Extension method

Learn to write extension methods.

Extension method

An extension method is a method declared inside an extension. It is accessed via this on an instance of the target type. The extension StringRepeat on String { String repeat(int n) => ... } declaration enables the call 'ab'.repeat(3).

Extension methods are used like class methods. However, they cannot be overridden or modified in subclasses. This is because extensions are resolved statically at compile time; there is no dynamic dispatch (polymorphism).

this and extension

Inside an extension method, this refers to the instance the method was called on. In extension on int { int doubled() => this * 2; }, this is the int value called. In 5.doubled(), this is 5.

Extension methods can call other methods in the same extension. Also, multiple methods can be declared inside an extension. The extension name is only for documentation and conflict avoidance; it doesn't affect the call.

Exercise A ListSum extension

Define a ListSum extension on List<int> with an int total() => fold(0, (a, b) => a + b); method. In main add print([1, 2, 3].total());.

Show example solution

extension ListSum on List<int> {
  int total() => fold(0, (a, b) => a + b);
}

void main() {
  print([1, 2, 3].total());
}

Test yourself

An extension method call

final result = 'ab'.____(3);
print(result);

Show answer

final result = 'ab'.repeat(3);
print(result);

How are extension methods resolved?
- Statically at compile time
- Only in subclasses
- Dynamically at runtime
- Never resolved
Show answer

Statically at compile time — extension methods are resolved statically at compile time.
Match the extension method parts

this

Called instance
on Type

Target type
static

Resolution style
override

Invalid

Next lesson: Extension operator

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