C# Academy · Lesson

Null-Coalescing Operators

?? and ??= defaults.

Lesson 3 of 413 steps

Null-Coalescing Operators is a free C# Academy lesson on CoddyKit. This is lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

Supplying a Fallback

Often you want to use a value if it exists, or a default if it is null. The null-coalescing operator ?? does exactly this.

a ?? b means: use a if it is not null, otherwise use b.

string input = null;
string name = input ?? "Guest";
System.Console.WriteLine(name);

How ?? Evaluates

The left side is evaluated first. If it is non-null, the right side is never evaluated at all.

This short-circuiting means an expensive fallback only runs when actually needed.

string cached = "data";
string value = cached ?? Compute();
System.Console.WriteLine(value);
// Compute() is never called here

Works with Nullable Value Types

?? also unwraps nullable value types. int? maybe combined with ?? 0 produces a plain non-nullable int.

This is a clean way to convert from int? to int with a default.

int? maybe = null;
int sure = maybe ?? 0;
System.Console.WriteLine(sure);

A Runnable Example

This program reads a possibly-null setting and falls back to a default. Run it to see the fallback in action.

using System;

class Program
{
    static void Main()
    {
        string theme = null;
        string active = theme ?? "light";
        Console.WriteLine("Theme: " + active);
    }
}

Chaining ??

You can chain several fallbacks. The expression picks the first non-null value from left to right.

a ?? b ?? c tries a, then b, then finally c as the last resort.

string a = null;
string b = null;
string c = "default";
System.Console.WriteLine(a ?? b ?? c);

The ??= Operator

The null-coalescing assignment ??= assigns the right side only if the left is currently null.

x ??= y is shorthand for if (x == null) x = y; and leaves x unchanged when it already has a value.

string config = null;
config ??= "default";
System.Console.WriteLine(config);
config ??= "ignored";
System.Console.WriteLine(config);

Lazy Initialization with ??=

A common use of ??= is lazy initialization: create an object only the first time it is needed.

On later calls the field is already set, so ??= does nothing.

System.Collections.Generic.List<int> items = null;
items ??= new System.Collections.Generic.List<int>();
items.Add(1);
System.Console.WriteLine(items.Count);

Combining ?. and ??

Pairing ?. with ?? is extremely common. The ?. avoids the exception, and ?? supplies a value when the chain is null.

user?.Name ?? "Unknown" reads the name safely, defaulting to "Unknown".

string s = null;
int length = s?.Length ?? -1;
System.Console.WriteLine(length);

?? in a Full Program

Here both ?. and ?? work together to print a safe length. Run it with a null string to see the fallback.

using System;

class Program
{
    static void Main()
    {
        string username = null;
        string display = username ?? "anonymous";
        Console.WriteLine(display);
        Console.WriteLine(username?.Length ?? 0);
    }
}

?? Throwing on Null

The right side of ?? can be any expression, including throw. This makes ?? a compact way to fail fast on an unexpected null.

name ?? throw new ArgumentNullException(nameof(name)) returns the name, or throws if it is null.

string Validate(string name) =>
    name ?? throw new System.ArgumentNullException(nameof(name));

Right-Associative Behavior

?? is right-associative, so a ?? b ?? c groups as a ?? (b ?? c).

In practice this still means "first non-null wins", but it is good to know when reasoning about complex expressions.

int? a = null;
int? b = null;
int result = a ?? b ?? 99;
System.Console.WriteLine(result);

Quick Check

Test your understanding of the null-coalescing operators.

Answer it in the interactive lesson

Recap

?? returns its left side if non-null, otherwise the right side, and can be chained. ??= assigns only when the target is null, ideal for lazy init.

Combine them with ?. for safe, concise null handling, and even use ?? throw to fail fast.

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Frequently Asked Questions

Is the “Null-Coalescing Operators” lesson free?

Yes — the full text of “Null-Coalescing Operators” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO. The C# Academy course includes 4 lessons in total.

What will I learn in “Null-Coalescing Operators”?

?? and ??= defaults. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

How long does the “Null-Coalescing Operators” lesson take?

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