EventHandler delegate pattern in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an event using the EventHandler delegate.

C Sharp (C#)

public class Button {
    public event [1] Click;
}

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

AFunc<int>

BAction

CEventHandler

DEventArgs

Attempts:

3 left

💡 Hint

Common Mistakes

Using Action or Func instead of EventHandler for events.

Using EventArgs as the delegate type.

The EventHandler delegate is used to declare events that follow the standard pattern with sender and EventArgs parameters.

2fill in blank

medium

Complete the code to raise the Click event safely.

C Sharp (C#)

protected virtual void OnClick() {
    [1]?.Invoke(this, EventArgs.Empty);
}

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AClick

BClickEvent

COnClick

DRaiseClick

Attempts:

3 left

💡 Hint

Common Mistakes

Calling OnClick recursively instead of invoking the event.

Using a wrong event name.

The event named Click is invoked safely using the null-conditional operator to check if there are subscribers.

3fill in blank

hard

Fix the error in the event subscription code.

C Sharp (C#)

Button btn = new Button();
btn.Click += [1];

void Btn_Click(object sender, EventArgs e) {
    Console.WriteLine("Button clicked");
}

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ABtn_Click()

BBtn_Click(sender, e)

CBtn_Click(object, EventArgs)

DBtn_Click

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses when subscribing to the event.

Trying to pass parameters in the subscription.

When subscribing to an event, use the method name without parentheses or parameters.

4fill in blank

hard

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

C Sharp (C#)

public class Alarm {
    public event [1] AlarmRaised;

    public void RaiseAlarm() {
        AlarmRaised[2].Invoke(this, EventArgs.Empty);
    }
}

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AEventHandler

BEventHandler<TEventArgs>

CClick

D?

Attempts:

3 left

💡 Hint

Common Mistakes

Using EventHandler without a custom EventArgs class.

Omitting the null check operator when raising the event.

The event is declared with EventHandler and raised safely using the null-conditional operator ?.

5fill in blank

hard

Fill all three blanks to create a dictionary of event handlers and invoke one safely.

C Sharp (C#)

var handlers = new Dictionary<string, [1]>();
handlers["start"] = (sender, e) => Console.WriteLine("Started");

if (handlers.TryGetValue("start", out var handler)) {
    handler[2].Invoke([3], EventArgs.Empty);
}

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AEventHandler

B?

Cthis

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect delegate types in the dictionary.

Not using the null-conditional operator when invoking.

The dictionary stores EventHandler delegates. The handler is invoked safely using the null-conditional operator, and null is passed as sender.