Composite Pattern

Composite Pattern 

Gamma Categorization: Structural Design Pattern
Summary: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Sample Code

Problem: We want to store a trips photoset hierarchy.

Code

public abstract class Photo

{

    protected string name;

    public Photo(string name)

    {

        this.name = name;

    }

    public abstract void Add(Photo p);

    public abstract void Remove(Photo p);

    public abstract void Display(int depth);

}

public class SinglePhoto : Photo

{

    public SinglePhoto(string name) : base(name) { }

    public override void Add(Photo p) => Console.WriteLine("Cannot add to a leaf");

    public override void Remove(Photo p) => Console.WriteLine("Cannot remove from a leaf");

    public override void Display(int depth)

    {

        Console.WriteLine(new String('-', depth) + name);

    }

}

public class PhotoSet : Photo

{

    private List<Photo> _children = new List<Photo>();

    public PhotoSet(string name) : base(name) { }

    public override void Add(Photo p) => _children.Add(p);

    public override void Remove(Photo p) => _children.Remove(p);

    public override void Display(int depth)

    {

        Console.WriteLine(new String('-', depth) + name);

        foreach (Photo i in _children)

        {

            i.Display(depth + 2);

        }

    }

}

Usage

var lisbonPhotoSet = new PhotoSet("Trip to Lisbon");

lisbonPhotoSet.Add(new SinglePhoto("Belém Tower"));

var alfamaPhotoSet = new PhotoSet("Alfama area");

alfamaPhotoSet.Add(new SinglePhoto("Se Cathedral"));

alfamaPhotoSet.Add(new SinglePhoto("Portas do Sol"));

alfamaPhotoSet.Add(new SinglePhoto("Lisbon Castle"));

alfamaPhotoSet.Add(new SinglePhoto("Casa dos Bicos"));

lisbonPhotoSet.Add(alfamaPhotoSet);

lisbonPhotoSet.Add(new SinglePhoto("Jerónimos Monastery"));

lisbonPhotoSet.Display(0);

Output

Trip to Lisbon
--Belém Tower
--Alfama area
----Se Cathedral
----Portas do Sol
----Lisbon Castle
----Casa dos Bicos
--Jerónimos Monastery