using System.Text.Json.Serialization; // [JsonInclude]

public class Book
{
  // constructor to set non-nullable property
  public Book(string title)
  {
    Title = title;
  }

  // properties
  public string Title { get; set; }
  public string? Author { get; set; }

  // fields
  [JsonInclude] // include this field
  public DateTime PublishDate;

  [JsonInclude] // include this field
  public DateTimeOffset Created;

  public ushort Pages;
}