I have a set of ASP.NET 4 projects that culminate in an MVC (3 RC2) app. The solution uses Unity and EntLib Validation for cross-cutting dependency injection and validation. Both are working great for injecting repository and service layer implementations.

However, I can't figure out how to do duplicate key validation. For example, when a user registers, we want to make sure they don't pick a UserID that someone else is already using. For this type of validation, the validating object must have a repository reference... or some other way to get an IQueryable / IEnumerable reference to check against other rows already in the DB.

What I have is a UserMetadata class that has all of the property setters and getters for a user, along with all of the appropriate DataAnnotations and EntLib Validation attributes. There is also a UserEntity class implemented using EF4 POCO Entity Generator templates. The UserEntity depends on UserMetadata, because it has a MetadataTypeAttribute. I also have a UserViewModel class that has the same exact MetadataType attribute. This way, I can apply the same validation rules, via attributes, to both the entity and viewmodel.

There are no concrete references to the Repository classes whatsoever. All repositories are injected using Unity. There is also a service layer that gets dependency injection. In the MVC project, service layer implementation classes are injected into the Controller classes (the controller classes only contain service layer interface references). Unity then injects the Repository implementations into the service layer classes (service classes also only contain interface references).

I've experimented with the DataAnnotations CustomValidationAttribute in the metadata class. The problem with this is the validation method must be static, and the method cannot instantiate a repository implementation directly. My repository interface is IRepository, and I have only one single repository implementation class defined as EntityRepository for all domain objects. To instantiate a repository explicitly I would need to say new EntityRepository(), which would result in a circular dependency graph: UserMetadata [depends on] DuplicateUserIDValidator [depends on] UserEntity [depends on] UserMetadata.

I've also tried creating a custom EntLib Validator along with a custom validation attribute. Here I don't have the same problem with a static method. I think I could get this to work if I could just figure out how to make Unity inject my EntityRepository into the validator class... which I can't. Right now, all of the validation code is in my Metadata class library, since that's where the custom validation attribute would go.

Any ideas on how to perform validations that need to check against the current repository state? Can Unity be used to inject a dependency into a lower-layer class library?