作为官方文档演示，如果要防止Null引用异常，应始终在实体构造函数内部初始化集合导航属性.

As the official documentation demonstrates, you should always initialize your collection navigation properties inside the entity constructor if you want to prevent a Null reference exception.

public Subject()
{
    Students = new HashSet<Student>(); // you may also use List<Student>, but HashSet will guarantee that you are not adding the same Student mistakenly twice
}

只有在至少有一个学生的情况下，实体框架才会使用代理填充 Students 属性(否则使用属性)(如果尚未初始化，则为null).

Entity framework will fill Students property (using a proxy) only if there is at least a student, else it will leave the property as is (null if you have not initialized it).

当实体不是代理时，则只有当使用上下文的原始 entity 进行比较时，Entity Framework才在上下文中调用 SaveChanges()来跟踪其更改.此答案将进一步阐明这种行为.

When the entity is not a proxy, then Entity Framework tracks its changes only when calling SaveChanges() on the context, using its original entity state for comparison. This answer will further clarify this behavior.