recordsv3+builders.cs

using System;

public class Person
{
    public int Age { get; }
    public string Name { get; }

    public Person(int age, string name)
        : this(null, new Builder { Age = age, Name = name })
    {
    }

    protected Person(Person prototype, in Builder builder)
    {
        if (prototype != null)
        {
            Age = builder.HasAge ? builder.Age : prototype.Age;
            Name = builder.HasName ? builder.Name : prototype.Name;
        }
        else
        {
            Age = builder.Age;
            Name = builder.Name;
        }

        // extra constructor logic/validation.
    }

    // Can have custom logic in here to return 'this' if builder wouldn't
    // change what we already are.
    public virtual Person With(in Builder builder)
        => new Person(this, builder);

    public ref struct Builder
    {
        private byte _set;
        private int _age;
        private string _name;

        public int Age
        {
            get => _age;
            set { _age = value; _set |= 1; }
        }

        public string Name
        {
            get => _name;
            set { _name = value; _set |= 2; }
        }

        public bool HasAge => (_set & 1) == 1;
        public bool HasName => (_set & 2) == 2;
    }
}

public class Student : Person
{
    public double Gpa { get; }

    public Student(int age, string name, double gpa) 
        : this(null, new Builder { Age = age, Name = name, Gpa = gpa })
    {
    }

    protected Student(Student prototype, in Builder builder)
        : base(prototype, builder.Parent)
    {
        if (prototype != null)
        {
            Gpa = builder.HasGpa ? builder.Gpa : prototype.Gpa;
        }
        else
        {
            Gpa = builder.Gpa;
        }

        // extra constructor logic/validation.
    }

    public override Person With(in Person.Builder builder)
        => With(new Builder { Parent = builder });

    public virtual Student With(in Builder builder)
        => new Student(this, builder);

    public new ref struct Builder
    {
        public Person.Builder Parent;
        private byte _set;
        private double _gpa;

        public int Age
        {
            get => Parent.Age;
            set => Parent.Age = value;
        }

        public string Name
        {
            get => Parent.Name;
            set => Parent.Name = value;
        }

        public double Gpa
        {
            get => _gpa;
            set { _gpa = value; _set |= 1; }
        }

        public bool HasAge => Parent.HasAge;
        public bool HasName => Parent.HasName;
        public bool HasGpa => (_set & 1) == 1;
    }
}

class Usage
{
    static void Main()
    {
        var madsStudent = new Student(age: 39, name: "mads", gpa: 4.0);
        var cyrusStudent = madsStudent.With(new Student.Builder
        {
            Name = "cyrus",
        });

        var madsPerson = (Person)madsStudent;
        var dustinPerson = madsPerson.With(new Person.Builder { Name = "dustin " });
    }
}