Chapter 89: Events

EventArguments An instance of the EventArgsT type that contains the event parameters.

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.

Section 89.1: Declaring and Raising Events

Declaring an Event

You can declare an event on any class or struct using the following syntax:

public class MyClass

{

//Declares the event for MyClass public event EventHandler MyEvent;

//Raises the MyEvent event

public void RaiseEvent()

{

OnMyEvent();

}

}

There is an expanded syntax for declaring events, where you hold a private instance of the event, and define a public instance using add and set accessors. The syntax is very similar to C# properties. In all cases, the syntax demonstrated above should be preferred, because the compiler emits code to help ensure that multiple threads can safely add and remove event handlers to the event on your class.

Raising the Event

Version ≥ 6.0

private void OnMyEvent()

{

EventName?.Invoke(this, EventArgs.Empty);

}

Version < 6.0

private void OnMyEvent()

{

//Use a local for EventName, because another thread can modify the

//public EventName between when we check it for null, and when we

//raise the event.

var eventName = EventName;

safety).

Section 89.3: Creating cancelable event

A cancelable event can be raised by a class when it is about to perform an action that can be canceled, such as the

FormClosing event of a Form.

To create such event:

Create a new event arg deriving from CancelEventArgs and add additional properties for event data.

Create an event using EventHandler<T> and use the new cancel event arg class which you created.

Example

In the below example, we create a PriceChangingEventArgs event for Price property of a class. The event data class contains a Value which let the consumer know about the new . The event raises when you assign a new value to Price property and lets the consumer know the value is changing and let them to cancel the event. If the consumer cancels the event, the previous value for Price will be used:

PriceChangingEventArgs

public class PriceChangingEventArgs : CancelEventArgs

{

int value; public int Value

{

get { return value; }

}

public PriceChangingEventArgs(int value)

{

this.value = value;

}

}

Product

public class Product

{

int price; public int Price

{

get { return price; } set

{

var e = new PriceChangingEventArgs(value); OnPriceChanging(e);

if (!e.Cancel) price = value;

}

}

public event EventHandler<PriceChangingEventArgs> PropertyChanging; protected void OnPriceChanging(PriceChangingEventArgs e)

{

var handler = PropertyChanging; if (handler != null)

PropertyChanging(this, e);

}