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 (eg, 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 eg

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

Answer 2

You should implement an Interface . For example

public interface IHasLength
{
    int Length;
}

Then in the implementation you can do

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

FInally, in your list:

List<IHasLength> shapesWithSomeLength = new List<IHasLength>();

Now your list can hold ANYTHING that implements IHasLength whether it's a QCircle , QShape , or even a QDuck if you want as long as it implements IHasLength .

Answer 3

You could store them in a List<QShape> but this would mean that you could not access type-specific properties.

Generally, you might approach this by providing a common interface in your base class, and overriding behaviour in subclasses. In this way, a common interface can hide a diverse bunch of behaviours. For instance a Grow method could hide the complexities of growing items of different shape and could be called without explicit knowlege of the shape upon which it is operating.

public abstract class QShape {
    public abstract void Grow(int amt);
}
public class QSquare : QShape {
    private int sideLength;
    public override void Grow(int amt)
    {
        sideLength+=amt;
    }
}
public class QCircle : QShape {
    private int radius;
    public override void Grow(int amt)
    {
        radius+=amt;
    }
}

Answer 4

Is this what you want?

public class QShape
{
    protected 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;
    }
}
class Program
{
    static void Main(string[] args)
    {
        List<QShape> list = new List<QShape>();
        list.Add(new QCircle(100, 50, 50, "Red"));
        list.Add(new QCircle(100, 400, 400, "Red"));
        list.Add(new QSquare(50, 300, 100, "Blue"));


        foreach (var item in list.OfType<QCircle>())
        {
            item.radius += 10;
        }

        foreach (var item in list.OfType<QSquare>())
        {
            item.sideLength += 10;
        }
    }
}

Answer 5

I feel like i'm missing something but...

List<QCircle> circleObjects = new List<QCircle>();

and

List<QSquare> squareObjects = new List<QSquare>();

will work perfectly well.

EDIT:

Ah, I didn't understand what was being asked.

Yes, as your QCircle and QSquare classes inherit from QShape , you can just do.

List<QShape> shapes= new List<QShape>();

It's worth noting that if you want to access the radius property of all the QCircle 's in that list, then you are going to have to filter the list based on type.

Answer 6

You can use Ian Mercer's comment List<QShape>

And here's how you would fill it:

List<QShape> shapes = new List<QShape>();
QCircle circle = new QCircle(); 

shapes.Add(circle);

To unbox it:

QCircle circle = (QCircle) shapes[0];

If you need to call a method off the base class, no need to unbox, just use it.

Answer 7

Storing

You're already on the right track with your class definitions. What you have to do is make a List of the superclass (in this case, QShape ), which will be able to hold all of your shapes.

Here's an example of how you would make it:

List<QShape> objects = new List<QShape>();

objects.add(new QCircle(...));
objects.add(new QSquare(...));

Accessing

The problem here is differentiating what is what once everything is in the list. That's done with the getType() and typeof() methods of C#. ( Jon Skeet has an excellent answer about how to do this ). Basically, it looks like this:

if(objects.get(some_position).getType() == typeof(QCircle))
  QCircle circle = objects.get(some_position);
else if(/* like above with QSquare */)
  QSquare square = objects.get(some_position);

After you do this, you can resume using your objects like normal. But if you try accessing them from the list, you can only use the methods and variables that QShape has, as every object put in the list will be cast to it.

Answer 8

public Class abstract  Base<T>
{
    public abstract List<T>GetList();
}

then do this

public class className:Base<ObjectName>
{
    public override List<T>GetList()
    {
        //do work here 
    }
}