ABP框架系列之六：(Value-Objects-值对象)

Introduction

"An object that represents a descriptive aspect of the domain with no conceptual identity is called a VALUE OBJECT." (Eric Evans).

“表示没有概念标识的域的描述方面的对象称为值对象。”（Eric Evans）。

As opposite to Entities, which have their identities (Id), a Value Object has not it's identity. If identities of two Entities are different, they are considered as different objects/entities even if all other properties of those entities are same. Think two different persons have same Name, Surname and Age but they are different people if their identity numbers are different. But, for an Address (which is a classic Value Object) class, if two addresses has same Country, City, Street number... etc. they are considered as the same address.

与具有身份（id）的实体相反，值对象没有它的标识。如果两个实体的身份不同，即使这些实体的所有其他属性相同，它们也被视为不同的对象/实体。认为两个不同的人有相同的名字，姓氏和年龄，但他们是不同的人，如果他们的身份号码是不同的。但是，对于一个地址（这是一个经典值对象）类，如果两个地址具有相同的国家，城市，街道号…等等，它们被认为是同一个地址。

In Domain Driven Design (DDD), Value Object is another type of domain object which can include business logic and is an essential part of the domain.

在领域驱动设计（DDD）中，值对象是另一种类型的域对象，它可以包含业务逻辑，是域的重要组成部分。

Value Object Base Class

ABP has a ValueObject<T> base class which can be inherited in order to easily create Value Object types. Example Address Value Object type:

public class Address : ValueObject<Address>
{
    public Guid CityId { get; private set; } //A reference to a City entity.

    public string Street { get; private set; }

    public int Number { get; private set; }

    public Address(Guid cityId, string street, int number)
    {
        CityId = cityId;
        Street = street;
        Number = number;
    }
}

ValueObject base class overrides equality operator (and other related operator and methods) to compare two value object and assumes that they are identical if all properties are identical.

值类型继承基础类重载相等运算符（以及其他相关的操作和方法）比较两个值类型，假设所有的特性都是相同的他们都是相同的。

So, all of these tests pass:

var address1 = new Address(new Guid("21C67A65-ED5A-4512-AA29-66308FAAB5AF"), "Baris Manco Street", 42);
var address2 = new Address(new Guid("21C67A65-ED5A-4512-AA29-66308FAAB5AF"), "Baris Manco Street", 42);

Assert.Equal(address1, address2);
Assert.Equal(address1.GetHashCode(), address2.GetHashCode());
Assert.True(address1 == address2);
Assert.False(address1 != address2);

Even they are different objects in memory, they are identical for our domain.

即使它们是内存中不同的对象，它们对于我们的域也是相同的。

Best Practices

Here, some best practices for Value Objects:

• Design a value object as immutable (as like the Address above) if there is not a very good reason for designing it as mutable.
• The properties that make up a Value Object should form a conceptual whole. For example, CityId, Street and Number shouldn't be serarate properties of a Person entity. Also, this makes Person entity simpler.

• 如果没有很好的理由将值对象设计为可变的，那么就要设计一个不可变的值对象（如上面的地址）。
  构成一个值对象的属性应该构成一个概念整体。例如，cityid，街道和号码不应该被一个人员实体分离。此外，这使实体更简单。