Null-conditional operator in C Sharp (C#) - Mini Project: Build & Apply

Using the Null-conditional Operator in C#

📖 Scenario: Imagine you are building a simple contact list app. Each contact may or may not have a phone number. You want to safely check if a contact has a phone number without causing errors if the phone number is missing.

🎯 Goal: Learn how to use the null-conditional operator (?.) in C# to safely access properties that might be null.

📋 What You'll Learn

Create a class called Contact with a string property PhoneNumber that can be null.

Create a variable called contact and assign it a new Contact object with no phone number.

Use the null-conditional operator ?. to safely get the length of the phone number string.

Print the length or null if the phone number is missing.

💡 Why This Matters

🌍 Real World

In real apps, data can be missing or optional. The null-conditional operator helps avoid errors when accessing such data.

💼 Career

Understanding null safety is important for writing robust C# code, especially when working with APIs or databases where data might be missing.

1

Create the Contact class and a contact with no phone number

Create a class called Contact with a public string property PhoneNumber that can be null. Then create a variable called contact and assign it a new Contact object with PhoneNumber set to null.

C Sharp (C#)

// Define the Contact class and create a contact with no phone number
// Your code here

Hint

Use string? to allow PhoneNumber to be null.

2

Create a variable to hold the phone number length safely

Create an integer variable called phoneLength and assign it the length of contact.PhoneNumber using the null-conditional operator ?.. Use the null-coalescing operator ?? to assign 0 if the phone number is null.

C Sharp (C#)

public class Contact
{
    public string? PhoneNumber { get; set; }
}

Contact contact = new Contact { PhoneNumber = null };

// Create phoneLength using null-conditional and null-coalescing operators
// Your code here

Hint

Use contact.PhoneNumber?.Length to get length safely, then ?? 0 to default to zero.

3

Change the contact to have a phone number

Change the contact variable so that PhoneNumber is set to the string "123-4567" instead of null.

C Sharp (C#)

public class Contact
{
    public string? PhoneNumber { get; set; }
}

// Change contact to have a phone number
Contact contact = new Contact { PhoneNumber = "123-4567" };

int phoneLength = contact.PhoneNumber?.Length ?? 0;

Hint

Assign the string "123-4567" to PhoneNumber.

4

Print the phone number length

Write a Console.WriteLine statement to print the value of phoneLength.

C Sharp (C#)

public class Contact
{
    public string? PhoneNumber { get; set; }
}

Contact contact = new Contact { PhoneNumber = "123-4567" };

int phoneLength = contact.PhoneNumber?.Length ?? 0;

// Print phoneLength
// Your code here

Hint

Use Console.WriteLine(phoneLength); to show the length.

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