public double Area
    {
        get { return Width * Height; }

        set
        {
            Width = Math.Sqrt(value);
            Height = Width;
        }
    }

You can also use expression bodied members (that's the => syntax) with get/set. There's a shorthand if you only need to define the getter (i.e. read-only):

    public double Area => Width * Height;

Swift has the same syntax too except the last line.

Have they introducted willSet and didSet too?

Neat stuff, i may try the language out in a future project on windows.

There's no direct equivalent to willSet or didSet. The closest equivalent (aside from adding code before/after your own get/set code) might be INotifyPropertyChanged [1]/INotifyPropertyChanging [2]? But they are more for external observers and add a lot of boilerplate to your code. It's possible to manage the boilerplate, however [3].

[1] https://msdn.microsoft.com/en-us/library/system.componentmod...

[2] https://msdn.microsoft.com/en-us/library/system.componentmod...

[3] https://github.com/Fody/PropertyChanged