设计模式：组合模式

组合模式（Composite）：将对象组合成树形结构以表示‘部分-整体’的层次结构。组合模式使得用户对单个对象和组合对象的使用具有一致性。

namespace CompositeDesign
{
    public abstract class Component
    {
        protected string name;
        public Component(string name)
        {
            this.name = name;
        }
        public abstract void Add(Component c);
        public abstract void Remove(Component c);
        public abstract void Display(int depth);
    }
    public class Leaf : Component
    {
        public Leaf(string name) : base(name) { }
        public override void Add(Component c)
        {
            Console.WriteLine("Cannot add to a leaf");
        }

        public override void Display(int depth)
        {
            Console.WriteLine(new String('-',depth)+name);
        }

        public override void Remove(Component c)
        {
            Console.WriteLine("Cannot remove from a leaf");
        }
    }
    public class Composite : Component
    {
        private List<Component> children = new List<Component>();
        public Composite(string name) : base(name) { }
        public override void Add(Component c)
        {
            children.Add(c);
        }

        public override void Display(int depth)
        {
            Console.WriteLine(new String('-', depth) + name);
            foreach(Component com in children)
            {
                com.Display(depth + 2);
            }
        }

        public override void Remove(Component c)
        {
            children.Remove(c);
        }
    }
}

测试代码：

            Composite root = new Composite("root");
            root.Add(new Leaf("Leaf A"));
            root.Add(new Leaf("Leaf B"));
            Composite comp = new Composite("Composite X");
            comp.Add(new Leaf("Leaf XA"));
            comp.Add(new Leaf("Leaf XB"));
            root.Add(comp);
            Composite comp2 = new Composite("Composite XY");
            comp2.Add(new Leaf("Leaf XYA"));
            comp2.Add(new Leaf("Leaf XYB"));
            comp.Add(comp2);
            root.Add(new Leaf("Leaf C"));
            Leaf leaf = new Leaf("Leaf D");
            root.Add(leaf);
            root.Remove(leaf);
            root.Display(1);

XElement和XmlNode就是这种结构。