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'.