Composite Design Pattern -- Design Pattern

        Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. 

Participants

    The classes and/or objects participating in this pattern are:

• Component(Drawing Element)

        Declares the interface for objects in the composition. Implements default behavior for the interface common to all classes, as appropriate.

       Declares an interface for accessing and managing its child components.

       (Optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate.

• Leaf(Primitive Element)

        Represents leaf objects in the composition. A leaf has no children. Defines behavior for primitive objects in the composition.

• Composite(Composite Element)

      Defines behavior for components having children. Stores child components.

      

      Implements child-related operations in the Component interface.

Client  (Composite App)

      

      Manipulates objects in the composition through the Component interface.

Example for Composite Design pattern

using System; using System.Collections.Generic;  namespace  Composite.Structural {   /// <summary>   /// MainApp startup class for Structural   /// Composite Design Pattern.   /// </summary>   class MainApp   {     /// <summary>     /// Entry point into console application.     /// </summary>     static void Main()     {       // Create a tree structure       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);       root.Add(new Leaf("Leaf C"));        // Add and remove a leaf       Leaf leaf = new Leaf("Leaf D");       root.Add(leaf);       root.Remove(leaf);        // Recursively display tree       root.Display(1);        // Wait for user       Console.ReadKey();     }   }    /// <summary>   /// The 'Component' abstract class   /// </summary>   abstract class Component   {     protected string name;      // Constructor     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);   }    /// <summary>   /// The 'Composite' class   /// </summary>   class Composite : Component   {     private List<Component> _children = new List<Component>();      // Constructor     public Composite(string name)       : base(name)     {     }      public override void Add(Component component)     {       _children.Add(component);     }      public override void Remove(Component component)     {       _children.Remove(component);     }      public override void Display(int depth)     {       Console.WriteLine(new String('-', depth) + name);       // Recursively display child nodes       foreach (Component component in _children)       {         component.Display(depth + 2);       }     }   }    /// <summary>   /// The 'Leaf' class   /// </summary>   class Leaf : Component   {     // Constructor     public Leaf(string name)       : base(name)     {     }      public override void Add(Component c)     {       Console.WriteLine("Cannot add to a leaf");     }      public override void Remove(Component c)     {       Console.WriteLine("Cannot remove from a leaf");     }      public override void Display(int depth)     {       Console.WriteLine(new String('-', depth) + name);     }   } }

Output -root ---Leaf A ---Leaf B ---Composite X -----Leaf XA -----Leaf XB ---Leaf C