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Question

I have create a generic Node class which will be used to build a tree object. I want to inherit the attributes of this class in another. The job class represents a SQL job which is included in a chain (tree) of jobs. The following code is giving me an error and I am not sure why.

public class Node<T>
{
    public int id { get; set; }
    public Node<T> parent { get; set; }
    public List<Node<T>> Children = new List<Node<T>>();

    public bool isRoot
    {
        get { return parent == null; }
    }

    public static Node<T> createTree(List<Node<T>> nodes)
    {
        if (nodes.Count() == 0)
            return new Node<T>();
     //Build parent / Child relationships
    }
}

public class Job : Node<Job>
{
    public string name {get; set;}

    public Job(String name)
    {
        this.name = name;
    }
}


   List<Job> joblist = JobDict.Select(j => new Job(j.Key)).ToList();
   Node<Job>.createTree(joblist);

I am Unable to call createTree with the List of Jobs. I realize changing it from List < Job > to List < Node< Job > > works but why am I unable to do the former? I figured because I am inheriting the node class, a List of Jobs would in fact be equivalent to a List of Node. I am sorry if this is a very basic question but I just began with generics and inheritance and am having a hard time grasping it entirely.

Answer 1

The problem is that List<Node<Job>> and List<Job> are not co-variant.

If you're using .NET 4 you can do this.

Node<Job>.createTree((IEnumerable<Node<Job>>)joblist);

or, you can modify the creeatetree method definition as follows.

public static Node<T> createTree(IList  nodes)
{
    if (nodes.Count == 0)
    return new Node<T>();       

    //Build parent / Child relationships
} 

Answer 2

I realize changing it from List<Job> to List<Node<Job>> works but why am I unable to do the former?

Because List<Job> does not inherit List<Node<Job>> even if Job inherits Node<Job> . In other words, A inherits B does not mean List<A> inherits List<B> .

You may need to cast each Job object to Node<Job> first:

var jobNodeList = joblist.Select(j => (Node<Job>)j).ToList();

Node<Job>.createTree(jobNodeList);

Answer 3

This isn't a direct answer to the question, but it might give you some ideas on how to make your code simpler.

It's often a good idea to keep code simple. The simplest tree structure you can make is this:

public class Tree<T> : List<Tree<T>> { }

In your case, you would extend it slightly as:

public class Node<T> : List<Node<T>>
{
    public int id { get; set; }
    public Node<T> parent { get; set; }

    public bool isRoot
    {
        get { return parent == null; }
    }
}

Effectively you get all of the Children properties for free.