c# manage progressive identity on domain entity in a client application

Question

i have this problem modelling one of my entities:

1. My entity must have a mandatory ID field that user use to recognize single object
2. User don't want to manual enter ID but it must be generate by Application incrementing by 1 every time
3. Appllication resides only on clients machine, on server there is only the db

According to 1. I create Id field as mandatory (and not nullable), in this way it must be provided to create an instance but how can program know which ID has to use?

Ideally i have to query db to get max id, add 1 and use this value but how can do it? To mee it seem a matter of my domain object, and i want to isolate domain layer from data access layer so i cant call directly repository from by entity.

Possible solutions:

1. I look to Interlocked.Increment but how can use it if any user can create a new instance on his client?
2. Move the logic to Application Logic layer: there i can call the repository, get the ID, create the class and call repository to save class. in this way a move a domain logic (the incremental id is a specific user request) out of domain layer
3. Define repository interface in Domain Layer, call the interface in constructor method of my entity. All doing with dependecy injection, but how? I use ninject in my bootstrapper and all references goes from top to down, how can i revert it for domain layer making it call a superior layer avoiding service locator antipattern (otherwise a can simply write NinjectKernel.Get() in my ctor)
4. i found this article and it's interesting but he use my solution in a bad way (without interface) and don't define the field as mandatory so he can create a new instance withoud specify id

Are there a better solutions?

EDIT

some code to better explain tha case:

Public MyClass {
 public int Id{get;set;}
 .....

 public MyClass(int id){
  if(id != null)
   this.Id = id;
  else
   throw (new ArgumentException)

  .....
 }
}

So i cant do MyClass mc = new MyClass(null); but i need to have the id before

in Data Access Layer i use NHibernate and have Uow / Repository pattern

after i can get a valid ID a can write

UnitOfWork.MyClassRepository.Add(mc);

Answer 1

There's another solution, used by some high load distributed apps. The idea is to create a separate service (web service) to get you a new ID. Inside, implementation could be as simple as:

public int GetNextId()
{
    return Interlocked.Increment(ref _lastId);
}

Although you obviously have to fetch last id from database at service start.

Then, you just make each of your client apps to call this service before creating an entity.

But honestly I'd start with database generated Id or using Guids if possible.

UPDATE: Another option. Assign a unique id (int) to each client. Then use internal counter each time you need a new Id, but mix it with client as as ClientId * 10000 + LastId. This is similar to Hi/Lo generator.

Answer 2

In your domain model you can create repository which takes id from DB. And you must create factory of your domain objects which uses repository for generating id.