Is it possible to change an object's type from inside itself

Question

I have a shape class with square and triangle as inheriting classes

Square square = new Square();
square.SideCount = 4;
//Logic
square.SideCount = 3;
 
if(square is Triangle)
   {
   //...

If I instantiate a new square with SideCount = 4, then if I change it's SideCount to 3, can I put code into the SideCount's Setter to convert Square to Triangle if sides are set to 3?

Answer 1

Not directly no, an instance of Square cannot be changed to an instance of Triangle.

You could have a method, which returns an instance of Shape defined on the base class Shape:

public abstract class Shape
{
    public Shape Mutate(int numberOfSides)
    {
        // over simplified example:
        if(numberOfSides == 3)
           return new Triangle();
    }
}

public class Triangle : Shape {}
public class Square : Shape {}

usage

var squ = new Square();
var triangle = squ.Mutate(3);