Raise an event in C# [duplicate]

问题

This question already has answers here:

Checking for null before event dispatching… thread safe? (6 answers)

Closed 3 months ago.

I came across this question in a Microsoft Practice Test and I got confused. Here is the question:

Which of the following C# code samples is the proper way to raise an event, assuming that the Alarm event, the AlarmEventArgs class, and the AlarmEventHandler delegate have been declared?

Here is the "correct" answer they provided:

AlarmEventArgs e = new AlarmEventArgs(1, 2);
AlarmEventHandler handler = Alarm; 
if (handler != null) 
{ 
    handler(this, e);
}

However, there is also another answer which seems correct.

AlarmEventArgs e = new AlarmEventArgs(1, 2);
if (Alarm!= null) 
{ 
    Alarm (this, e);
}

I personally, always use the second method. It works just fine. Can someone please tell me why I should use the first method instead of second?

回答1:

In a multi-threaded environment, it's possible that the event handler may be updated while your event is being dispatched. To avoid this scenario, you assign the handler to a local variable before checking for null and dispatching the message.

来源：https://stackoverflow.com/questions/1344972/raise-an-event-in-c-sharp