Builder pattern

The Builder design pattern is one of the Design Patterns[1] that describe how to solve recurring design problems in object-oriented software.

The Builder design pattern solves problems like:[2]

• How can a class (the same construction process) create different representations of a complex object?
• How can a class that includes creating a complex object be simplified?

Creating and assembling the parts of a complex object directly within a class is inflexible. It commits the class to creating a particular representation of the complex object and makes it impossible to change the representation later independently from (without having to change) the class.

The Builder design pattern describes how to solve such problems:

• Encapsulate creating and assembling the parts of a complex object in a separate Builder object.
• A class delegates object creation to a Builder object instead of creating the objects directly.

A class (the same construction process) can delegate to different Builder objects to create different representations of a complex object.

The intent of the Builder design pattern is to separate the construction of a complex object from its representation. By doing so, the same construction process can create different representations.[1]

Advantages of the Builder pattern include:[3]

• Allows you to vary a product's internal representation.
• Encapsulates code for construction and representation.
• Provides control over steps of construction process.

Disadvantages of the Builder pattern include:[3]

• A distinct ConcreteBuilder must be created for each type of product.
• Builder classes must be mutable.
• May hamper/complicate dependency injection.

A C# example:

/// <summary>
/// Represents a product created by the builder.
/// </summary>
public class Bicycle
{
    public Bicycle(string make, string model, string colour, int height)
    {
        Make = make;
        Model = model;
        Colour = colour;
        Height = height;
    }

    public string Make { get; set; }
    public string Model { get; set; }
    public int Height { get; set; }
    public string Colour { get; set; }
}

/// <summary>
/// The builder abstraction.
/// </summary>
public interface IBicycleBuilder
{
    Bicycle GetResult();

    string Colour { get; set; }
    int Height { get; set; }
}

/// <summary>
/// Concrete builder implementation.
/// </summary>
public class GTBuilder : IBicycleBuilder
{
    public Bicycle GetResult()
    {
        return Height == 29 ? new Bicycle("GT", "Avalanche", Colour, Height) : null;        
    }

    public string Colour { get; set; }
    public int Height { get; set; }
}

/// <summary>
/// The director.
/// </summary>
public class MountainBikeBuildDirector
{
    private IBicycleBuilder _builder;

    public MountainBikeBuildDirector(IBicycleBuilder builder) 
    {
        _builder = builder;
    }

    public void Construct()
    {
        _builder.Colour = "Red";
        _builder.Height = 29;
    }

    public Bicycle GetResult()
	{
		return this._builder.GetResult();
	}
}

public class Client
{
    public void DoSomethingWithBicycles()
    {
        var director = new MountainBikeBuildDirector(new GTBuilder());
        // Director controls the stepwise creation of product and returns the result.
        director.Construct();
        Bicycle myMountainBike = director.GetResult();
    }
}

The Director assembles a bicycle instance in the example above, delegating the construction to a separate builder object that has been given to the Director by the Client.

• Currying