.NET Design Patterns

Composite


 Definition
 UML diagram
 Participants
 Structural code in C#
 Real-world code in C#
 .NET Optimized code in C#



Definition

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

Frequency of use:
Medium high




UML class diagram






Participants


    The classes and objects participating in this pattern are:

  • Component   (DrawingElement)
    • 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   (PrimitiveElement)
    • represents leaf objects in the composition. A leaf has no children.
    • defines behavior for primitive objects in the composition.
  • Composite   (CompositeElement)
    • defines behavior for components having children.
    • stores child components.
    • implements child-related operations in the Component interface.
  • Client  (CompositeApp)
    • manipulates objects in the composition through the Component interface.



Structural code in C#


This structural code demonstrates the Composite pattern which allows the creation of a tree structure in which individual nodes are accessed uniformly whether they are leaf nodes or branch (composite) nodes.

                 

using System;

using System.Collections.Generic;

 

namespace DoFactory.GangOfFour.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




Real-world code in C#


This real-world code demonstrates the Composite pattern used in building a graphical tree structure made up of primitive nodes (lines, circles, etc) and composite nodes (groups of drawing elements that make up more complex elements).

                 

using System;

using System.Collections.Generic;

 

namespace DoFactory.GangOfFour.Composite.RealWorld

{

  /// <summary>

  /// MainApp startup class for Real-World

  /// Composite Design Pattern.

  /// </summary>

  class MainApp

  {

    /// <summary>

    /// Entry point into console application.

    /// </summary>

    static void Main()

    {

      // Create a tree structure

      CompositeElement root =

        new CompositeElement("Picture");

      root.Add(new PrimitiveElement("Red Line"));

      root.Add(new PrimitiveElement("Blue Circle"));

      root.Add(new PrimitiveElement("Green Box"));

 

      // Create a branch

      CompositeElement comp =

        new CompositeElement("Two Circles");

      comp.Add(new PrimitiveElement("Black Circle"));

      comp.Add(new PrimitiveElement("White Circle"));

      root.Add(comp);

 

      // Add and remove a PrimitiveElement

      PrimitiveElement pe =

        new PrimitiveElement("Yellow Line");

      root.Add(pe);

      root.Remove(pe);

 

      // Recursively display nodes

      root.Display(1);

 

      // Wait for user

      Console.ReadKey();

    }

  }

 

  /// <summary>

  /// The 'Component' Treenode

  /// </summary>

  abstract class DrawingElement

  {

    protected string _name;

 

    // Constructor

    public DrawingElement(string name)

    {

      this._name = name;

    }

 

    public abstract void Add(DrawingElement d);

    public abstract void Remove(DrawingElement d);

    public abstract void Display(int indent);

  }

 

  /// <summary>

  /// The 'Leaf' class

  /// </summary>

  class PrimitiveElement : DrawingElement

  {

    // Constructor

    public PrimitiveElement(string name)

      : base(name)

    {

    }

 

    public override void Add(DrawingElement c)

    {

      Console.WriteLine(

        "Cannot add to a PrimitiveElement");

    }

 

    public override void Remove(DrawingElement c)

    {

      Console.WriteLine(

        "Cannot remove from a PrimitiveElement");

    }

 

    public override void Display(int indent)

    {

      Console.WriteLine(

        new String('-', indent) + " " + _name);

    }

  }

 

  /// <summary>

  /// The 'Composite' class

  /// </summary>

  class CompositeElement : DrawingElement

  {

    private List<DrawingElement> elements =

      new List<DrawingElement>();

 

    // Constructor

    public CompositeElement(string name)

      : base(name)

    {

    }

 

    public override void Add(DrawingElement d)

    {

      elements.Add(d);

    }

 

    public override void Remove(DrawingElement d)

    {

      elements.Remove(d);

    }

 

    public override void Display(int indent)

    {

      Console.WriteLine(new String('-', indent) +

        "+ " + _name);

 

      // Display each child element on this node

      foreach (DrawingElement d in elements)

      {

        d.Display(indent + 2);

      }

    }

  }

}


Output
-+ Picture
--- Red Line
--- Blue Circle
--- Green Box
---+ Two Circles
----- Black Circle
----- White Circle




.NET Optimized code in C#


The .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, object initializers, automatic properties, etc. You can find an example on our Singleton pattern page.

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