Event-driven design pattern in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an event in C#.

C Sharp (C#)

public event EventHandler [1];

Drag options to blanks, or click blank then click option'

AOnClick

BClicked

CClick

DClickEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using method-like names such as 'OnClick' instead of event names.

Using past tense like 'Clicked' which is less common for event names.

The event name is typically a noun or noun phrase like 'Click'. Here, 'Click' is the correct event name.

2fill in blank

medium

Complete the code to subscribe to an event named 'DataReceived'.

C Sharp (C#)

someObject.DataReceived [1]= HandlerMethod;

Drag options to blanks, or click blank then click option'

A+=

B==

C+

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which overwrites existing handlers.

Using '+' which is invalid syntax for event subscription.

In C#, you subscribe to an event using the '+=' operator.

3fill in blank

hard

Fix the error in the event invocation code.

C Sharp (C#)

if ([1] != null)
{
    [1](this, EventArgs.Empty);
}

Drag options to blanks, or click blank then click option'

ADataReceived

BDataReceived()

CDataReceived?.Invoke

DDataReceived.Invoke

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses in the null check condition.

Using the null-conditional operator '?.Invoke' inside the if condition.

Before invoking an event, check if it is not null, then call it like a method with parameters.

4fill in blank

hard

Fill both blanks to declare and raise a custom event with EventHandler.

C Sharp (C#)

public event EventHandler [1];

protected virtual void On[2](EventArgs e)
{
    [1]?.Invoke(this, e);
}

Drag options to blanks, or click blank then click option'

ADataProcessed

BDataProcessedEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for event and method.

Adding 'Event' suffix inconsistently.

The event name and the method name usually share the same base name. The event is 'DataProcessed' and the method is 'OnDataProcessed'.

5fill in blank

hard

Fill all three blanks to create a custom event with a delegate and raise it.

C Sharp (C#)

public delegate void [1](object sender, EventArgs e);

public event [1] [2];

protected virtual void On[2](EventArgs e)
{
    [2]?.Invoke(this, e);
}

Drag options to blanks, or click blank then click option'

ADataChangedHandler

BDataChanged

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing delegate and event names.

Not matching the method name with the event name.

The delegate is named 'DataChangedHandler', the event uses that delegate type, and the event name is 'DataChanged'. The method raising the event is 'OnDataChanged'.

Practice

1. What is the main role of an event subscriber in the event-driven design pattern?

easy

A. To manage the program's user interface layout

B. To listen for events and respond when they occur

C. To store data for the program's use

D. To create and send events to other parts of the program

Solution

1. Step 1: Understand the roles in event-driven design

   In event-driven design, publishers send events and subscribers listen for these events.

2. Step 2: Identify subscriber's responsibility

   The subscriber's job is to react or respond when an event happens, meaning it listens and acts accordingly.

3. Final Answer:

   To listen for events and respond when they occur -> Option B

4. Quick Check:

   Subscriber listens and reacts = D [OK]

Hint: Subscribers always listen and react to events [OK]

Common Mistakes:

• Confusing subscriber with publisher
• Thinking subscriber creates events
• Assuming subscriber manages UI layout

2. Which of the following is the correct syntax to declare an event in C#?

easy

A. public void event MyEvent();

B. event public MyEvent EventHandler;

C. public event EventHandler MyEvent;

D. EventHandler public event MyEvent;

Solution

1. Step 1: Recall event declaration syntax in C#

   Events are declared with the keyword 'event' followed by a delegate type, then the event name.

2. Step 2: Check each option

   public event EventHandler MyEvent; matches the correct syntax: 'public event EventHandler MyEvent;'. Others have wrong order or keywords.

3. Final Answer:

   public event EventHandler MyEvent; -> Option C

4. Quick Check:

   Correct event declaration = B [OK]

Hint: Event declaration: 'public event DelegateType EventName;' [OK]

Common Mistakes:

• Mixing method and event syntax
• Wrong keyword order
• Missing 'event' keyword

3. What will be the output of the following C# code?

using System;
class Program {
  public static event Action OnNotify;
  static void Main() {
    OnNotify += () => Console.WriteLine("Event triggered");
    OnNotify?.Invoke();
  }
}

medium

A. Event triggered

B. No output

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand event subscription and invocation

   The event OnNotify is subscribed with a lambda that prints "Event triggered". The event is then invoked safely with ?.Invoke().

2. Step 2: Predict output

   Since OnNotify has a subscriber, invoking it calls the lambda, printing "Event triggered" to the console.

3. Final Answer:

   Event triggered -> Option A

4. Quick Check:

   Subscribed event invoked prints message = A [OK]

Hint: Subscribed event invocation prints output [OK]

Common Mistakes:

• Ignoring null-conditional operator ?.Invoke()
• Expecting no output without subscription
• Confusing event with method call syntax

4. Identify the error in this C# event-driven code snippet:

public class Publisher {
  public event EventHandler OnChange;
  public void Raise() {
    OnChange();
  }
}

medium

A. OnChange event is invoked without checking for null

B. EventHandler delegate is not declared

C. Raise method should be static

D. Event should be private

Solution

1. Step 1: Review event invocation rules

   Events can be null if no subscribers exist. Invoking without null check causes runtime error.

2. Step 2: Identify the error in code

   OnChange() is called directly without checking if OnChange is null, risking NullReferenceException.

3. Final Answer:

   OnChange event is invoked without checking for null -> Option A

4. Quick Check:

   Invoke event safely with null check = C [OK]

Hint: Always check event for null before invoking [OK]

Common Mistakes:

• Calling event without null check
• Assuming event is never null
• Misunderstanding event access modifiers

5. You want to design a system where multiple subscribers react differently to a button click event. Which approach best uses the event-driven pattern in C# to achieve this?

hard

A. Make subscribers poll the button state continuously in a loop

B. Create a list of methods and call them manually without events

C. Use a static method to handle all button clicks centrally

D. Declare a public event, let subscribers add their handlers, and invoke event on button click

Solution

1. Step 1: Understand event-driven design for multiple subscribers

   Events allow multiple subscribers to register handlers that get called automatically when the event fires.

2. Step 2: Evaluate options for best practice

   Declare a public event, let subscribers add their handlers, and invoke event on button click uses a public event and lets subscribers add handlers, which is the standard event-driven approach. Others are less efficient or not event-driven.

3. Final Answer:

   Declare a public event, let subscribers add their handlers, and invoke event on button click -> Option D

4. Quick Check:

   Use events for multiple subscriber reactions = A [OK]

Hint: Use events to notify multiple subscribers automatically [OK]

Common Mistakes:

• Manually calling subscriber methods instead of using events
• Using polling instead of event notifications
• Centralizing all logic in static methods