Null-conditional operator in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of null-conditional operator with null object

What is the output of this C# code snippet using the null-conditional operator?

C Sharp (C#)

class Program {
    static void Main() {
        string? text = null;
        int? length = text?.Length;
        System.Console.WriteLine(length == null ? "null" : length.ToString());
    }
}

A null

B Throws NullReferenceException

C 0

D Empty string

💡 Hint

The null-conditional operator returns null if the object is null instead of throwing an exception.

Since 'text' is null, 'text?.Length' evaluates to null, so the output is 'null'. No exception is thrown.

❓ Predict Output

intermediate

Null-conditional operator with method invocation

What will this C# program print?

C Sharp (C#)

class Program {
    static string? GetName(bool returnNull) {
        if (returnNull) return null;
        return "Alice";
    }
    static void Main() {
        string? name = GetName(true);
        int? length = name?.Length;
        System.Console.WriteLine(length ?? -1);
    }
}

A Throws NullReferenceException

B 5

C null

D -1

💡 Hint

The null-coalescing operator '??' provides a default value if the left side is null.

GetName(true) returns null, so 'name?.Length' is null. The '?? -1' returns -1 as default.

🔧 Debug

advanced

Identify the error with null-conditional operator usage

What error does this C# code produce?

C Sharp (C#)

class Program {
    static void Main() {
        string? text = null;
        int length = text?.Length;
        System.Console.WriteLine(length);
    }
}

A NullReferenceException at runtime

B Syntax error: missing semicolon

C Cannot implicitly convert type 'int?' to 'int'

D No error, prints 0

💡 Hint

The null-conditional operator returns a nullable type, but the variable is non-nullable.

The expression 'text?.Length' returns 'int?', but 'length' is declared as 'int'. This causes a compile-time error.

🧠 Conceptual

advanced

Behavior of null-conditional operator with events

Consider this C# code snippet. What is the purpose of using the null-conditional operator when invoking the event?

C Sharp (C#)

public class Publisher {
    public event System.Action? OnChange;
    public void Raise() {
        OnChange?.Invoke();
    }
}

A It unsubscribes all event handlers before invoking.

B It prevents a NullReferenceException if no subscribers are attached to the event.

C It forces the event to always have at least one subscriber.

D It throws an exception if the event has no subscribers.

💡 Hint

Events can be null if no methods are subscribed.

Using 'OnChange?.Invoke()' safely calls the event only if it has subscribers, avoiding exceptions.

❓ Predict Output

expert

Output with chained null-conditional operators and null-coalescing

What is the output of this C# program?

C Sharp (C#)

class Person {
    public string? Name { get; set; }
    public Person? Friend { get; set; }
}
class Program {
    static void Main() {
        Person? p = new Person { Name = "Bob", Friend = null };
        string friendName = p?.Friend?.Name ?? "No friend";
        System.Console.WriteLine(friendName);
    }
}

A No friend

B Bob

C null

D Throws NullReferenceException

💡 Hint

The null-conditional operator returns null if any part of the chain is null, then the null-coalescing operator provides a default.

Since 'p.Friend' is null, 'p?.Friend?.Name' is null, so the output is the default string 'No friend'.

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