Two Question Marks in C#: What It Is and How to Use It

Two Question Marks in C#: Not a Mistake, But a Powerful Tool

Imagine: you are writing code in C#, and suddenly you see the ?? construct. Your first thought — a typo? An extra character? In reality, this is one of the most elegant and useful operators in the language — the null-coalescing operator. If you have ever caught a NullReferenceException and cursed everything in sight — this operator will be your lifesaver.

In this article, we will break down what two question marks mean in C#, how to use them correctly, how they differ from the ternary operator, and show real-world examples from practice. Let's go!

What is the ?? Operator in C#?

The ?? operator (two question marks) is a binary operator that returns the left operand if it is not null; otherwise, it returns the right operand. Simply put: "If the left side is not null — take the left side, otherwise take the right side."

The syntax is extremely simple:

var result = left ?? right;

Where:

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• left — a value that can be null;
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• right — a default value (cannot be null if the right side type is nullable).
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First Example: Saving Yourself from null

Suppose you have a variable string? name (nullable string). You want to display the user's name, and if it's absent — "Guest":

string? name = GetUserName(); // may return null\nstring displayName = name ?? "Guest";\nConsole.WriteLine(displayName);

Without the ?? operator, you would have to write:

string displayName = (name != null) ? name : "Guest";

Admit it, the first option reads like a normal sentence: "name or guest". Elegant, right?

The ??= Operator: The Next Generation of Null-Coalescing

Starting with C# 8.0, the ??= operator appeared — this is a shorthand assignment form with null-coalescing. It assigns the value of the right operand to the left operand only if the left operand is null.

Syntax:

left ??= right;

This is equivalent to:

if (left == null) left = right;

Example with ??=

List<int>? numbers = null;\nnumbers ??= new List<int>(); // numbers is now = new List<int>()\nnumbers.Add(42);\nConsole.WriteLine(numbers.Count); // 1

Note: if numbers were already initialized (not null), the assignment would not occur. This is perfect for lazy initialization.

Comparison with the Ternary Operator (? :)

Many beginners confuse ?? with the ternary operator ? :. Let's settle this once and for all:

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• Ternary operator — is a shorthand form of if-else. It checks ANY boolean condition: condition ? value_if_true : value_if_false.
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• The ?? operator — checks ONLY for null. It is a specific case, tailored for one task.
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Example of the ternary operator:

int age = 18;\nstring status = (age >= 18) ? "Adult" : "Minor";

And here is what happens if you try to replace ?? with the ternary:

string? name = null;\nstring displayName = name != null ? name : "Guest"; // works, but longer\nstring displayName2 = name ?? "Guest"; // shorter and clearer

Conclusion: use ?? when you simply need to substitute a default value instead of null. For all other conditions — use the ternary operator.

Chaining the ?? Operator

You can chain multiple ?? operators together. This is convenient when there are several possible sources for a value:

string? firstChoice = GetFromCache