C# Language 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
| Parameter | Details |
|---|---|
| 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 aNullReferenceException.
6.0
- 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);
6.0
- Use the null-conditional operator (?.) for raising the method instead of null-checking the delegate for subscribers in an
ifstatement: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.
