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An event is a notification that something has occurred (such as a mouse click) or, in some cases, is about to occur (such as a price change).

Classes can define events and their instances (objects) may raise these events. For instance, a Button may contain a Click event that gets raised when a user has clicked it.

Event handlers are then methods that get called when their corresponding event is raised. A form may contain a Clicked event handler for every Button it contains, for instance.



When raising an event:


if (Changed != null)      // Changed has 1 subscriber at this point
                             // In another thread, that one subscriber decided to unsubscribe
        Changed(this, args); // `Changed` is now null, `NullReferenceException` is thrown.


// Cache the "Changed" event as a local. If it is not null, then use
    // the LOCAL variable (handler) to raise the event, NOT the event itself.
    var handler = Changed;
    if(handler != null)
        handler(this, args);

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