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## 值对象
> 一个对象,表示领域的描述方面,没有概念上的身份被称为 值对象.
>
> (Eric Evans)
属性相同但`Id`不同的两个[实体](https://docs.abp.io/zh-Hans/abp/latest/Entities) 被视为不同的实体.但是,值对象没有`Id`
## 值对象的类
值对象是一个抽象类,可以继承它来创建值对象类
** 示例: An Address class**
```csharp
public class Address : ValueObject
{
public Guid CityId { get ; private set ; }
public string Street { get ; private set ; }
public int Number { get ; private set ; }
private Address ()
{
}
public Address (
Guid cityId ,
string street ,
int number )
{
CityId = cityId ;
Street = street ;
Number = number ;
}
protected override IEnumerable < object > GetAtomicValues ()
{
yield return Street ;
yield return CityId ;
yield return Number ;
}
}
```
- 值对象类必须实现 `GetAtomicValues()` 方法来返回原始值
### ValueEquals
`ValueObject.ValueEquals(...)` 用于检测两个值是否相等
** 示例: Check if two addresses are equals**
```csharp
Address address1 = ...
Address address2 = ...
if ( address1 . ValueEquals ( address2 )) //Check equality
{
...
}
```
## 最佳实践
以下是使用值对象时的一些最佳实践:
- 如果没有充分的理由将值对象设计为可变的,则将其设计为**不可变**（如上面的地址）.
- 构成一个值对象的属性应该形成一个概念整体.例如：
## 另请参阅
- [实体 ](Entities.md )
