Dart Lesson 16 of 102 2 min read

switch and case in Dart

Learn the switch statement in Dart: matching values, shared cases, default, guards with when, and how Dart 3 removed the need for break.

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A switch compares one value against a list of cases and runs the first one that matches. It is tidier than a long else if chain when every branch tests the same value.

A basic switch #

void main() {
  var day = 3;

  switch (day) {
    case 1:
      print('Monday');
    case 2:
      print('Tuesday');
    case 3:
      print('Wednesday');
    default:
      print('Another day');
  }
}

Wednesday

Since Dart 3, a case that contains code ends on its own. You do not need break, and one case never falls into the next.

Several values, one result #

An empty case shares the body of the case below it. You can also join values with ||.

void main() {
  var day = 'sat';

  switch (day) {
    case 'sat':
    case 'sun':
      print('Weekend');
    case 'mon' || 'tue' || 'wed' || 'thu' || 'fri':
      print('Weekday');
    default:
      print('Not a day');
  }
}

Weekend

Ranges and comparisons #

Cases are patterns, so they can test more than equality.

void main() {
  var marks = 84;

  switch (marks) {
    case >= 90:
      print('A+');
    case >= 80:
      print('A');
    case >= 60 && < 80:
      print('B');
    default:
      print('C');
  }
}

Guards with when #

Add when to attach an extra condition to a case.

void main() {
  var temperature = 35;
  var isRaining = false;

  switch (temperature) {
    case > 30 when isRaining:
      print('Hot and wet');
    case > 30:
      print('Hot and dry');
    default:
      print('Mild');
  }
}

Hot and dry

Switching on an enum #

With an enum, Dart checks that you handled every value. If you forget one, the code does not compile, so no default is needed.

enum Light { red, yellow, green }

void main() {
  var light = Light.yellow;

  switch (light) {
    case Light.red:
      print('Stop');
    case Light.yellow:
      print('Get ready');
    case Light.green:
      print('Go');
  }
}

Get ready

Switch as an expression #

When each branch just produces a value, use the expression form. It is covered fully in switch expressions.

void main() {
  var code = 404;

  var message = switch (code) {
    200 => 'OK',
    404 => 'Not found',
    500 => 'Server error',
    _ => 'Unknown',
  };

  print(message);
}

Not found

Try it yourself #

Write a switch on a month number (1 to 12) that prints how many days the month has. Group the months with 31 days into one case and the months with 30 into another.