GenericTree<T> can't be properly extended

[bretthysuik]

InsertItem and RemoveItem can't be properly overridden in a derived class because Add returns a GeneralTree<T> instance. Consumers of the derived type have to explicitly cast the GeneralTree instance returned by Add to their derived type in order for the overridden functionality to be called.

Example:

public class DerivedGeneralTree<T> : GeneralTree<T>
{
    protected override void InsertItem(int index, GeneralTree<T> item)
    {
        // Do something special...
    }

    protected override void RemoveItem(T item)
    {
        // Do something special...
    }
}

var derivedTree = new DerivedGeneralTree<string>("root");
// Returns GeneralTree<T>! Overridden code won't be called unless consumer 
//  explicitly casts to DerivedGeneralTree<T> each and every time!
GeneralTree<T> a = derivedTree.Add("a");

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I hate to say it, but the entire GeneralTree<T> abstraction is broken because of all the ITree<T> explicit implementations and the usage of GeneralTree<T> in method signatures instead of the base interface.

Declaring var derivedTree as an ITree<string> isn't useful because you lose out on all the nice methods in GeneralTree<string>....

[SimonCropp]

@bretthysuik so how would you fix this?

[bretthysuik]

I haven't looked into the other tree implementations so I don't know if the following suggestions would break something outside the scope of GeneralTree<T> and ITree<T>:

ITree

• Add the following property and methods:

ReadOnlyCollection<ITree<T>> Children { get; }

ITree<T> Add(T item);

BreadthFirstTraverse(IVisitor<ITree<T>> visitor);

DepthFirstTraverse(OrderedVisitor<ITree<T>> visitor);

I find IVisitor<ITree<T>> much more useful than IVisitor<T> because it allows me access to more information during traversal.

---

GeneralTree

• Eliminate as much explicit interface implementations as possible.
• Reference the base ITree<T> interface as much as possible instead of GeneralTree<T> (e.g. replace public GeneralTree<T> Add(T item) with public ITree<T> Add(T item))
• Make the Add(...) methods virtual so they can be overridden to return the correct ITree<T> instance.
• Make the traversal methods virtual so they can be overridden.
• Check if the visitor has completed before every Visit() call in the traversal methods so they can be short-circuited.

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If I have time in the next few days I'll submit a pull request with some or all of these changes.

[bretthysuik]

I haven't had time to look into fixing this, and it's unlikely I'll have time in the near future. You can close this bug if you want.