Nullable value types in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a nullable integer variable.

C Sharp (C#)

int[1] number = null;

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

A!

B?

C*

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!' instead of '?' to declare nullable types.

Trying to assign null to a non-nullable value type.

In C#, adding a ? after a value type makes it nullable.

2fill in blank

medium

Complete the code to check if a nullable integer has a value.

C Sharp (C#)

if (number[1]) {
    Console.WriteLine("Has value");
}

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

A.HasValue

B== null

C!= null

D.Value

Attempts:

3 left

💡 Hint

Common Mistakes

Using '== null' to check for value presence instead of 'HasValue'.

Using '.Value' without checking if the value exists.

The HasValue property returns true if the nullable variable contains a value.

3fill in blank

hard

Fix the error in accessing the value of a nullable integer safely.

C Sharp (C#)

int value = number[1];

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

A.Value

B== null

C!

D?? 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using '.Value' without checking for null, causing exceptions.

Using '!' which is not valid here.

The null-coalescing operator ?? provides a default value if number is null, avoiding exceptions.

4fill in blank

hard

Fill both blanks to declare a nullable double and assign a value if it is null.

C Sharp (C#)

double[1] temperature = null;
double actualTemp = temperature [2] 25.0;

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

A?

B??

C+

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '??' for default value.

Forgetting to make the type nullable.

The ? makes the double nullable, and ?? assigns 25.0 if temperature is null.

5fill in blank

hard

Fill all three blanks to create a nullable bool, check if it has value, and get the value or default to false.

C Sharp (C#)

bool[1] isActive = null;
if (isActive[2]) {
    bool status = isActive[3] false;
}

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

A?

B.HasValue

C??

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!' instead of '?' for nullable.

Checking null with '== null' instead of '.HasValue'.

Using '+' instead of '??' for default value.

The ? makes the bool nullable, .HasValue checks if it has a value, and ?? provides a default if null.

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