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Composite

Composite in C#

Composite is a structural design pattern that allows composing objects into a tree-like structure and work with the it as if it was a singular object.

Composite became a pretty popular solution for the most problems that require building a tree structure. Composite’s great feature is the ability to run methods recursively over the whole tree structure and sum up the results.

Learn more about Composite

Usage of the pattern in C#

Complexity:

Popularity:

Usage examples: The Composite pattern is pretty common in C# code. It’s often used to represent hierarchies of user interface components or the code that works with graphs.

Identification: The composite is easy to recognize by behavioral methods taking an instance of the same abstract/interface type into a tree structure.

Conceptual Example

This example illustrates the structure of the Composite design pattern. It focuses on answering these questions:

  • What classes does it consist of?
  • What roles do these classes play?
  • In what way the elements of the pattern are related?

Program.cs: Conceptual Example

using System;
using System.Collections.Generic;

namespace RefactoringGuru.DesignPatterns.Composite.Conceptual
{
    // The base Component class declares common operations for both simple and
    // complex objects of a composition.
    abstract class Component
    {
        public Component() { }

        // The base Component may implement some default behavior or leave it to
        // concrete classes (by declaring the method containing the behavior as
        // "abstract").
        public abstract string Operation();

        // In some cases, it would be beneficial to define the child-management
        // operations right in the base Component class. This way, you won't
        // need to expose any concrete component classes to the client code,
        // even during the object tree assembly. The downside is that these
        // methods will be empty for the leaf-level components.
        public virtual void Add(Component component)
        {
            throw new NotImplementedException();
        }

        public virtual void Remove(Component component)
        {
            throw new NotImplementedException();
        }

        // You can provide a method that lets the client code figure out whether
        // a component can bear children.
        public virtual bool IsComposite()
        {
            return true;
        }
    }

    // The Leaf class represents the end objects of a composition. A leaf can't
    // have any children.
    //
    // Usually, it's the Leaf objects that do the actual work, whereas Composite
    // objects only delegate to their sub-components.
    class Leaf : Component
    {
        public override string Operation()
        {
            return "Leaf";
        }

        public override bool IsComposite()
        {
            return false;
        }
    }

    // The Composite class represents the complex components that may have
    // children. Usually, the Composite objects delegate the actual work to
    // their children and then "sum-up" the result.
    class Composite : Component
    {
        protected List<Component> _children = new List<Component>();
        
        public override void Add(Component component)
        {
            this._children.Add(component);
        }

        public override void Remove(Component component)
        {
            this._children.Remove(component);
        }

        // The Composite executes its primary logic in a particular way. It
        // traverses recursively through all its children, collecting and
        // summing their results. Since the composite's children pass these
        // calls to their children and so forth, the whole object tree is
        // traversed as a result.
        public override string Operation()
        {
            int i = 0;
            string result = "Branch(";

            foreach (Component component in this._children)
            {
                result += component.Operation();
                if (i != this._children.Count - 1)
                {
                    result += "+";
                }
                i++;
            }
            
            return result + ")";
        }
    }

    class Client
    {
        // The client code works with all of the components via the base
        // interface.
        public void ClientCode(Component leaf)
        {
            Console.WriteLine($"RESULT: {leaf.Operation()}\n");
        }

        // Thanks to the fact that the child-management operations are declared
        // in the base Component class, the client code can work with any
        // component, simple or complex, without depending on their concrete
        // classes.
        public void ClientCode2(Component component1, Component component2)
        {
            if (component1.IsComposite())
            {
                component1.Add(component2);
            }
            
            Console.WriteLine($"RESULT: {component1.Operation()}");
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            Client client = new Client();

            // This way the client code can support the simple leaf
            // components...
            Leaf leaf = new Leaf();
            Console.WriteLine("Client: I get a simple component:");
            client.ClientCode(leaf);

            // ...as well as the complex composites.
            Composite tree = new Composite();
            Composite branch1 = new Composite();
            branch1.Add(new Leaf());
            branch1.Add(new Leaf());
            Composite branch2 = new Composite();
            branch2.Add(new Leaf());
            tree.Add(branch1);
            tree.Add(branch2);
            Console.WriteLine("Client: Now I've got a composite tree:");
            client.ClientCode(tree);

            Console.Write("Client: I don't need to check the components classes even when managing the tree:\n");
            client.ClientCode2(tree, leaf);
        }
    }
}

Output.txt: Execution result

Client: I get a simple component:
RESULT: Leaf

Client: Now I've got a composite tree:
RESULT: Branch(Branch(Leaf+Leaf)+Branch(Leaf))

Client: I don't need to check the components classes even when managing the tree:
RESULT: Branch(Branch(Leaf+Leaf)+Branch(Leaf)+Leaf)

Composite in Other Languages

Design Patterns: Composite in Java Design Patterns: Composite in PHP Design Patterns: Composite in Python Design Patterns: Composite in Ruby Design Patterns: Composite in Swift Design Patterns: Composite in TypeScript