gistfile1.txt

class Complex {
  num _real;
  num _imaginary;

  num get real => _real;
  num get imaginary => _imaginary;
  set real(num r) {
    if(r < 0) return;
    _real = r;
  }
}

void main() async {
  var a = Complex();
  a.real = 123;
  print(a.real);
}