Generic extension

Learn to write generic extensions.

Generic extension

A generic extension is an extension that takes a type parameter. The extension ListFirst<T> on List<T> { T? firstOrNull() => isEmpty ? null : first; } declaration safely gets the first element of any list.

Generic extensions preserve type safety: [1, 2].firstOrNull() returns int?, ['a'].firstOrNull() returns String?. The type parameter T is inferred from the target type.

Type inference

In generic extensions, the type parameter is automatically inferred from the target. In extension BoxUnwrap<T> on Box<T> { T unwrap() => value; }, when called on a Box<int>, T becomes int.

The type parameter can also be specified explicitly: box<int>.unwrap(). But in most cases inference is sufficient. Generic extensions are ideal for adding functionality to generic classes or types.

Exercise A ListMiddle extension

Define a ListMiddle<T> extension on List<T> with a T? middleOrNull() => isEmpty ? null : this[length ~/ 2]; method. In main add print([1, 2, 3].middleOrNull());.

Show example solution

extension ListMiddle<T> on List<T> {
  T? middleOrNull() => isEmpty ? null : this[length ~/ 2];
}

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

Test yourself

What does a generic extension provide?
- Function only for int
- Function only for String
- Type-safe multi-type function
- Runtime type checking
Show answer

Type-safe multi-type function — a generic extension works type-safely with multiple types.
Match the generic extension parts

<T>

Type parameter
on List<T>

Target type
T?

Nullable return
first

First element

A generic extension declaration

extension ListFirst<____> on List<T> {
  T? firstOrNull() => isEmpty ? null : first;
}

Show answer

extension ListFirst<T> on List<T> {
  T? firstOrNull() => isEmpty ? null : first;
}

Next lesson: Extension vs subclass

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