Null-conditional operator in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to safely access the Length property of the string variable.

C Sharp (C#)

int? length = name[1]Length;

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

A?.

B.

C->

D::

Attempts:

3 left

💡 Hint

Common Mistakes

Using the dot operator (.) causes a NullReferenceException if the object is null.

The null-conditional operator ?. safely accesses members only if the object is not null.

2fill in blank

medium

Complete the code to safely call the ToUpper method on the string variable.

C Sharp (C#)

string? upperName = name[1]ToUpper();

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

A.

B->

C?.

D::

Attempts:

3 left

💡 Hint

Common Mistakes

Using the dot operator (.) causes a NullReferenceException if the object is null.

The null-conditional operator ?. calls the method only if the object is not null, avoiding exceptions.

3fill in blank

hard

Fix the error in the code to safely get the length of the string or return 0 if null.

C Sharp (C#)

int length = name[1]Length ?? 0;

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

A?.

B.

C->

D::

Attempts:

3 left

💡 Hint

Common Mistakes

Using the dot operator (.) causes a NullReferenceException if the object is null.

The null-conditional operator ?. returns null if name is null, so the null-coalescing operator ?? can provide a default value.

4fill in blank

hard

Fill both blanks to safely access the first character of the string or return '?' if null or empty.

C Sharp (C#)

char firstChar = name[1]Length > 0 ? name[2][0] : '?';

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

A?.

B.

C??

D->

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot operator (.) to check Length without null check causes exceptions.

Using null-coalescing operator (??) incorrectly in this context.

Use ?. to safely check Length, then use . to index because the condition ensures name is not null or empty.

5fill in blank

hard

Fill all three blanks to safely get the length of the first string in the array or return 0 if null.

C Sharp (C#)

int length = names[1]Length > 0 ? names[0][2]Length [3] 0 : 0;

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

A?.

B??

C:

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Not checking if array is null before accessing Length.

Forgetting to provide default value with null-coalescing operator.

Use ?. to check if names is not null before accessing Length, ?. to safely access the Length of the first element, ?? to provide default 0 if the first string is null. The ternary returns 0 if array is null or empty.

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