A name. An error message. A welcome screen. A tweet.

They are all strings.

You have already seen String in Lesson 3. But there is one Dart feature for strings that changes everything: interpolation.

Quotes

You can write a string with single quotes or double quotes:

void main() {
  var a = 'Hello';
  var b = "Hello";
  print(a); // Hello
  print(b); // Hello
}

They are identical. Pick one and be consistent. This course uses single quotes throughout.

Concatenation

Before interpolation, there is an older way to build strings from multiple values: the + operator.

void main() {
  var name = 'Ali';
  print('Hello, ' + name + '!'); // Hello, Ali!
}

It works. But it is ugly. Every value needs a + on either side, and the quotes pile up fast.

There is a better way.

Interpolation

Dart lets you embed a variable directly inside a string with $:

void main() {
  var name = 'Ali';
  print('Hello, $name!'); // Hello, Ali!
}

The $ tells Dart: replace this with the value of the variable. The result is the same as concatenation. But the string reads like a sentence.

Expression interpolation

$name works for a single variable. What if you want something more complex?

Use ${} and put any expression inside the curly braces:

void main() {
  var age = 20;
  print('You are $age years old.');               // You are 20 years old.
  print('In ten years you will be ${age + 10}.'); // In ten years you will be 30.
}

The rule is simple. A single variable gets $. Everything else gets ${}.

Multi-line strings

Sometimes a string spans more than one line. Triple quotes handle that:

void main() {
  var message = '''Hello, Ali.
Welcome back.
You have 3 new messages.''';
 
  print(message);
}

Output:

Hello, Ali.
Welcome back.
You have 3 new messages.

The content between ''' and ''' is taken exactly as written, line breaks included. Triple double quotes work too: """...""". Same behavior.

Raw strings

In a normal Dart string, a backslash has special meaning. \n becomes a newline:

void main() {
  print('Line one.\nLine two.');
}

Output:

Line one.
Line two.

But sometimes you want the backslash to be a literal backslash. File paths are a common case.

Prefix the string with r to make it raw:

void main() {
  print(r'C:\Users\Ali\Documents'); // C:\Users\Ali\Documents
}

In a raw string, backslashes are just backslashes. No special processing happens.

There is a lot more you can do with strings: checking their length, converting to uppercase, searching for substrings. But those require method calls, and method calls require classes. We cover them in Lesson 13.

Summary