How to map a dictionary property of an entity to a string field of a table in Entity Framework Core v2.1+

MyDbContext.cs

public class MyDbContext : DbContext
{
    public DbSet<User> Users { get; set; }

    public MyDbContext(DbContextOptions<MyDbContext> options)
        : base(options)
    {

    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<User>(b =>
        {
            b.Property(u => u.UserName).IsRequired().HasMaxLength(128);

            b.Property(u => u.ExtraProperties)
                .HasConversion(
                    d => JsonConvert.SerializeObject(d, Formatting.None),
                    s => JsonConvert.DeserializeObject<Dictionary<string, object>>(s)
                )
                .HasMaxLength(4000)
                .IsRequired();
        });
    }
}

Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        using (var context = new MyDbContext(CreateDbContextOptions()))
        {
            var userJohn = new User("john");
            var userNeo = new User("neo")
            {
                ExtraProperties =
                {
                    ["Age"] = 42
                }
            };

            context.Users.Add(userJohn);
            context.Users.Add(userNeo);
            context.SaveChanges();
        }

        using (var context = new MyDbContext(CreateDbContextOptions()))
        {
            foreach (var user in context.Users)
            {
                Console.WriteLine(user);
            }
        }

        Console.ReadLine();
    }

    public static DbContextOptions<MyDbContext> CreateDbContextOptions()
    {
        return new DbContextOptionsBuilder<MyDbContext>()
            .UseSqlServer("Server=localhost;Database=EfCoreValueConverterDemo;Trusted_Connection=True;MultipleActiveResultSets=true")
            .Options;
    }
}

User.cs

public class User
{
    public Guid Id { get; set; }
    
    public string UserName { get; set; }

    public Dictionary<string, object> ExtraProperties { get; set; }

    public User()
    {
        Id = Guid.NewGuid();
        ExtraProperties = new Dictionary<string, object>();
    }
    
    public override string ToString()
    {
        return "# User: " + JsonConvert.SerializeObject(this, Formatting.Indented);
    }
}