 at 12:03

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In a multi-threading application you should store the eventhandlers in a local variable before invoking. See this SO answer, this blog post from Eric Lippert and this SO answer for more details.

void SomeEventInvoke(object sender, EventArgs args) {
    EventHandler ev = SomeEvent;
    if (ev != null) ev(sender, args);
}

edited May 23 '17 at 12:03

Community

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answered Feb 06 '14 at 10:56

Maarten

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Thank you. Links seem to be really useful. – now he who must not be named. Feb 06 '14 at 11:04

Why is an additional if statement to check if an event is null before invoking it?

5 Answers5