实体标识 - 使用字符串而非类型 [英] Entity Identity - use of strings instead of type
问题描述
我已经看到了一些DDD的帖子确实书籍，其中实体类是由某种形式的基类是对实体身份类型泛型参数派生的：
I have seen a number of DDD posts and indeed books where entity classes are derived from some form of base class that has a generic parameter for the Entity Identity type:
public interface IEntity<out TKey>
{
TKey Id { get; }
}
public interface IComplexEntity<out TKey, in TEntity>
{
TKey Id { get; }
bool IsSameAs(TEntity entity);
}
//Object Definition
public class TaxPayer : IComplexEntity<string, User>
{
...
}
在弗农的实施领域驱动设计，特定类型用作身份创建：
On Vernon's Implementing Domain Driven design, specific types are created for use as identities:
public class TaxPayerIdentity : Identity
{
public TaxPayerIdentity() { }
public TaxPayerIdentity(string id)
: base(id)
{
}
}
最近，我一直在努力的事件总线外部监听器上进行通信的事件。该问题我是我需要一个通用的消息格式事件信封发送：
More recently I have been working on communicating events on an event bus to external listeners. The "issue" I have is I need a common message format for sending of the event envelope:
public EventEnvelope
{
long EventStoreSequence; // from the event store
bool IsReplay; // if event store is replaying from position 0 of stream
object EventBeingSent; // this is the actual event, i.e. class AddressChanged { string Old; string New; DateTime On; }
object IdentityOfSender; // this is the identity of the entity who raised the event
}
上面的 IdentityOfSender 是一个对象，但实际价值将是字符串， INT ，的Guid 等根据对象的身份类型。
Above the IdentityOfSender is an object, but the actual value would be string, int, Guid etc depending on the objects identity type.
我的问题是，为什么不能简单地用为标识字符串？毕竟，的GUID，整数，名称，数量都可以重新presented为字符串，他们是在一个共同的格式字符串容易比较 - 这不仅会使EventEnvelope一个字符串更容易为通用格式，它会使实体更容易，而不需要基类或特殊类型的处理？
My question is why not simply use a string for the identity? After all, Guids, integer, names, numbers can all be represented as strings and they are easily comparable as strings in a common format - this not only would make the EventEnvelope easier with a string as a common format, it would make Entities easier to handle without need for base classes or special types?
因此，在总结，为什么人们不建议使用的字符串进行身份为通用格式（或者我还没有看到它），而不是说说基类和泛型类型的身份？
推荐答案
由于在跨文化的字符集字符串比较是不容易的。可与字符串进行比较，最近的数据类型是GUID。但是，即使GUID可以有不同的字符串重新presentation。
Because string comparison in cross-culture charset is not easy. The closest data type that can be compared with string is GUID. But even GUID can have different string representation.
例如：
{FD49D6AE-019C-4118-B7D1-A4DA54E4474F},
fd49d6ae-019c-4118-b7d1-a4da54e4474f,
FD49D6AE019C4118B7D1A4DA54E4474F
这三家公司均相同的GUID值，但不同的字符串比较。号码是著名的千位分隔符/小数点格式的区别。
All three have same GUID value but different string comparison. Number is famous for the thousand separator / decimal point format difference.
此外，我认为，对于身份的自定义类是提供明确的身份比较。例如：信用卡号码的有六位发行人识别号码（IIN）在第6位（适用的维基百科）。也就是说，让你在领域驱动设计更高的灵活性。是的，这设计prevent您使用我为人人的数据类型。
Moreover, I believe that the custom class for identity is to provide explicit identity comparison. Example: Credit card number's has a six-digit Issuer Identification Number (IIN) in the first six digit (wikipedia). That is, enable you higher flexibility in domain-driven design. And yes, that design prevent you from using "one for all" data type.
但是，您可以通过对象映射连接使用外部总线的code-DE code型虽然。这样，就可以避免打破了DDD，并在同一时间在整个外部总线提供相同的格式。
However you can use encode-decode model for external bus through object mapping though. That way you can avoid breaking the DDD and at the same time providing same format throughout external buses.
这一边，它是一个一般准则。而在软件工程的每一个准则都有优点和缺点。使用一个你找到最合适的，知道最好的，但保持它在你的设计是一致的。
That aside, it is a general guidelines. And every guidelines in software engineering has pros and cons. Use one you find most suitable and know the best, but keep it consistent across your design.
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