Fluent API Builder Quirks

Program.cs

using System;
using System.Linq.Expressions;
using System.Reflection;

namespace BuilderPattern
{
    class Program
    {
        static void Main(string[] args)
        {

            RunBuilderExample2();

            Console.WriteLine("Done!!");
        }

        private static void RunBuilderExample2()
        {
            Adventurer dink = Adventurer.Create
                   .WithName("Some name")
                   .WithSex(Sex.Female)
                   .WithHP(100)
                   .Stats
                       .Assign(a => a.Stats.Intelligence, 80)
                       .Assign(a => a.Stats.Strength, 10)
                   .Build();
                   
            Console.WriteLine(dink);
        }
       
    }

    public interface IAdventurerBuilder
    {
        IAdventurerBuilder WithName(string name);
        IAdventurerBuilder WithSex(Sex sex);
        IAdventurerBuilder WithHP(int hp);

        IAdventurerStatsBuilder Stats { get; }

        Adventurer Build();
    }

    public interface IAdventurerStatsBuilder : IAdventurerBuilder
    {
        IAdventurerStatsBuilder Assign<TProp>(Expression<Func<Adventurer, TProp>> statSelector, int value);
    }


    public enum Sex { Male, Female }

    public class Adventurer
    {
        private Adventurer() { }
        public Stats Stats { get; set; } = new Stats();
        public string Name { get; set; }
        public int HitPoints { get; set; }
        public Sex Sex { get; set; }
        public static IAdventurerBuilder Create => new AdventureBuilderImpl();
        private class AdventureBuilderImpl : IAdventurerBuilder
        {
            protected Adventurer adventurer = new Adventurer();

            public IAdventurerStatsBuilder Stats => new AdventureStatsBuilderImpl(adventurer);

            public Adventurer Build()
            {
                return adventurer;
            }

            public IAdventurerBuilder WithHP(int hp)
            {
                adventurer.HitPoints = hp;
                return this;
            }

            public IAdventurerBuilder WithName(string name)
            {
                adventurer.Name = name;
                return this;
            }

            public IAdventurerBuilder WithSex(Sex sex)
            {
                adventurer.Sex = sex;
                return this;
            }
        }

        private class AdventureStatsBuilderImpl : AdventureBuilderImpl, IAdventurerStatsBuilder
        {
            //private Adventurer adventurer;
            public AdventureStatsBuilderImpl(Adventurer adventurer) => this.adventurer = adventurer;

            public IAdventurerStatsBuilder Assign<TProp>(Expression<Func<Adventurer, TProp>> statSelector, int value)
            {
                // a => a.Stats.Intelligence

                PropertyInfo property = ((MemberExpression)statSelector.Body).Member as PropertyInfo;
                property.SetValue(adventurer.Stats, value);

                return this;
            }
        }
        
        public override string ToString()
        {
            return $"Name : {Name} | Gender : {Sex.ToString()} | HitPoints : {HitPoints} | Stats : I-{Stats.Intelligence}/S-{Stats.Strength}";
        }
    }

    public class Stats
    {
        public int Intelligence { get; set; }
        public int Strength { get; set; }
    }
}