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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of null-coalescing with nullable int

What is the output of this C# code snippet?

C Sharp (C#)

int? a = null;
int b = 5;
int result = a ?? b;
Console.WriteLine(result);

A null

B 0

C Compilation error

D 5

💡 Hint

The null-coalescing operator returns the left value if it is not null; otherwise, it returns the right value.

Since 'a' is null, the operator returns 'b', which is 5.

❓ Predict Output

intermediate

Null-coalescing with strings

What will this program print?

C Sharp (C#)

string s = null;
string result = s ?? "default";
Console.WriteLine(result);

A default

B null

C Compilation error

D Empty string

💡 Hint

If the left side is null, the operator returns the right side.

Since 's' is null, the expression returns "default".

❓ Predict Output

advanced

Chained null-coalescing operators

What is the output of this code?

C Sharp (C#)

string? a = null;
string? b = null;
string c = "hello";
string result = a ?? b ?? c;
Console.WriteLine(result);

A hello

B null

C Compilation error

D Empty string

💡 Hint

The operator evaluates left to right and returns the first non-null value.

Both 'a' and 'b' are null, so the operator returns 'c', which is "hello".

❓ Predict Output

advanced

Null-coalescing with method call

What will this code print?

C Sharp (C#)

string? GetName() => null;

string name = GetName() ?? "Unknown";
Console.WriteLine(name);

A null

B Unknown

C Compilation error

D Empty string

💡 Hint

The method returns null, so the operator returns the fallback string.

GetName() returns null, so the null-coalescing operator returns "Unknown".

❓ Predict Output

expert

Null-coalescing with nullable struct and property

What is the output of this program?

C Sharp (C#)

struct Point { public int X; public int Y; }

Point? p = null;
int x = p?.X ?? -1;
Console.WriteLine(x);

A Compilation error

B 0

C -1

D NullReferenceException

💡 Hint

The null-conditional operator returns null if 'p' is null, so the null-coalescing operator uses -1.

Since 'p' is null, 'p?.X' is null, so the expression evaluates to -1.

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