using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var list = new List<X>

        {

            new() { Name = "Max" },

            new() { Name = "Bram", Bsn = "12345678" }

        };

        var groupedByValid =

            list

            .GroupBy(x => !string.IsNullOrWhiteSpace(x.Bsn));

        if (groupedByValid.Any(g => !g.Key))

        {

            Console.WriteLine($"invalid: {string.Join(',', groupedByValid.Where(g => !g.Key).SelectMany(g => g.Select(x => x.Name)))}");

        }

        Console.WriteLine($"valid: {string.Join(',', groupedByValid.Where(g => g.Key).SelectMany(g => g.Select(x => x.Name)))}");

    }

    private class X

    {

        public string Bsn { get; set; }

        public string Name { get; set; }

    }

}