Creating a generic list of objects in C#

Question

By way of an intro, I'm creating a basic Quadtree engine for personal learning purposes. I'm wanting this engine to have the capability of working with many different types of shapes (at the moment I'm going with circles and squares) that will all move around in a window and perform some sort of action when collision occurs.

Here are my shape objects as I have them so far:

public class QShape {
    public int x { get; set; }
    public int y { get; set; }
    public string colour { get; set; }
}

public class QCircle : QShape {
    public int radius;
    public QCircle(int theRadius, int theX, int theY, string theColour) {
        this.radius = theRadius;
        this.x = theX;
        this.y = theY;
        this.colour = theColour;
    }
}

public class QSquare : QShape {
    public int sideLength;
    public QSquare(int theSideLength, int theX, int theY, string theColour) {
        this.sideLength = theSideLength;
        this.x = theX;
        this.y = theY;
        this.colour = theColour;
    }
}

Now my question is, how do I create a generic list (List<T> QObjectList = new List<T>();) in C# so I can have one list containing all these various shapes that may have different properties (e.g., QCircle has the "radius" property while QSquare has the "sideLength" property)? An example of implementation would be helpful as well.

I just know that there is a stupidly obvious answer to this question but I'd appreciate any help anyway. I'm trying to get back into C#; it has obviously been a while...

Answer 1

You need to use downcasting

Store the objects in a list with the base class

 List<QShape> shapes = new List<QShape>

You can then upcast the object safely if you know what it is e.g.

if(shapes[0] is QSquare)
{
     QSquare square = (QSquare)shapes[0]
} 

You can also implicitly downcast objects

QSquare square = new Square(5,0,0,"Blue");
QShape shape =  square

For more information read the Upcasting and Downcasting sections here