• Builder pattern isolates a view from its logic.
  • It separates the construction of a complex object (Process) from its representation (Data).
  • Therefore, the same construction process can create classes with different data.
  • This pattern is concerned with how a single object is made up by the different factories.
• The client can still direct the steps taken by the Builder, without knowing how the actual work is accomplished.
• Builders frequently encapsulate the construction of "Composite" objects because the procedures involved are often repetitive and complex.
• A scenario where the Builder can be helpful is when building a code generator.
  • Say you’re writing an application that generates stored procedures for different target databases (SQL Server, Oracle, Db2).
  • The actual output is quite different but the different steps of creating the separate procedures that implement the CRUD statements are similar.
• Factory patterns create objects in one single step, while Builder gives you more control in that each step in the construction process can be customized

Participants

• Product (Vehicle)
  • Represents the complex object under construction.
  • Each implementation of the ConcreteBuilder builds the product's internal representation and defines the process by which it's assembled.
• Builder (IVehicleBuilder)
  • Specifies an abstract interface for creating parts of a **Product** object.
  • This interfaces defines how to a **ConcreteBuilder** instance represents itself.
• ConcreteBuilder (CarBuilder, MotorCycleBuilder)
  • 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 (VehicleMaker)
  • Constructs an object using the Builder interface.

var vehicleMaker1 = new VehicleMaker(new MotorCycleBuilder());
vehicleMaker1.BuildVehicle();
var vehicle1 = vehicleMaker1.GetVehicle();
Console.WriteLine(vehicle1.GetFeatures());

var vehicleMaker2 = new VehicleMaker(new CarBuilder());
vehicleMaker2.BuildVehicle();
var vehicle2 = vehicleMaker2.GetVehicle();
Console.WriteLine(vehicle2.GetFeatures());

public interface IVehicle
{
    string VehicleType { get; set; }
    string this[string key] { get; set; }
    string GetFeatures();
}

public class Vehicle : IVehicle
{
    public string VehicleType { get; set; }

    private readonly Dictionary<string, string> _parts = new Dictionary<string, string>();
    public string this[string key]
    {
        get => _parts[key];
        set => _parts[key] = value;
    }

    public string GetFeatures() => string.Join('\\n', _parts.Select(x => $"{x.Key}: {x.Value}"));
}

public interface IVehicleBuilder
{
    void Construct();
    IVehicle GetVehicle();
}

public class MotorCycleBuilder : IVehicleBuilder
{
    private IVehicle _vehicle;

    public void Construct()
    {
        _vehicle = new Vehicle
        {
            VehicleType = "MotorCycle",
            ["frame"] = "MotorCycle Frame",
            ["engine"] = "500 cc",
            ["wheels"] = "2",
            ["doors"] = "0"
        };
    }

    public IVehicle GetVehicle() => _vehicle;
}

public class CarBuilder : IVehicleBuilder
{
    private IVehicle _vehicle;

    public void Construct()
    {
        _vehicle = new Vehicle
        {
            VehicleType = "Car",
            ["frame"] = "Car Frame",
            ["engine"] = "2500 cc",
            ["wheels"] = "4",
            ["doors"] = "4"
        };
    }

    public IVehicle GetVehicle() => _vehicle;
}

public class VehicleMaker
{
    private IVehicleBuilder _builder;

    public VehicleMaker(IVehicleBuilder builder) => _builder = builder;

    public void BuildVehicle() => _builder.Construct();
    public IVehicle GetVehicle() => _builder.GetVehicle();
}

Rules of Thumb

• Sometimes creational patterns are complementary: Builder can use one of the other patterns to implement which components get built.
  • Abstract Factory, Builder, and Prototype can use Singleton in their implementations.
• Builder focuses on constructing a complex object step by step.
• Abstract Factory emphasizes a family of product objects (either simple or complex).
• Builder returns the product as a final step, but as far as the Abstract Factory is concerned, the product gets returned immediately.
• Builder often builds a Composite.

C# Design Patterns - Faceted Builder - Code Maze