Unity IoC throws invalid cast exception

Question

I'm trying to register a generic interface / implementation via Unity IoC, but the constructor parameter in the ViewModel, throws an invalid cast exception, and I cannot figure out why.

I have an interface, which all models implement:

public interface IEntity {}

A typical model would look like this:

public class Dashboard: IEntity { .... }

I have a database service, which can be of any model type, and an associated interface, for example:

public interface IDbService<T> where T: IEntity { .... }

public class DbService<T> where T : IEntity, IDbService<T> { .... }

The container registration looks like this:

container.RegisterType(typeof(IDbService<>), typeof(DbService<>));

which works up to this point.

However, when I try and load this into my ViewModel class, I get an invalid cast exception, my implementation on the constructor, for argument sake, in the DashboardPageViewModel, which looks like this:

private readonly IDbService<Dashboard> _dbService;

public DashboardPageViewModel(IDbService<Dashboard> dbService)
{
     _dbService = dbService;
}

This throws the exception.

Why can I not cast Dashboard to in this regard? Trying to resolve the cast after I've registered the type, does not work either.

Answer 1

The class has been defined incorrectly.

In

public class DbService<T> where T : IEntity, IDbService<T> { .... }

DbService<T> only has type constraints where T is derived from IEntity and also IDbService<T>

You need to have the class derived from IDbService<T> interface and then apply the generic type constraint.

public interface IDbService<T> where T: IEntity { .... }

public class DbService<T> : IDbService<T> where T : IEntity { .... }