c# declare subclass as type superclass

Question

I can't seem to find a satisfactory answer to this inheritance question. Why do you declare a subtype like this sometimes:

Shape myCircle = new Circle();

and this other times?

Circle myShape = new Circle();

Where Circle is a child/sub-class of Shape .....

There doesn't seem to be consistency across the OO books I've been reading and an explanation I can get my head around. People have shown me examples of a Shapes class being instantiated for circles, squares, etc... But I don't understand when you use the 1st declaration and the second.

Answer 1

It is often desirable to treat objects of concrete class polymorphically to avoid creating dependencies on specific types where you can avoid them. For example, if you are creating a Circle with the intention of drawing it, and Draw() is a method of a Shape, then it is better to declare myCircle as a shape, in case you decide to replace it with a square later on.

On the other hand, if your intention is to do something specific to Circle, say, to set its radius, then you need to declare myCircle as Circle, because SetRadius() may not be available on the Shape.

Answer 2

Perhaps you have a list of shapes, and you don't particularly care what kind of shapes they are because what you need to do with that list is general to all shapes (ie draw), you would want to add them to the list like this

List<Shape> shapeList = new List<Shape>();
shapeList.Add(new Circle());
shapeList.Add(new Rectangle());
foreach (Shape shape in shapeList)
{
    shape.Draw();
}

This way you can access the general methods of shapes on objects that are actually different types but share a common parent. Then if you want to do something specific to circles you can use the same list like this.

foreach (Shape shape in shapeList)
{
    if (!(shape is Circle)) continue;
    Circle circle = shape as Circle;
    circle.Radius = 100;
}