Introduction

        Composite Pattern is another Structural Design Pattern which let us treat single object and group of objects in same way. means no need to write different implementations for single and composite objects. It can be understood by a tree where we have leaf node and branch node and both can be treated in same way.

Implementation

        To understand Composite Pattern we will create an interface IComputer with a method Print that prints whether computer is a laptop or desktop, we will implement the method in subclsses:

interface IComputer
{
    void Print();
}
class Laptop : IComputer
{
    public void Print()
    {
        Console.WriteLine("Laptop");
    }
}
class Desktop : IComputer
{
    public void Print()
    {
        Console.WriteLine("Desktop");
    }
}

Now we will create composite class which will hold composition of IComputer class and provides some extra methods to manipulate this composition:

class CompositeComputer : IComputer
{
    //Composition
    List<IComputer> computers = new List<IComputer>();

    //Additional methods
    public void Add(IComputer computer)
    {
        computers.Add(computer);
    }
    public void AddRange(params IComputer[] computer)
    {
        computers.AddRange(computer);
    }
    public void Remove(IComputer computer)
    {
        computers.Remove(computer);
    }

    //Implements from interface
    public void Print()
    {
        foreach (IComputer computer in computers)
        {
            computer.Print();
        }
    }
}

Above class is composition class which has composite type and implements some methods like Add, Remove and Implements Print. Now let's execute this code:

CompositeComputer computer = new CompositeComputer();
CompositeComputer computer1 = new CompositeComputer();
computer1.Add(new Laptop());

computer.Add(new Desktop());
computer.Add(new Desktop());
computer.Add(new Laptop());
computer.Add(computer1);

computer.Print();

Above code adds desktop, laptop as well composite type in list and print all of them using same method, output:

Desktop

Desktop

Laptop

Laptop