Events

Introduction

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.

Parameters

• EventArgsT : The type that derives from EventArgs and contains the event parameters.
• EventName : The name of the event.
• HandlerName : The name of the event handler.
• SenderObject : The object that’s invoking the event.
• EventArguments : An instance of the EventArgsT type that contains the event parameters.|

Remarks

When raising an event:

• Always check if the delegate is null. A null delegate means the event has no subscribers. Raising an event with no subscribers will result in a NullReferenceException.
• Copy the delegate (e.g. EventName) to a local variable (e.g. eventName) before checking for null / raising the event. This avoids race conditions in multi-threaded environments:

Wrong:

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.

Right:

// 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);

• Use the null-conditional operator (?.) for raising the method instead of null-checking the delegate for subscribers in an if statement: EventName?.Invoke(SenderObject, new EventArgsT());
• When using Action<> to declare delegate types, the anonymous method / event handler signature must be the same as the declared anonymous delegate type in the event declaration.