Does Dart have extension methods like c#?

6 years later and extension functions are finally on the horizon: https://github.com/dart-lang/language/issues/41#issuecomment-539251446

They will be included in dart 2.6

Not yet. See issue 9991 - scoped object extensions.

Yes, this was introduced in Dart 2.6.

The syntax is the following:

extension YourExtension on YourClass {
  void yourFunction() {
    this.anyMember(); // You can access `anyMember` of `YourClass` using `this`.
    anyMember(); // You can access `anyMember` of `YourClass` without `this`.
  }
}

void main() {
  YourClass yourVariable;

  yourVariable.yourFunction(); // valid
}

You can also extend types like int:

void main() {
  5.fancyPrint();
  3.minusThree; // returns 0
}

extension FancyInt on int {
  void fancyPrint() => print('The magic number is $this.');

  int get minusThree => this - 3;
}

You can learn more here.