Question

Every entity class in my application must implement the following interface:

public interface IEntity<T> {
    T Id { get; set; }
}

Almost 100% of the time the type of T will be an int. However I have to deal with cases were the id may be a composite id. E.g. I could have the following implementations:

public class User : IEntity<int> {
    public int Id { get; set; }
    ...
}

public class Site : IEntity<int> {
    public int Id { get; set; }
    ...
}

public class UserSite : IEntity<UserSiteIdentifier> {
    public UserSiteIdentifier Id { get; set; }
    ...
}

// Note: IIdentifier doesn't have any members
public class UserSiteIdentifier : IIdentifier {
    public User User { get; set; }
    public Site Site { get; set; }
    ...

    public override ToString() {
        return User.Id + "|" + Site.Id;
    }
}

Now given an entity instance (where the type is unknown) I need to retrieve the id and convert it to a string. I could say:

object entity = ???;
string id;

if (entity is IEntity<int>)
    id = ((IEntity<int>)entity).Id.ToString();
else if (entity is IEntity<IIdentifier>)
    id = ((IEntity<IIdentifier>)entity).Id.ToString();

But this code doesn’t sit right with me as I have to repeat almost the same code just to handle composite id’s.

I’d appreciate it if someone could show me a cleaner solution. This application is still a prototype and is completely open to suggestions. Thanks

Answer 1

You can introduce IEntity interface:

public interface IEntity<T>:IEntity {
    new T Id { get; set; }
}

 public interface IEntity {
    object Id { get; }
}

There will be some inconvenience while implementing two properties which are supposed to return the same value but this can be overcome by introducing base class Entity<T> which will hide the object Id property.

public class Entity<T>:IEntity<T> {
    public T Id { get; set; }
    object IEntity.Id { get{ return Id;}  }
}

then your problematic code could be:

object value =...;
var entity = value as IEntity;
string id;
if(entity != null)
    id = entity.Id.ToString();