Builder.md

Builder

Definition

Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Frequency of use:

Medium low

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UML class diagram

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Participants

The classes and objects participating in this pattern are:

• Builder (VehicleBuilder)

• specifies an abstract interface for creating parts of a Product object

• ConcreteBuilder (MotorCycleBuilder, CarBuilder, ScooterBuilder)

• constructs and assembles parts of the product by implementing the Builder interface

• defines and keeps track of the representation it creates

• provides an interface for retrieving the product

• Director (Shop)

  • constructs an object using the Builder interface

• Product (Vehicle)

  • represents the complex object under construction. ConcreteBuilder builds the product's internal representation and defines the process by which it's assembled
  • includes classes that define the constituent parts, including interfaces for assembling the parts into the final result

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Structural code in C#

This structural code demonstrates the Builder pattern in which complex objects are created in a step-by-step fashion. The construction process can create different object representations and provides a high level of control over the assembly of the objects.

using System;
using System.Collections.Generic;

namespace DoFactory.GangOfFour.Builder.Structural
{
    /// <summary>
    /// MainApp startup class for Structural
    /// Builder Design Pattern.
    /// </summary>
    public class MainApp
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        public static void Main()
        {
            // Create director and builders
            Director director = new Director();

            Builder b1 = new ConcreteBuilder1();
            Builder b2 = new ConcreteBuilder2();

            // Construct two products
            director.Construct(b1);
            Product p1 = b1.GetResult();
            p1.Show();

            director.Construct(b2);
            Product p2 = b2.GetResult();
            p2.Show();

            // Wait for user
            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Director' class
    /// </summary>
    class Director
    {
        // Builder uses a complex series of steps
        public void Construct(Builder builder)
        {
            builder.BuildPartA();
            builder.BuildPartB();
        }
    }

    /// <summary>
    /// The 'Builder' abstract class
    /// </summary>
    abstract class Builder
    {
        public abstract void BuildPartA();
        public abstract void BuildPartB();
        public abstract Product GetResult();
    }

    /// <summary>
    /// The 'ConcreteBuilder1' class
    /// </summary>
    class ConcreteBuilder1 : Builder
    {
        private Product _product = new Product();

        public override void BuildPartA()
        {
            _product.Add("PartA");
        }

        public override void BuildPartB()
        {
            _product.Add("PartB");
        }

        public override Product GetResult()
        {
            return _product;
        }
    }

    /// <summary>
    /// The 'ConcreteBuilder2' class
    /// </summary>
    class ConcreteBuilder2 : Builder
    {
        private Product _product = new Product();

        public override void BuildPartA()
        {
            _product.Add("PartX");
        }

        public override void BuildPartB()
        {
            _product.Add("PartY");
        }

        public override Product GetResult()
        {
            return _product;
        }
    }

    /// <summary>
    /// The 'Product' class
    /// </summary>
    class Product
    {
        private List<string> _parts = new List<string>();

        public void Add(string part)
        {
            _parts.Add(part);
        }

        public void Show()
        {
            Console.WriteLine("\nProduct Parts -------");
            foreach (string part in _parts)
                Console.WriteLine(part);
        }
    }
}

Output

Product Parts -------\
PartA\
PartB

Product Parts -------\
PartX\
PartY