What is the Null-Coalescing Operator ?? in C#
💡 Concept: Null-Coalescing Operator (??)
The null-coalescing operator ?? in C# provides a concise way to return a default value when a nullable value is null. It simplifies null checks and improves code readability.
📘 Quick Intro
The ?? operator returns the left-hand operand if it is not null; otherwise, it returns the right-hand operand. It is widely used for fallback logic.
🧠 Analogy
Imagine ordering coffee. If your favorite blend is unavailable (null), the barista gives you a backup option (default). That’s exactly what the ?? operator does—fallback when your primary choice is null.
🔧 Technical Explanation
- Syntax:
var result = value ?? defaultValue; - Only evaluates the right-hand side if the left-hand is null.
- Works with nullable types and reference types.
- Helps eliminate verbose null-checking logic.
🎯 Use Cases
- Set fallback values for nullable variables.
- Simplify property assignments when using user inputs.
- Reduce if-else statements when checking for null.
💻 Real Code Example
string userInput = null;
string result = userInput ?? "Default Value";
Console.WriteLine(result); // Output: Default Value
int? score = null;
int finalScore = score ?? 0;
Console.WriteLine(finalScore); // Output: 0
❓ Interview Q&A
Q1: What does the null-coalescing operator do?
A: It returns the left-hand operand if it’s not null; otherwise, it returns the right-hand operand.
Q2: What is the syntax for the null-coalescing operator?
A: var result = value ?? defaultValue;
Q3: Can it be used with value types?
A: Yes, if the value type is nullable (e.g., int?).
Q4: Does it short-circuit evaluation?
A: Yes, it only evaluates the right-hand if the left is null.
Q5: Is this operator available in all C# versions?
A: It’s available since C# 2.0.
Q6: Can you use it inside expressions or assignments?
A: Yes, it’s often used in assignments and method arguments.
Q7: What is the difference between ?? and ??: (null-coalescing assignment)?
A: ?? returns a fallback; ??: assigns if null.
Q8: Can it chain with multiple operands?
A: Yes: a ?? b ?? c returns the first non-null value.
Q9: What happens if both operands are null?
A: The result is null.
Q10: Does it throw exception on null left operand?
A: No, it handles it safely by evaluating the right-hand side.
📝 MCQs
Q1. What is the null-coalescing operator in C#?
- ?:
- ??
- ::
- --
Q2. What does 'x ?? y' return if x is not null?
- y
- x
- null
- Exception
Q3. Which C# version introduced ?? operator?
- C# 1.0
- C# 2.0
- C# 3.0
- C# 5.0
Q4. Can ?? be used with nullable int?
- No
- Only with reference types
- Yes
- Only in C# 11
Q5. Which is correct usage of ??
- input ? 'default';
- if (input) then default;
- result = input ?? 'default';
- input =? default
Q6. Is the right operand of ?? always evaluated?
- Always
- Never
- Only if left is null
- Only in value types
Q7. What does ?? do with null left and right?
- Throws error
- Returns right
- Returns null
- Returns left
Q8. Can you chain multiple ???
- No
- Yes
- Only twice
- Not in expressions
Q9. What is the benefit of using ??
- Faster loops
- Cleaner null-check logic
- Memory allocation
- Boxing
Q10. What is difference between ?? and ??=
- Same
- ?? returns fallback, ??= assigns fallback
- ??= is deprecated
- None
💡 Bonus Insight
The null-coalescing operator enhances code readability and safety by removing the need for explicit null checks. It’s essential for clean, defensive programming in modern C#.
📄 PDF Download
Need a handy summary for your notes? Download this topic as a PDF!