EF 测试：不同对象的同一外键是否指向同一对象

问题

在一对多关系中，例如一个 Class 对多个 Student ，使用 ToList() 取出一个 List 的 Student 对象，
那么 List 中的 Student 对象外键对应的 Class 到底是各自独立，还是会共用一个对象？
那么分开取出的 Student 们对应的 Class 共用吗？
加上 AsNoTracking() 方法呢？

测试代码

model 定义

public class Class
{
    public int Id { get; set; }

    public string Number { get; set; }

    public List<Student> Students { get; set; } = new List<Student>();
}

public class Student
{
    public int Id { get; set; }

    public string Name { get; set; }

    public Class Class { get; set; }
}

DbContext 的继承和重写

public class AppDbContext : DbContext
{
    public DbSet<Class> Classes { get; set; }
    public DbSet<Student> Students { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFGetStarted.ConsoleAppFortest.DbForTest;Trusted_Connection=True");
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
    }
}

主程序

public static void Main(string[] args)
{
    var dbContext = new AppDbContext();
    dbContext.Database.EnsureCreated();

    dbContext.Classes.RemoveRange(dbContext.Classes);
    dbContext.Students.RemoveRange(dbContext.Students);
    dbContext.SaveChanges();

    var c = new Class { Number = "001" };
    c.Students.Add(new Student()
    {
        Name = "Hale Lu"
    });
    c.Students.Add(new Student()
    {
        Name = "PM Extra"
    });
    dbContext.Classes.Add(c);
    dbContext.SaveChanges();

    Console.WriteLine("AsNoTracking:");
    var studentsAsNoTracking = dbContext.Students.Include(s => s.Class).AsNoTracking().ToList();
    Console.WriteLine("修改前：");
    Console.WriteLine("students[0].Class.Number = " + studentsAsNoTracking[0].Class.Number);
    Console.WriteLine("students[1].Class.Number = " + studentsAsNoTracking[1].Class.Number);
    studentsAsNoTracking[0].Class.Number = "002";
    Console.WriteLine("修改后：");
    Console.WriteLine("students[0].Class.Number = " + studentsAsNoTracking[0].Class.Number);
    Console.WriteLine("students[1].Class.Number = " + studentsAsNoTracking[1].Class.Number);
    Console.WriteLine("ToList方法查询结果的外键对象" + (studentsAsNoTracking[0].Class.Number == studentsAsNoTracking[1].Class.Number ? "" : "不") + "共用一个对象");
    Console.WriteLine();

    var aAsNoTracking = dbContext.Students.Include(s => s.Class).AsNoTracking().First();
    var bAsNoTracking = dbContext.Students.Include(s => s.Class).AsNoTracking().Last();
    Console.WriteLine("修改前：");
    Console.WriteLine("a.Class.Number = " + aAsNoTracking.Class.Number);
    Console.WriteLine("b.Class.Number = " + bAsNoTracking.Class.Number);
    aAsNoTracking.Class.Number = "003";
    Console.WriteLine("修改后：");
    Console.WriteLine("a.Class.Number = " + aAsNoTracking.Class.Number);
    Console.WriteLine("b.Class.Number = " + bAsNoTracking.Class.Number);
    Console.WriteLine("不同的方法查询结果的外键对象" + (aAsNoTracking.Class.Number == bAsNoTracking.Class.Number ? "" : "不") + "共用一个对象");
    Console.WriteLine();

    Console.WriteLine();
    Console.WriteLine("不加AsNoTracking:");
    var students = dbContext.Students.Include(s => s.Class).ToList();
    Console.WriteLine("修改前：");
    Console.WriteLine("students[0].Class.Number = " + students[0].Class.Number);
    Console.WriteLine("students[1].Class.Number = " + students[1].Class.Number);
    students[0].Class.Number = "002";
    Console.WriteLine("修改后：");
    Console.WriteLine("students[0].Class.Number = " + students[0].Class.Number);
    Console.WriteLine("students[1].Class.Number = " + students[1].Class.Number);
    Console.WriteLine("ToList方法查询结果的外键对象" + (students[0].Class.Number == students[1].Class.Number ? "" : "不") + "共用一个对象");
    Console.WriteLine();

    var a = dbContext.Students.Include(s => s.Class).First();
    var b = dbContext.Students.Include(s => s.Class).Last();
    Console.WriteLine("修改前：");
    Console.WriteLine("a.Class.Number = " + a.Class.Number);
    Console.WriteLine("b.Class.Number = " + b.Class.Number);
    a.Class.Number = "003";
    Console.WriteLine("修改后：");
    Console.WriteLine("a.Class.Number = " + a.Class.Number);
    Console.WriteLine("b.Class.Number = " + b.Class.Number);
    Console.WriteLine("不同的方法查询结果的外键对象" + (a.Class.Number == b.Class.Number ? "" : "不") + "共用一个对象");
    Console.WriteLine();
}

运行结果

AsNoTracking:
修改前：
students[0].Class.Number = 001
students[1].Class.Number = 001
修改后：
students[0].Class.Number = 002
students[1].Class.Number = 002
ToList方法查询结果的外键对象共用一个对象

修改前：
a.Class.Number = 001
b.Class.Number = 001
修改后：
a.Class.Number = 003
b.Class.Number = 001
不同的方法查询结果的外键对象不共用一个对象


不加AsNoTracking:
修改前：
students[0].Class.Number = 001
students[1].Class.Number = 001
修改后：
students[0].Class.Number = 002
students[1].Class.Number = 002
ToList方法查询结果的外键对象共用一个对象

修改前：
a.Class.Number = 002
b.Class.Number = 002
修改后：
a.Class.Number = 003
b.Class.Number = 003
不同的方法查询结果的外键对象共用一个对象

结论

不加 AsNoTracking() 的情况下，所有的同一外键指向同一对象；
加 AsNoTracking() 的情况下，分开取出的同一外键指向不同的对象，通过 List 方式取出的同一外键指向相同对象。