Null-coalescing operator in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It returns the left-hand operand if it is not null; otherwise, it returns the right-hand operand.

beginner

How would you use the null-coalescing operator to provide a default value for a nullable variable?

Example: int? x = null; int y = x ?? 10; Here, y will be 10 because x is null.

intermediate

Can the null-coalescing operator be chained? If yes, how?

Yes, you can chain it like a ?? b ?? c. It returns the first non-null value from left to right.

intermediate

What is the difference between the null-coalescing operator (??) and the ternary operator when checking for null?

The null-coalescing operator is a concise way to check for null and provide a default, while the ternary operator requires a full condition like x != null ? x : defaultValue.

beginner

Write a simple C# code snippet using the null-coalescing operator to assign a string variable a default value if it is null.

string name = null;
string displayName = name ?? "Guest";
// displayName will be "Guest" if name is null

What will the expression int? a = null; int b = a ?? 5; assign to b?

A 5

B null

C 0

D Compilation error

Since 'a' is null, the null-coalescing operator returns the right side value 5.

Which operator is used in C# to return the first non-null value between two expressions?

A ??

B ?:

C &&

D ||

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

What will string s = null; var result = s ?? "default"; set result to?

A Compilation error

B "default"

C Empty string

D null

Since s is null, the operator returns the right side string "default".

Can the null-coalescing operator be used with non-nullable value types directly?

A Yes, with any type

B Only with integers

C Only with strings

D No, only with nullable types or reference types

It works with nullable types or reference types because they can be null.

What is the output of string? name = "Alice"; var display = name ?? "Guest";?

A null

B "Guest"

C "Alice"

D Compilation error

Since name is not null, the operator returns "Alice".

Explain how the null-coalescing operator (??) works in C# and give a simple example.

Think about providing a default value when something might be null.

Describe a scenario where chaining the null-coalescing operator is useful and how it works.

Imagine checking several options until you find one that is not null.

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