Null-conditional operator in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the null-conditional operator in C#?

The null-conditional operator is ?. in C#. It allows safe access to members or elements of an object that might be null, preventing exceptions by returning null instead.

beginner

How does ?. help prevent exceptions?

When you use ?., if the object before it is null, the whole expression returns null instead of throwing a NullReferenceException. This makes your code safer and cleaner.

beginner

Example: What does person?.Name do if person is null?

If person is null, person?.Name returns null instead of throwing an error. If person is not null, it returns the Name property.

intermediate

Can the null-conditional operator be used with methods? Give an example.

Yes, you can call a method safely using ?.. For example, person?.GetAge() calls GetAge() only if person is not null. Otherwise, it returns null.

intermediate

What happens when you use the null-conditional operator with an event, like MyEvent?.Invoke()?

Using ?. with events safely invokes the event only if it has subscribers (is not null). This prevents exceptions when no handlers are attached.

What does the null-conditional operator ?. do in C#?

A Safely accesses members of an object that might be null

B Throws an exception if the object is null

C Assigns null to an object

D Checks if an object is not null

The null-conditional operator ?. safely accesses members or methods of an object that might be null, returning null instead of throwing an exception.

What will person?.Name return if person is null?

A Default value of Name's type

B Empty string

C Throws NullReferenceException

D null

If person is null, person?.Name returns null safely without throwing an exception.

Which of these is a valid use of the null-conditional operator?

A person?.GetAge()

B person.GetAge()?

C person?GetAge()

D person.GetAge?.()

person?.GetAge() safely calls the method only if person is not null.

How does MyEvent?.Invoke() help when raising events?

A It always invokes the event

B It unsubscribes all handlers

C It invokes the event only if there are subscribers

D It throws an exception if no subscribers

Using ?. with events invokes the event only if it has subscribers, preventing exceptions.

What type of value does the null-conditional operator return when the object is null?

A Default value of the member

B null

C Empty string

D Zero

When the object is null, the null-conditional operator returns null instead of throwing an error.

Explain how the null-conditional operator ?. works and why it is useful in C#.

Think about how it helps avoid errors when objects might be missing.

Describe a scenario where using ?. with an event is beneficial.

Consider what happens if you try to raise an event with no listeners.

Practice

1. What does the null-conditional operator ?. do in C#?

easy

A. It safely accesses a member only if the object is not null, otherwise returns null.

B. It throws an exception if the object is null.

C. It converts a null object to an empty string.

D. It forces the object to be non-null.

Solution

1. Step 1: Understand the purpose of the null-conditional operator

   The operator ?. is used to access members of an object only if the object is not null.

2. Step 2: Behavior when object is null

   If the object is null, instead of throwing an exception, it returns null safely.

3. Final Answer:

   It safely accesses a member only if the object is not null, otherwise returns null. -> Option A

4. Quick Check:

   Null-conditional operator = safe member access [OK]

Hint: Remember: ?. returns null instead of error if object is null [OK]

Common Mistakes:

• Thinking it throws an exception on null
• Confusing it with null-coalescing operator
• Assuming it converts null to empty string

2. Which of the following is the correct syntax to safely access the Length property of a string variable text that might be null?

easy

A. int length = text.Length?;

B. int? length = text?.Length;

C. int length = text?.Length;

D. int? length = text.Length?.;

Solution

1. Step 1: Identify correct null-conditional syntax

   The null-conditional operator is ?. placed before the member name, like text?.Length.

2. Step 2: Match variable type with possible null

   Since text might be null, the result text?.Length is nullable, so use int? to hold it.

3. Final Answer:

   int? length = text?.Length; -> Option B

4. Quick Check:

   Correct syntax = int? length = text?.Length; [OK]

Hint: Use int? for nullable results with ?. operator [OK]

Common Mistakes:

• Placing ? after Length instead of before
• Using non-nullable int for nullable result
• Adding extra dots or question marks

3. What is the output of the following code?

string? name = null;
int? length = name?.Length;
Console.WriteLine(length == null ? "Null" : length.ToString());

medium

A. Null

B. Empty string

C. Throws NullReferenceException

D. 0

Solution

1. Step 1: Evaluate null-conditional operator on null string

   The variable name is null, so name?.Length returns null safely.

2. Step 2: Check output logic

   The code prints "Null" if length is null, otherwise prints the length. Since length is null, it prints "Null".

3. Final Answer:

   Null -> Option A

4. Quick Check:

   Null string length = null, prints "Null" [OK]

Hint: ?. returns null for null object, so output is "Null" [OK]

Common Mistakes:

• Expecting 0 instead of null
• Thinking it throws exception
• Confusing null with empty string

4. Identify the error in this code snippet:

Person? person = null;
int age = person?.Age;
Console.WriteLine(age);

medium

A. Missing semicolon after Console.WriteLine.

B. Null-conditional operator cannot be used on objects.

C. Cannot assign nullable int to non-nullable int without cast.

D. Person class must be non-nullable.

Solution

1. Step 1: Analyze the type of person?.Age

   The expression person?.Age returns int? (nullable int) because person might be null.

2. Step 2: Check assignment compatibility

   Variable age is declared as int (non-nullable), so assigning int? to int without conversion causes a compile error.

3. Final Answer:

   Cannot assign nullable int to non-nullable int without cast. -> Option C

4. Quick Check:

   Nullable to non-nullable assignment needs cast [OK]

Hint: Nullable result needs nullable variable or cast [OK]

Common Mistakes:

• Ignoring nullable type returned by ?. operator
• Assuming no error on assignment
• Thinking null-conditional operator is invalid here

5. Given a list of Person objects where some may be null, how can you safely get a List<int> of their ages (with 0 for null persons) using the null-conditional operator?

List<Person?> people = new() { new Person(30), null, new Person(25) };
var ages = ???;

Choose the correct code to replace ???.

hard

A. people.Select(p => p?.Age).ToList();

B. people.Select(p => p.Age).ToList();

C. people.Select(p => p.Age ?? 0).ToList();

D. people.Select(p => p?.Age ?? 0).ToList();

Solution

1. Step 1: Understand the list and nulls

   The list contains Person? objects, some are null, so accessing Age needs safe access.

2. Step 2: Use null-conditional with fallback

   p?.Age safely accesses Age or returns null; using ?? 0 replaces null ages with 0 to avoid nullable list.

3. Step 3: Check other options

   people.Select(p => p?.Age).ToList(); returns nullable ints, people.Select(p => p.Age).ToList(); throws exception on null, people.Select(p => p.Age ?? 0).ToList(); is invalid because p can be null.

4. Final Answer:

   people.Select(p => p?.Age ?? 0).ToList(); -> Option D

5. Quick Check:

   Safe access with fallback = people.Select(p => p?.Age ?? 0).ToList(); [OK]

Hint: Use ?. with ?? to handle nulls and provide default [OK]

Common Mistakes:

• Not handling null Person objects
• Forgetting to provide fallback value
• Assuming Select handles nulls automatically