C# Feature - Null-coalescing assignment

Null-coalescing assignment

The ??= operator assigns the right-hand value only if the left-hand variable, property, or indexer currently evaluates to null.

Introduced In

This feature was introduced in C# 8.0 (2019).

Why This Feature Exists

Before ??=, lazy initialization often required repeated variable names and a full if block. ??= makes the intent explicit: initialize only once, only if missing.

Syntax

target ??= fallbackValue;

When To Use It

Use it for lazy initialization of collections, caches, settings objects, or dependency-created instances.
Use it when the fallback should only be created if the current value is missing.
Prefer it over verbose null-check blocks when the logic is just “initialize if null.”

Before

if (cache == null)
{
  cache = new Dictionary<string, string>();
}

After

cache ??= new Dictionary<string, string>();

Practical Example

private List<string>? _messages;

public void Add(string message)
{
  (_messages ??= new List<string>()).Add(message);
}

Common Pitfalls

Use ??= only when lazy initialization is the real goal, not as a substitute for broader validation logic.
The operator only checks for null; it does not handle empty strings, zero values, or empty collections.
Avoid hiding expensive object creation inside ??= unless that lazy creation is intentional.

Version Notes

C# 8 added ??= as the assignment form of null coalescing.