Null-coalescing operator in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign a default value if 'name' is null.

C Sharp (C#)

string displayName = name [1] "Guest";

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

A||

B??

C&&

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '||' which is a logical OR and does not work with null values.

Using '&&' which is a logical AND and incorrect here.

Using '==' which is a comparison operator, not for assignment.

The null-coalescing operator '??' returns the left-hand operand if it is not null; otherwise, it returns the right-hand operand.

2fill in blank

medium

Complete the code to assign 'result' to 'input' or 0 if 'input' is null.

C Sharp (C#)

int result = input [1] 0;

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

A?:

B&&

C||

D??

Attempts:

3 left

💡 Hint

Common Mistakes

Using '||' which is a logical OR and not for null checks.

Using '?:' which is the ternary operator but more verbose here.

Using '&&' which is logical AND and incorrect.

The '??' operator returns the left operand if it is not null; otherwise, it returns the right operand.

3fill in blank

hard

Fix the error in the code by completing the null-coalescing expression.

C Sharp (C#)

string message = GetMessage() [1] "No message";

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

A||

B&&

C??

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '||' which is a logical OR and not for null fallback.

Using '==' which is a comparison, not assignment.

Using '&&' which is logical AND and incorrect.

The '??' operator correctly provides a fallback value if GetMessage() returns null.

4fill in blank

hard

Fill both blanks to create a dictionary with word lengths only if length is greater than 3.

C Sharp (C#)

var lengths = new Dictionary<string, int> { {"apple", "apple".Length}, {"cat", "cat".Length} };
var filtered = lengths.Where(kv => kv.Value [1] 3).ToDictionary(kv => kv.Key, kv => kv.Value [2] 1);

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

A>

B+

C<

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for filtering.

Using '-' instead of '+' for incrementing values.

We filter lengths greater than 3 and add 1 to each value in the new dictionary.

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase keys and values only if value is not null.

C Sharp (C#)

var data = new Dictionary<string, string?> { {"a", "apple"}, {"b", null} };
var result = data.Where(kv => kv.Value [1] null).ToDictionary(kv => kv.Key.[2](), kv => kv.Value.[3]());

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

A!=

BToUpper

CToLower

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '!=' to filter nulls.

Using 'ToLower()' instead of 'ToUpper()' for uppercase conversion.

We filter out null values using '!=', then convert keys and values to uppercase using 'ToUpper()'.

Practice

1.

What does the null-coalescing operator ?? do in C#?

easy

A. Returns the left value if it is not null; otherwise, returns the right value.

B. Checks if two values are equal.

C. Assigns a value only if the variable is null.

D. Throws an exception if the left value is null.

Solution

1. Step 1: Understand the operator's purpose

   The null-coalescing operator ?? is used to provide a fallback value when the left-hand side is null.

2. Step 2: Analyze behavior

   If the left value is not null, it returns that value; otherwise, it returns the right value.

3. Final Answer:

   Returns the left value if it is not null; otherwise, returns the right value. -> Option A

4. Quick Check:

   Null-coalescing operator = fallback value [OK]

Hint: Think: "Use right value only if left is null" [OK]

Common Mistakes:

• Confusing it with equality check
• Thinking it assigns values automatically
• Assuming it throws exceptions on null

2.

Which of the following is the correct syntax to use the null-coalescing operator in C#?

string name = null;
string displayName = ??? "Guest";

easy

A. name ??

B. name ?? "Guest"

C. name ? "Guest"

D. name ?: "Guest"

Solution

1. Step 1: Identify correct operator usage

   The null-coalescing operator syntax is left ?? right, where right is the fallback value.

2. Step 2: Match syntax to code

   Given string displayName = name ?? "Guest";, it correctly assigns "Guest" if name is null.

3. Final Answer:

   name ?? "Guest" -> Option B

4. Quick Check:

   Correct syntax uses ?? between variables [OK]

Hint: Remember: null-coalescing uses two question marks ?? [OK]

Common Mistakes:

• Using single question mark ? instead of ??
• Placing ?? after the variable without right value
• Confusing with ternary operator syntax

3.

What will be the output of the following C# code?

string input = null;
string result = input ?? "Default";
Console.WriteLine(result);

medium

A. null

B. input

C. Default

D. Compilation error

Solution

1. Step 1: Check the value of input

   The variable input is assigned null.

2. Step 2: Apply null-coalescing operator

   The expression input ?? "Default" returns "Default" because input is null.

3. Final Answer:

   Default -> Option C

4. Quick Check:

   Null input returns fallback string [OK]

Hint: If left is null, output right value [OK]

Common Mistakes:

• Expecting output to be 'null' string
• Confusing with ternary operator output
• Thinking it causes compile error

4.

Identify the error in the following code snippet using the null-coalescing operator:

int? number = null;
int result = number ?? null;
Console.WriteLine(result);

medium

A. Variable 'number' must be non-nullable.

B. The null-coalescing operator is used incorrectly; should be &&.

C. No error; code runs fine and prints 0.

D. Cannot use null as right operand for int result.

Solution

1. Step 1: Analyze variable types

   number is nullable int (int?), but result is non-nullable int.

2. Step 2: Check right operand of null-coalescing

   The right operand is null, which cannot be assigned to non-nullable int result.

3. Final Answer:

   Cannot use null as right operand for int result. -> Option D

4. Quick Check:

   Right side must be non-null for non-nullable target [OK]

Hint: Right side of ?? must match target type, not null [OK]

Common Mistakes:

• Using null as fallback for non-nullable variable
• Confusing ?? with logical operators
• Ignoring nullable type rules

5.

Given the following code, what will be the output?

string? firstName = null;
string? lastName = "Smith";
string fullName = firstName ?? lastName ?? "Unknown";
Console.WriteLine(fullName);

hard

A. Smith

B. null

C. Unknown

D. Compilation error

Solution

1. Step 1: Evaluate first null-coalescing operation

   firstName is null, so firstName ?? lastName evaluates to lastName, which is "Smith".

2. Step 2: Evaluate second null-coalescing operation

   Since lastName is not null, the second ?? "Unknown" is not used.

3. Final Answer:

   Smith -> Option A

4. Quick Check:

   Chained ?? returns first non-null value [OK]

Hint: Chain ?? to pick first non-null value [OK]

Common Mistakes:

• Expecting 'Unknown' when lastName is not null
• Thinking null propagates through chain
• Assuming compilation error with nullable strings