Curiously Recurring Template Pattern and generics

问题:

I would like to create a method in a base generic class to return a specialized collection of derived objects and perform some operations on them, like in the following example:

using System;
using System.Collections.Generic;

namespace test {

    class Base<T> {

        public static List<T> DoSomething() {
            List<T> objects = new List<T>();
            // fill the list somehow...
            foreach (T t in objects) {
                if (t.DoSomeTest()) { // error !!!
                    // ...
                }
            }
            return objects;
        }

        public virtual bool DoSomeTest() {
            return true;
        }

    }

    class Derived : Base<Derived> {
        public override bool DoSomeTest() {
            // return a random bool value
            return (0 == new Random().Next() % 2);
        }
    }

    class Program {
        static void Main(string[] args) {
            List<Derived> list = Derived.DoSomething();
        }
    }
}

My problem is that to do such a thing I would need to specify a constraint like

class Base<T> where T : Base {
}

Is it possible to specify a constraint like that somehow?

回答1:

This might work for you:

class Base<T> where T : Base<T>

You can't constrain T to an open generic type. If you need to constrain T to Base<whatever>, you'll need to construct something like:

abstract class Base { }

class Base<T> : Base where T : Base { ... }

回答2:

I have used the following to create not a linked list, but a generecic linked tree. It works wonderfully nicely.

public class Tree<T> where T : Tree<T>
{
    T parent;
    List<T> children;

    public Tree(T parent)
    {
        this.parent = parent;
        this.children = new List<T>();
        if( parent!=null ) { parent.children.Add(this as T); }
    }
    public bool IsRoot { get { return parent == null; } }
    public bool IsLeaf { get { return children.Count == 0; } }
}

Example usage from Mechanics (Coordinate System Hierarchy)

class Coord3 : Tree<Coord3>
{
    Vector3 position;
    Matrix3 rotation;
    private Coord3() : this(Vector3.Zero, Matrix3.Identity) { }
    private Coord3(Vector3 position, Matrix3 rotation) : base(null) 
    {  
       this.position = position;
       this.rotation = rotation;
    }
    public Coord3(Coord3 parent, Vector3 position, Matrix3 rotation) 
       : base(parent)
    {
       this.position = position;
       this.rotation = rotation;
    }
    public static readonly Coord3 World = new Coord3();

    public Coord3 ToGlobalCoordinate()
    {
       if( IsRoot )
       { 
            return this;
       } else {
            Coord3 base_cs = parent.ToGlobalCoordinate();
            Vector3 global_pos = 
                      base_cs.position + base_cs.rotation * this.position;
            Matrix3 global_rot = base_cs.rotation * this.rotation;
            return new Coord3(global_pos, global_ori );
       }
    }
}

The trick is to initialize the root object with null parent. Remember you cannot do Coord3() : base(this) { }.