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

Question

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; }

    }

Justin Ethier · Accepted Answer · 2010-05-11 01:14:22Z

3

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.

answered May 11, 2010 at 1:14

Justin Ethier

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1 Comment

Jim L Over a year ago

dang it, you've got faster fingers.

Jon Skeet · Accepted Answer · 2010-05-11 01:16:44Z

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.

answered May 11, 2010 at 1:16

Jon Skeet

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1 Comment

Greg Over a year ago

Sounds good. Will the caller using the method have to pass the type when they are using the mehod however?

Keith Nicholas · Accepted Answer · 2010-05-11 01:17:19Z

0

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

CreateNodeImpl or something.

answered May 11, 2010 at 1:17

Keith Nicholas

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