How can I create object in abstract class without having knowledge of implementation?

Question

Hi,

Is there a way to implement the "CreateNode" method in my library abstract below? Or can this only be done in client code outside the library? I current get the error "Cannot create an instance of the abstract class or interface 'ToplogyLibrary.AbstractNode"

public abstract class AbstractTopology<T>
{
    // Properties
    public Dictionary<T, AbstractNode<T>> Nodes { get; private set; }
    public List<AbstractRelationship<T>> Relationships { get; private set; }

    // Constructors
    protected AbstractTopology()
    {
        Nodes = new Dictionary<T, AbstractNode<T>>();
    }

    // Methods
    public AbstractNode<T> CreateNode()
    {
        var node = new AbstractNode<T>();  // ** Does not work **
        Nodes.Add(node.Key, node);
        }
    }
 }

    public abstract class AbstractNode<T>
    {
        public T Key { get; set; }
    }

    public abstract class AbstractRelationship<T>
    {
        public AbstractNode<T> Parent { get; set; }
        public AbstractNode<T> Child { get; set; }

    }

Answer 1

You cannot create an instance of an abstract class, which is why you are receiving the error.

What you could do instead is declare CreateNode as an abstract method, and implement it in any descendant classes.

Answer 2

Well what concrete node class do you want to be created? Is this perhaps something which should be left up to the concrete topology class? If so, you might want to make it abstract:

public abstract AbstractNode<T> CreateNode();

Then provide a concrete implementation in the concrete topology class.

Alternatively, you could make your class generic in the node type as well:

public abstract class AbstractTopology<TValue, TNode>
    where TNode : AbstractNode<TValue>, new()

Then you could implement CreateNode like this:

public AbstractNode<T> CreateNode()
{
    var node = new TNode();
    Nodes.Add(node.Key, node);
    return node;   
}

Personally I get a little suspicious of designs involving this many abstract classes, but perhaps you've got good reasons.

Answer 3

define a protected pure abstract virtual method that CreateNode will call that the subclass return a new object

CreateNodeImpl or something.