Composite Pattern

• Use it to represent a hierarchy of objects and treat the objects in the hierarchy the same way.

Scenario

• Powerpoint with individual object and group of objects

Classical Structure

Problem

• Need to check the type of the object and cast it to new type and call method on it
• You need to repeat the same logic (iterating) for all methods

public class Shape
{
    public void Render()
    {
        Console.WriteLine("Rendering Shape");
    }
}
public class Group
{
    List<Object> objects = new List<object>();

    public void Add(Object obj)
    {
        objects.Add(obj);
    }

    public void Render()
    {
        foreach (var item in objects)
        {
            if (item is Shape)
            {
                ((Shape)item).Render();
            }
            else
            {
                ((Group)item).Render();
            }
        }
    }
}

Solution

public interface IComponent
{
    void Render();
}
public class Shape: IComponent
{
    public void Render()
    {
        Console.WriteLine("Rendering Shape");
    }
}
public class Group: IComponent
{
    List<IComponent> components = new List<IComponent>();

    public void Add(IComponent obj)
    {
        components.Add(obj);
    }

    public void Render()
    {
        foreach (var item in components)
        {
            item.Render();
        }
    }
}

public class Program 
{
	public static void Main(string[] args) 
	{
		var group1 = new Group();
		group1.Add(new Shape());
		group1.Add(new Shape());
		group1.Render();


		var group2 = new Group();
		group2.Add(new Shape());
		group2.Add(new Shape());
		group2.Render();

		var group = new Group();
		group.Add(group1);
		group.Add(group2);
	}
}

Example Structure

Example Code