Nullable value types in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Nullable Int Addition

What is the output of this C# code snippet?

C Sharp (C#)

int? a = 5;
int? b = null;
var result = a + b;
Console.WriteLine(result == null ? "null" : result.ToString());

A 0

B null

C 5

D Compilation error

💡 Hint

Adding a nullable int with null results in null.

When you add a nullable int with a null value, the result is null because the operation propagates null.

🧠 Conceptual

intermediate

Nullable Boolean Default Value

What is the default value of a nullable bool declared as bool? flag; before any assignment?

A null

B false

C true

D Compilation error

💡 Hint

Nullable types default to null if not assigned.

Nullable value types default to null when declared but not assigned any value.

🔧 Debug

advanced

Fix the NullReferenceException

This code throws a NullReferenceException. Which option fixes it?

C Sharp (C#)

int? x = null;
int y = x.Value;
Console.WriteLine(y);

A int y = x.GetValueOrDefault();

B int y = (int)x;

C int y = x ?? 0;

D int y = x.Value ?? 0;

💡 Hint

Use the null-coalescing operator to provide a default value.

Accessing Value on a null nullable throws an exception. Using 'x ?? 0' safely provides 0 if x is null.

📝 Syntax

advanced

Identify the Syntax Error

Which option contains a syntax error when declaring a nullable int?

A int? number = 10;

B Nullable<int> number = null;

C int? number = null;

D int? number = ?5;

💡 Hint

Check the placement of the question mark.

The question mark must come after the type, not before the value.

🚀 Application

expert

Count Non-Null Nullable Integers

Given this list of nullable integers, what is the count of non-null values?

List numbers = new() { 1, null, 3, null, 5 };

C Sharp (C#)

List<int?> numbers = new() { 1, null, 3, null, 5 };
int count = numbers.Count(n => n.HasValue);
Console.WriteLine(count);

A 3

B 2

C 5

D Compilation error

💡 Hint

Use the HasValue property to check for non-null values.

The list has three non-null values: 1, 3, and 5. Counting those gives 3.

Practice

1. What does a nullable value type in C# allow you to do?

easy

A. Store only non-null values

B. Store values only in reference types

C. Store multiple values at once

D. Store a value or no value (null)

Solution

1. Step 1: Understand nullable value types

   Nullable value types allow a value type to hold either a normal value or no value (null).

2. Step 2: Compare options

   Only Store a value or no value (null) correctly describes this behavior; others are incorrect because value types normally cannot be null.

3. Final Answer:

   Store a value or no value (null) -> Option D

4. Quick Check:

   Nullable value type = value or null [OK]

Hint: Nullable means value or null, not multiple values [OK]

Common Mistakes:

• Thinking nullable stores multiple values
• Confusing nullable with reference types
• Assuming nullable cannot hold null

2. Which of the following is the correct syntax to declare a nullable integer in C#?

easy

A. int number?;

B. int? number;

C. nullable int number;

D. Nullable number int;

Solution

1. Step 1: Recall nullable syntax

   In C#, you add a question mark after the value type to make it nullable, like int?.

2. Step 2: Check each option

   Only int? number; uses the correct syntax int? number;. Others are invalid syntax.

3. Final Answer:

   int? number; -> Option B

4. Quick Check:

   Nullable int syntax = int? [OK]

Hint: Use ? right after type for nullable, like int? [OK]

Common Mistakes:

• Placing ? after variable name
• Using 'nullable' keyword incorrectly
• Mixing order of type and nullable

3. What is the output of this C# code?

int? x = null;
int y = x ?? 5;
Console.WriteLine(y);

medium

A. 0

B. null

C. 5

D. Compilation error

Solution

1. Step 1: Understand the null-coalescing operator

   The operator ?? returns the left value if not null; otherwise, it returns the right value.

2. Step 2: Apply to given code

   Since x is null, y gets the value 5, so Console.WriteLine(y); prints 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   x is null, so y = 5 [OK]

Hint: ?? returns right value if left is null [OK]

Common Mistakes:

• Assuming null prints as 0
• Confusing ?? with || operator
• Expecting compilation error for null assignment

4. Identify the error in this code snippet:

int? a = 10;
int b = a;
Console.WriteLine(b);

medium

A. Cannot assign nullable int to int without checking for null

B. Variable 'a' must be initialized

C. Console.WriteLine cannot print int values

D. No error, code runs fine

Solution

1. Step 1: Check assignment from nullable to non-nullable

   Assigning a nullable int directly to an int variable without checking for null causes a compile error.

2. Step 2: Explain why error occurs

   Because a might be null, C# requires explicit null check or use of .Value or null-coalescing operator.

3. Final Answer:

   Cannot assign nullable int to int without checking for null -> Option A

4. Quick Check:

   Nullable to int needs null check [OK]

Hint: Check nullable has value before assigning to non-nullable [OK]

Common Mistakes:

• Ignoring null check before assignment
• Assuming implicit conversion works
• Thinking Console.WriteLine causes error

5. Given a nullable double double? d = null;, which code safely assigns its value to a non-nullable double result with default 0.0 if d is null?

hard

A. double result = d ?? 0.0;

B. double result = d.Value;

C. double result = (double)d;

D. double result = d != null ? d : null;

Solution

1. Step 1: Understand safe assignment from nullable

   To assign a nullable to non-nullable safely, provide a default value if null using ??.

2. Step 2: Evaluate each option

   double result = d.Value; throws exception if d is null. double result = d ?? 0.0; uses ?? to assign 0.0 if null, which is safe. double result = (double)d; casts but throws if null. double result = d != null ? d : null; tries to assign null to non-nullable, invalid.

3. Final Answer:

   double result = d ?? 0.0; -> Option A

4. Quick Check:

   Use ?? to provide default for nullable [OK]

Hint: Use ?? operator to assign default if nullable is null [OK]

Common Mistakes:

• Using .Value without null check
• Casting nullable directly without check
• Assigning null to non-nullable variable