Null-coalescing operator in C Sharp (C#) - Mini Project: Build & Apply

Using the Null-coalescing Operator in C#

📖 Scenario: Imagine you are building a simple program that greets users. Sometimes, the user's name might not be provided, so you want to use a default greeting name instead.

🎯 Goal: You will create a program that uses the null-coalescing operator (??) to provide a default name when the user's name is missing.

📋 What You'll Learn

Create a string variable called userName with a null value.

Create a string variable called defaultName with the value "Guest".

Use the null-coalescing operator ?? to assign a variable nameToUse either userName if it is not null, or defaultName if userName is null.

Print the greeting message Hello, {nameToUse}!.

💡 Why This Matters

🌍 Real World

In real applications, user input or data from databases can be missing or null. The null-coalescing operator helps provide safe default values to avoid errors.

💼 Career

Understanding how to handle null values safely is important for writing robust C# programs, especially in web development and data processing jobs.

1

Create the userName variable with a null value

Create a string variable called userName and set it to null.

C Sharp (C#)

# Your code here

Hint

Use string? userName = null; to allow userName to be null.

2

Create the defaultName variable

Create a string variable called defaultName and set it to "Guest".

C Sharp (C#)

string? userName = null;
# Your code here

Hint

Use string defaultName = "Guest"; to set the default name.

3

Use the null-coalescing operator to assign nameToUse

Create a string variable called nameToUse and assign it the value of userName ?? defaultName using the null-coalescing operator.

C Sharp (C#)

string? userName = null;
string defaultName = "Guest";
# Your code here

Hint

Use ?? to choose userName if not null, otherwise defaultName.

4

Print the greeting message

Write a Console.WriteLine statement to print Hello, {nameToUse}! using string interpolation.

C Sharp (C#)

string? userName = null;
string defaultName = "Guest";
string nameToUse = userName ?? defaultName;
# Your code here

Hint

Use Console.WriteLine($"Hello, {nameToUse}!"); to print the greeting.

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