EventHandler delegate pattern in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the EventHandler delegate in C#?

The EventHandler delegate defines a method signature for handling events. It allows methods to respond to events with a sender object and event data.

beginner

What are the parameters of the EventHandler delegate?

The EventHandler delegate has two parameters: object sender which is the source of the event, and EventArgs e which contains event data.

beginner

How do you subscribe to an event using the EventHandler delegate?

You subscribe by adding a method matching the EventHandler signature to the event using the += operator, for example: myObject.MyEvent += MyEventHandlerMethod;

beginner

What is the role of the sender parameter in an EventHandler method?

The sender parameter identifies the object that raised the event, allowing the event handler to know the source of the event.

intermediate

Why is it recommended to use EventHandler or EventHandler<T> delegates for events?

Using EventHandler or EventHandler<T> provides a standard, consistent pattern for events, making code easier to understand and maintain.

What is the correct signature of the EventHandler delegate?

A void Handler()

B int Handler(object sender)

C void Handler(object sender, EventArgs e)

D bool Handler(object sender, EventArgs e)

The EventHandler delegate expects a method that returns void and takes two parameters: an object sender and EventArgs e.

How do you raise an event using the EventHandler delegate safely?

A Check if the event is not null, then invoke it with sender and EventArgs

B Call the event directly without checking for null

C Assign null to the event before invoking

D Use a try-catch block without checking for null

You should check if the event is not null before invoking it to avoid NullReferenceException.

Which of these is a common use of EventHandler<T>?

A To handle events with no data

B To handle events with multiple senders

C To handle events asynchronously

D To handle events with custom event data

EventHandler<T> allows passing custom event data by specifying a type derived from EventArgs.

What does the += operator do in event subscription?

A Adds an event handler

B Removes an event handler

C Invokes an event handler

D Declares an event

The += operator adds a method to the event's invocation list.

What type must the second parameter of an EventHandler delegate be?

A string

B EventArgs or a subclass

C object

D int

The second parameter must be EventArgs or a type derived from EventArgs to carry event data.

Explain the EventHandler delegate pattern and how it is used to handle events in C#.

Think about how methods listen and respond to events using a standard signature.

Describe the steps to create, subscribe, and raise an event using the EventHandler delegate.

Imagine you want to notify other parts of your program when something happens.