Custom event arguments in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a custom event argument class inheriting from EventArgs.

C Sharp (C#)

public class MyEventArgs : [1] {
    public int Value { get; set; }
}

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

AEventArgs

BEventHandler

CEvent

DEventArgsBase

Attempts:

3 left

💡 Hint

Common Mistakes

Using EventHandler instead of EventArgs as base class.

Trying to inherit from a non-existent class like EventArgsBase.

Custom event argument classes should inherit from EventArgs to be used in events.

2fill in blank

medium

Complete the code to declare an event using the custom event argument class.

C Sharp (C#)

public event EventHandler<[1]> OnValueChanged;

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

AEventArgs

BMyEventArgs

CValueChangedArgs

DCustomArgs

Attempts:

3 left

💡 Hint

Common Mistakes

Using the base EventArgs instead of the custom class.

Using a class name that was not declared.

The event uses the custom event argument class MyEventArgs as the generic parameter.

3fill in blank

hard

Fix the error in the event invocation by completing the code.

C Sharp (C#)

OnValueChanged?.Invoke(this, new [1] { Value = 10 });

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

AEventArgs

BEventHandler

CMyEventArgs

DCustomEventArgs

Attempts:

3 left

💡 Hint

Common Mistakes

Using EventArgs instead of MyEventArgs when invoking the event.

Trying to invoke the event with EventHandler instead of event args.

The event must be invoked with an instance of the custom event argument class MyEventArgs.

4fill in blank

hard

Fill both blanks to complete the event handler method signature.

C Sharp (C#)

private void [1](object [2], MyEventArgs e) {
    Console.WriteLine($"Value changed to {e.Value}");
}

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

AOnValueChangedHandler

Bsender

Cargs

DHandleValueChange

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid method names.

Using incorrect parameter names that do not match event handler conventions.

The method name can be any valid identifier, here OnValueChangedHandler, and the sender parameter is conventionally named sender.

5fill in blank

hard

Fill all three blanks to subscribe to the event and trigger it.

C Sharp (C#)

MyClass obj = new MyClass();
obj.[1] += [2];
obj.TriggerEvent();

// Event handler method
private void [3](object sender, MyEventArgs e) {
    Console.WriteLine($"New value: {e.Value}");
}

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

AOnValueChanged

BOnValueChangedHandler

DValueChanged

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching event and handler names.

Using incorrect event names that do not exist.

Subscribe to the event OnValueChanged with the handler OnValueChangedHandler, which is defined below.

Practice

1. What is the main purpose of creating a custom class that inherits from EventArgs in C#?

easy

A. To replace the EventHandler delegate entirely.

B. To prevent events from being raised multiple times.

C. To send additional data with an event when it is raised.

D. To automatically handle exceptions in event handlers.

Solution

1. Step 1: Understand the role of EventArgs

   The EventArgs class is used to pass data with events. Creating a custom class inheriting from it allows adding extra information.

2. Step 2: Identify the purpose of custom event arguments

   Custom event argument classes let you send specific data related to the event, making event communication clearer.

3. Final Answer:

   To send additional data with an event when it is raised. -> Option C

4. Quick Check:

   Custom EventArgs = Extra event data [OK]

Hint: Custom EventArgs carry extra info with events [OK]

Common Mistakes:

• Thinking custom EventArgs replace EventHandler delegate
• Believing custom EventArgs stop event repetition
• Assuming custom EventArgs handle exceptions automatically

2. Which of the following is the correct way to declare a custom event argument class named MyEventArgs with a string property Message?

easy

A. public class MyEventArgs : EventArgs { public int Message { get; set; } }

B. public class MyEventArgs { public string Message { get; set; } }

C. public class MyEventArgs : EventHandler { public string Message; }

D. public class MyEventArgs : EventArgs { public string Message { get; set; } }

Solution

1. Step 1: Check inheritance from EventArgs

   The custom event argument class must inherit from EventArgs to be used properly in events.

2. Step 2: Verify property type and syntax

   The property Message should be a string with get and set accessors for data storage.

3. Final Answer:

   public class MyEventArgs : EventArgs { public string Message { get; set; } } -> Option D

4. Quick Check:

   Custom EventArgs inherit EventArgs + correct property type [OK]

Hint: Custom EventArgs must inherit EventArgs and use correct property types [OK]

Common Mistakes:

• Forgetting to inherit from EventArgs
• Using EventHandler as base class instead of EventArgs
• Declaring property with wrong type

3. Consider the following code snippet:

public class MyEventArgs : EventArgs { public int Number { get; set; } }

public class Publisher {
    public event EventHandler<MyEventArgs> OnNumberChanged;
    public void ChangeNumber(int num) {
        OnNumberChanged?.Invoke(this, new MyEventArgs { Number = num });
    }
}

public class Subscriber {
    public void Subscribe(Publisher pub) {
        pub.OnNumberChanged += HandleNumberChanged;
    }
    private void HandleNumberChanged(object sender, MyEventArgs e) {
        Console.WriteLine($"Number is {e.Number}");
    }
}

// Main method
var pub = new Publisher();
var sub = new Subscriber();
sub.Subscribe(pub);
pub.ChangeNumber(42);

What will be the output when this code runs?

medium

A. Number is 42

B. Number is 0

C. No output, event not raised

D. Compilation error due to event handler signature

Solution

1. Step 1: Understand event raising and subscription

   The Publisher raises OnNumberChanged event with MyEventArgs containing the number passed.

2. Step 2: Check subscriber's event handler output

   The subscriber prints the Number property from the event args, which is set to 42.

3. Final Answer:

   Number is 42 -> Option A

4. Quick Check:

   Event raised with Number=42 prints "Number is 42" [OK]

Hint: Event args data prints exactly as passed in event [OK]

Common Mistakes:

• Assuming event is not raised or subscribed
• Confusing default values with passed values
• Mistaking event handler signature causing errors

4. The following code attempts to raise a custom event with arguments but causes a runtime error. What is the most likely cause?

public class MyEventArgs : EventArgs { public string Info { get; set; } }

public class Publisher {
    public event EventHandler<MyEventArgs> OnInfoChanged;
    public void RaiseEvent() {
        OnInfoChanged(this, new MyEventArgs { Info = "Hello" });
    }
}

// Usage
var pub = new Publisher();
pub.RaiseEvent();

medium

A. OnInfoChanged event is null because no subscribers are attached.

B. MyEventArgs does not inherit from EventArgs.

C. Event handler signature does not match the event declaration.

D. The Info property is not initialized before raising the event.

Solution

1. Step 1: Check event invocation safety

   The event OnInfoChanged is invoked without checking if it has subscribers, so if null, it causes a runtime NullReferenceException.

2. Step 2: Identify missing subscriber check

   Before invoking, the code should check if OnInfoChanged is not null to avoid errors.

3. Final Answer:

   OnInfoChanged event is null because no subscribers are attached. -> Option A

4. Quick Check:

   Invoke event without subscribers = NullReferenceException [OK]

Hint: Always check event is not null before invoking [OK]

Common Mistakes:

• Forgetting to check event for null before invoking
• Assuming event has subscribers by default
• Confusing property initialization with event subscription

5. You want to create a custom event argument class ProgressEventArgs that includes a percentage (int) and a message (string). You also want to raise an event ProgressChanged in a class Downloader that sends this data. Which of the following code snippets correctly implements this scenario?

hard

A. public class ProgressEventArgs {
public int Percentage { get; set; }
public string Message { get; set; }
}

public class Downloader {
public event EventHandler ProgressChanged;
public void ReportProgress(int percent, string msg) {
ProgressChanged?.Invoke(this, new ProgressEventArgs { Percentage = percent, Message = msg });
}
}

B. public class ProgressEventArgs : EventArgs {
public int Percentage { get; }
public string Message { get; }
public ProgressEventArgs(int percentage, string message) {
Percentage = percentage;
Message = message;
}
}

public class Downloader {
public event EventHandler<ProgressEventArgs> ProgressChanged;
public void ReportProgress(int percent, string msg) {
ProgressChanged?.Invoke(this, new ProgressEventArgs(percent, msg));
}
}

C. public class ProgressEventArgs : EventArgs {
public int Percentage;
public string Message;
}

public class Downloader {
public event EventHandler<ProgressEventArgs> ProgressChanged;
public void ReportProgress(int percent, string msg) {
ProgressChanged(this, new ProgressEventArgs { Percentage = percent, Message = msg });
}
}

D. public class ProgressEventArgs : EventArgs {
public int Percentage { get; set; }
public string Message { get; set; }
}

public class Downloader {
public event EventHandler<ProgressEventArgs> ProgressChanged;
public void ReportProgress(int percent, string msg) {
ProgressChanged(this, new ProgressEventArgs { Percentage = percent, Message = msg });
}
}

Solution

1. Step 1: Define immutable custom event args with constructor

   public class ProgressEventArgs : EventArgs { public int Percentage { get; } public string Message { get; } public ProgressEventArgs(int percentage, string message) { Percentage = percentage; Message = message; } } public class Downloader { public event EventHandler<ProgressEventArgs> ProgressChanged; public void ReportProgress(int percent, string msg) { ProgressChanged?.Invoke(this, new ProgressEventArgs(percent, msg)); } } defines ProgressEventArgs inheriting from EventArgs with readonly properties set via constructor, which is a good practice.

2. Step 2: Raise event safely with null check

   public class Downloader { public event EventHandler<ProgressEventArgs> ProgressChanged; public void ReportProgress(int percent, string msg) { ProgressChanged?.Invoke(this, new ProgressEventArgs(percent, msg)); } } uses null-conditional operator ?. to safely raise the event with the new event args instance.

3. Final Answer:

   public class ProgressEventArgs : EventArgs { public int Percentage { get; } public string Message { get; } public ProgressEventArgs(int percentage, string message) { Percentage = percentage; Message = message; } } public class Downloader { public event EventHandler<ProgressEventArgs> ProgressChanged; public void ReportProgress(int percent, string msg) { ProgressChanged?.Invoke(this, new ProgressEventArgs(percent, msg)); } } -> Option B

4. Quick Check:

   Immutable EventArgs via ctor + safe ?.Invoke [OK]

Hint: Use constructor for EventArgs properties and null-check event before invoke [OK]

Common Mistakes:

• Not inheriting from EventArgs
• Using wrong event handler delegate type
• Invoking event without null check
• Using public fields instead of properties