static class 初始化 static 参考

Question

In the following code, why Library.genres[n].booklist are null at runtime for all items in Library.genres ? Just like if the definitions for dramabooks and kidsbooks were not being considered for the static initialization:

static class Library
{
   public static List<Genre> genres = new List<Genre>()
   {
      new Genre("Drama", dramabooks),
      new Genre("Kids", kidsbooks)
   };

   public static List<Book> dramabooks = new List<Book>() {
      new Book("book1"), 
      new Book("book2")
   };

   public static List<Book> kidsbooks = new List<Book>(){
      new Book("book3"),
      new Book("book4")
   };
}

class Genre
{
   string catname;
   List<Book> booklist;

   Genre(string catname, List<Book> booklist)
   {
      this.catname = catname;
      this.booklist = booklist;
   }
}

class Book
{
   string title;

   Book(string title)
   {
      this.title = title;
   }
}

Answer 1

Disregarding any other code errors (and there are many), This is due to how static fields initialise, which is in the textual order of which they appear. kidsbooks and dramabooks haven't been initialised when genres initialises

From the ECMA C# Specifications

15.5.6.2 Static field initialization

The static field variable initializers of a class correspond to a sequence of assignments that are executed in the textual order in which they appear in the class declaration (§15.5.6.1). Within a partial class, the meaning of "textual order" is specified by §15.5.6.1. If a static constructor (§15.12) exists in the class, execution of the static field initializers occurs immediately prior to executing that static constructor. Otherwise, the static field initializers are executed at an implementation-dependent time prior to the first use of a static field of that class.

In short, use a static constructor where you can express the order of initialisation explicitly, it's less prone to error and more declarative

static class Library
{
   static Library()
   {

      dramabooks = new List<Book>()
      {
         new Book("book1"),
         new Book("book1")
      };

      kidsbooks = new List<Book>()
      {
         new Book("book3"),
         new Book("book4")
      };
      genres = new List<Genre>()
      {
         new Genre("Drama", dramabooks),
         new Genre("Kids", kidsbooks)
      };

   }

   public static List<Genre> genres;

   public static List<Book> dramabooks;

   public static List<Book> kidsbooks;
}