Lesson 09 of 59

5 mins read

Switch Statements, Switch Expressions & Exhaustiveness

In Plain English

Dart 3 revolutionized `switch` with pattern matching and Switch Expressions (`=>`). Unlike traditional switch statements that require verbose `case` and `break` keywords, switch expressions return values directly, require no `break` statements, and enforce compile-time exhaustiveness checking (guaranteeing all enum or sealed class cases are handled).

The Mental Model

"A switch expression is like a vending machine button panel: you press button "B4" and a snack drops directly into the dispenser tray in a single motion."

Deep Dive: How It Works

Switch Expressions: `final label = switch (status) { Status.ok => "Success", Status.err => "Failed" };`.

Exhaustiveness Checking: The compiler verifies at build-time that every possible enum variant or sealed class subtype is covered.

Pattern Matching & Guards: `case int n when n > 100 => "Large"`.

Syntax Blueprint

// Dart 3 Switch Expression
final label = switch (role) {
  "admin" => "Administrator",
  "editor" => "Content Editor",
  _ => "Standard User",
};

Modern concise switch expression with wildcard fallback.

Core Rules to Remember

Expression Syntax (=>): Returns values directly without tedious case/break boilerplate.

Compile-Time Exhaustiveness: Missing enum cases trigger compiler errors before runtime.

Wildcard Fallback (_): The underscore `_` serves as the default fallback branch.

Common Beginner Traps & How to Fix Them

Mistake:Adding `break;` inside a modern Dart 3 switch expression.

Why it happens: Habit from classic C/Java switch statements.

How to fix: Switch expressions use `pattern => value,` syntax and do not use break.

Pro Tip: Use switch expressions to replace multi-tier ternary expressions for clean, readable state mapping.

Live Interactive Example

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Modern Dart 3 Switch Expression

Python 3.12

Output Console

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How it works: The switch expression matched 404 directly and returned the descriptive string.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Evaluate Day Category with Switch Expression

Given `String day = "SAT";`.
Use a switch expression to assign `final category = switch (day) { "SAT" || "SUN" => "Weekend", _ => "Weekday" };`.
Print `Category: $category`.

Sandbox Output

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