For a blog "Calculated properties – three strategies"

Code sample A

public class Square {
    private int _SideLength;

    public int SideLength {
        get => _SideLength;
        set => _SideLength = value;
    }

    public int Area {
        get => _SideLength * _SideLength;
    }
}

Code sample B

public int Area {
	get => _SideLength * _SideLength;
}

Code sample C

public class Square {
	private int _SideLength;
	private int _Area;
	
	/* a constructor */
	public Square() {
		UpdateArea();
	}
	
	public int SideLength {
		get => _SideLength;
		set {
			// we update related property
			_SideLength = value;

			// after that we recalculate _Area
			UpdateArea();
		}
	}
	
	private void UpdateArea() {
		_Area = _SideLength * _SideLength; 
	}
	
	public int Area {
		get => _Area;
	}
}

Code sample D

public class Square {
	private int _SideLength;
	private int _Area;
	
	// our internal indicator
	private bool _dirtyArea = true;
	
	public int SideLength {
		get => _SideLength;
		set {
			_SideLength = value;
			
			// here we set it to true,
			// _Area must be recalculated
			_dirtyArea = true;
		}
	}
	
	private void UpdateArea() {
		_Area = _SideLength * _SideLength;
		
		// here we set it to false,
		// _Area is ok to use now
		_dirtyArea = false;		
	}
	
	public int Area {
		get {
			// check our “dirty” indicator
			if (_dirtyArea) UpdateArea;	
		
			return _Area;
		}
	}
}

Code sample E

public Square(bool calculate) {
	if (calculate) UpdateArea();
}