Raising events safely in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an event named OnChange.

C Sharp (C#)

public event EventHandler [1];

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

AEventChanged

BChanged

CChangeEvent

DOnChange

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different event name than declared.

Forgetting to declare the event type.

The event name is OnChange, so the blank should be filled with OnChange.

2fill in blank

medium

Complete the code to safely raise the event OnChange using a local copy.

C Sharp (C#)

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

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

AOnChange

BHandler

ConChange

DChangeEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than the event name.

Not checking for null before invoking the event.

To safely raise the event, we copy OnChange to a local variable handler and check if it is not null.

3fill in blank

hard

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

C Sharp (C#)

OnChange[1](this, EventArgs.Empty);

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A.Invoke

B()

C.Call

D.Raise

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call the event like a method without .Invoke.

Using non-existent methods like .Raise or .Call.

In C#, to invoke an event delegate safely, use the .Invoke method.

4fill in blank

hard

Fill both blanks to create a method RaiseOnChange that safely raises the event.

C Sharp (C#)

protected virtual void RaiseOnChange()
{
    var handler = [1];
    if (handler [2] null)
        handler(this, EventArgs.Empty);
}

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

AOnChange

B==

C!=

DOnChanged

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of != in the if condition.

Copying the wrong event name.

We copy the event OnChange to handler and check if it is not equal (!=) to null before invoking.

5fill in blank

hard

Fill all three blanks to raise the event with a custom EventArgs subclass MyEventArgs.

C Sharp (C#)

protected virtual void RaiseOnChange(MyEventArgs e)
{
    var handler = [1];
    if (handler != null)
        handler([2], [3]);
}

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

AOnChange

Bthis

Ce

DEventArgs.Empty

Attempts:

3 left

💡 Hint

Common Mistakes

Passing EventArgs.Empty instead of the custom e.

Not checking if the event is null before invoking.

We copy the event OnChange to handler, check for null, then invoke it passing this and the custom event args e.