Builder class needs to create only a single instance of what its building

Question

I am trying to create a builder class that will build a single instance of Person object. Problem is that it creates three Person objects. I know this as it triggers the constructor three times (It writes three times to the console). How can I make it so it only creates the one?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;
using static System.Console;

namespace DotNetDesignPatternDemos.Creational.BuilderFacets
{
  public class Person
  {
    // address
    public string StreetAddress, Postcode, City;

    // employment
    public string CompanyName, Position;

    public int AnnualIncome;

    public Person()
    {
      Console.WriteLine("Person instanced");
    }

    public override string ToString()
    {
      return $"{nameof(StreetAddress)}: {StreetAddress}, {nameof(Postcode)}: {Postcode}, {nameof(City)}: {City}, {nameof(CompanyName)}: {CompanyName}, {nameof(Position)}: {Position}, {nameof(AnnualIncome)}: {AnnualIncome}";
    }
  }

  public class PersonBuilder  
  {
    protected Person person = new Person();
    public PersonAddressBuilder Lives => new PersonAddressBuilder(person);
    public PersonJobBuilder Works => new PersonJobBuilder(person);

    public Person Build()
    {
      return this.person;
    }
  }

  public class PersonJobBuilder : PersonBuilder
  {
    public PersonJobBuilder(Person person)
    {
      this.person = person;
    }

    public PersonJobBuilder At(string companyName)
    {
      person.CompanyName = companyName;
      return this;
    }

    public PersonJobBuilder AsA(string position)
    {
      person.Position = position;
      return this;
    }

    public PersonJobBuilder Earning(int annualIncome)
    {
      person.AnnualIncome = annualIncome;
      return this;
    }
  }

  public class PersonAddressBuilder : PersonBuilder
  {
    public PersonAddressBuilder(Person person)
    {
      this.person = person;
    }

    public PersonAddressBuilder At(string streetAddress)
    {
      person.StreetAddress = streetAddress;
      return this;
    }

    public PersonAddressBuilder WithPostcode(string postcode)
    {
      person.Postcode = postcode;
      return this;
    }

    public PersonAddressBuilder In(string city)
    {
      person.City = city;
      return this;
    }

  }

  public class Demo
  {
    static void Main(string[] args)
    {
      var pb = new PersonBuilder();
      Person person = pb
        .Lives
          .At("123 London Road")
          .In("London")
          .WithPostcode("SW12BC")
        .Works
          .At("Fabrikam")
          .AsA("Engineer")
          .Earning(123000).Build();

      WriteLine(person);
    }
  }
}

Results in

Person instanced
Person instanced
Person instanced
StreetAddress: 123 London Road, Postcode: SW12BC, City: London, CompanyName: Fab
rikam, Position: Engineer, AnnualIncome: 123000

Essentially it will need to create a single Person instance per instance of the Builder.

I dont think Singleton is the solution as I dont need a globally accessible object.

Answer 1

Every derived builder inherits from PersonBuilder so when you new up a PersonJobBuilder you're also new'ing up a Person

var pb = new PersonBuilder();

public PersonAddressBuilder Lives => new PersonAddressBuilder(person);
public PersonJobBuilder Works => new PersonJobBuilder(person);

Each new's up a Person thus, three instances.

Edit

A quick fix would be to make your Person instance static such that only one ever exists. Technically you're tutorials code does work it just doesn't guarantee that one person object is created but you're always operating on the current person.

public class Person
{
    // address
    public string StreetAddress, Postcode, City;

    // employment
    public string CompanyName, Position;

    public int AnnualIncome;

    public Person()
    {
        Console.WriteLine("Person instanced");
    }

    public override string ToString()
    {
        return $"{nameof(StreetAddress)}: {StreetAddress}, {nameof(Postcode)}: {Postcode}, {nameof(City)}: {City}, {nameof(CompanyName)}: {CompanyName}, {nameof(Position)}: {Position}, {nameof(AnnualIncome)}: {AnnualIncome}";
    }
}

public class PersonBuilder
{
    protected static Person _person = new Person();
    public PersonAddressBuilder Lives => new PersonAddressBuilder(_person);
    public PersonJobBuilder Works => new PersonJobBuilder(_person);

    public Person Build()
    {
        return _person;
    }
}

public class PersonJobBuilder : PersonBuilder
{
    public PersonJobBuilder(Person person)
    {
        _person = person;
    }

    public PersonJobBuilder At(string companyName)
    {
        _person.CompanyName = companyName;
        return this;
    }

    public PersonJobBuilder AsA(string position)
    {
        _person.Position = position;
        return this;
    }

    public PersonJobBuilder Earning(int annualIncome)
    {
        _person.AnnualIncome = annualIncome;
        return this;
    }
}

public class PersonAddressBuilder : PersonBuilder
{
    public PersonAddressBuilder(Person person)
    {
        _person = person;
    }

    public PersonAddressBuilder At(string streetAddress)
    {
        _person.StreetAddress = streetAddress;
        return this;
    }

    public PersonAddressBuilder WithPostcode(string postcode)
    {
        _person.Postcode = postcode;
        return this;
    }

    public PersonAddressBuilder In(string city)
    {
        _person.City = city;
        return this;
    }

}

public class Demo
{
    static void Main(string[] args)
    {
        var pb = new PersonBuilder();
        Person person = pb
          .Lives
            .At("123 London Road")
            .In("London")
            .WithPostcode("SW12BC")
          .Works
            .At("Fabrikam")
            .AsA("Engineer")
            .Earning(123000).Build();

        WriteLine(person);
        Console.ReadLine();
    }
}