Null-Coalescing Operators in C#

Vaibhav • September 11, 2025

In programming, dealing with missing or unknown values is a common challenge. Sometimes a variable might not have a value yet, or it might be intentionally left empty. In C#, this situation is represented using the special keyword null. But working with null can be tricky - if you try to use a null value without checking it, your program might crash. That’s where null-coalescing operators come in.

In this article, we’ll explore two powerful tools for handling null values safely and easily: ?? and ??=. These operators help you write cleaner, safer code by providing fallback values and assigning defaults when needed. We’ll stay within the concepts you’ve already learned - variables, data types, expressions - and avoid anything we haven’t covered yet, like methods or control structures.

What Is null?

In C#, null means “no value.” It’s like an empty box - the box exists, but there’s nothing inside. You can assign null to variables that are allowed to be empty, such as strings or nullable types.

string name = null;
int? age = null;

In this example, name is a string with no value, and age is a nullable integer that also has no value. These variables exist, but they don’t hold any actual data yet.

The ?? Operator - Providing a Fallback

The ?? operator is used to provide a fallback value when something is null. It’s like saying, “If this value is missing, use this other one instead.”

string userName = null;
string displayName = userName ?? "Guest";

Here, userName is null, so displayName becomes "Guest". If userName had a value, that value would be used instead.

The ?? operator checks the value on the left. If it’s not null, it returns that value. If it is null, it returns the value on the right.

Using ?? with Nullable Numbers

You can use ?? with numeric types that allow null, such as int?, double?, or decimal?.

int? score = null;
int finalScore = score ?? 0;

In this case, score is null, so finalScore becomes 0. If score had a value like 85, that value would be used instead.

Chaining ?? for Multiple Options

You can chain multiple ?? operators to check several values in order. This is useful when you have more than one possible source of data.

string first = null;
string second = null;
string third = "Fallback";

string result = first ?? second ?? third;

This expression checks first, then second, and finally third. Since the first two are null, result becomes "Fallback".

The ??= Operator - Assigning a Default

The ??= operator is used to assign a value to a variable only if it’s currently null. It’s like saying, “If this variable is empty, fill it with this.”

string title = null;
title ??= "Untitled";

After this line, title becomes "Untitled". If title already had a value, it would stay unchanged.

This operator is useful when you want to make sure a variable has a value, but you don’t want to overwrite it if it already does.

string caption = "Welcome";
caption ??= "Default"; // caption stays "Welcome"

In this case, caption already has a value, so ??= does nothing.

Using ??= with Nullable Numbers

Just like ??, the ??= operator works with nullable numeric types. You can use it to ensure a number has a value before using it.

int? quantity = null;
quantity ??= 1;

Now quantity becomes 1. If it had a value like 5, it would stay unchanged.

Combining ?? and ??=

You can use both operators together to check and assign values in a flexible way. First, use ??= to make sure a variable has a value. Then use ?? to provide a fallback when using it.

string label = null;
label ??= "Default";
string output = label ?? "Unknown";

In this example, label becomes "Default", and output uses that value. If label had stayed null, output would become "Unknown".

Real-World Analogy

Imagine you’re packing for a trip. You check your bag for a flashlight. If it’s missing, you pack a spare one. That’s like using ??. If your bag already has a flashlight, you leave it alone. That’s like using ??=. Both help you avoid surprises when you need something later.

Common Mistakes to Avoid

One common mistake is using ?? with types that can’t be null, like regular int or bool. These types always have a value, so ?? won’t do anything useful. Use nullable types like int? or bool? instead.

Another mistake is forgetting that ??= changes the variable. If you don’t want to modify the original value, use ?? instead.

Summary

The null-coalescing operators ?? and ??= make it easy to handle missing values in C#. Use ?? to provide a fallback when a value is null, and use ??= to assign a default only if needed. These tools help you write safer, cleaner code without extra checks or complexity. As you build more advanced programs, these operators will become essential for working with flexible and optional data.