Question

I have the following interfaces defined in a base library project:

public interface IWorkContext {
    T User<T>() where T : IUser;

    // Note this method is kind of a shortcut so i don't
    // have to keep passing in T
    IUser User();
}

public interface IUser {
    int Id { get; }
    string UserName { get; }
}

Now I have another project which references the one above with the following class (which implements IUser).

public class User : IUser {
   public virtual int Id { get; set; }
   public virtual string UserName { get; set; }
   // ... Code removed for brevity
}

Now i also need to implement IWorkContext. Here’s my first attempt:

public class DefaultWorkContext : IWorkContext {
    private readonly IUsersService _usersService;

    public DefaultWorkContext(IUsersService usersService) {
        _usersService = usersService;
    }

    public T User<T>() where T : IUser {
        return _usersService.GetUser<T>(1));
    }

    public User User() {
        return User<User>();
    }
}

However this gives the error:

The type ‘T’ cannot be used as type parameter ‘T’ in the generic type
or method ‘…GetUser(int)’. There is no boxing conversion or type
parameter conversion from ‘T’ to ‘User’.

I’m sure there’s something fundamental i’m doing wrong. I’d appreciate any advice on this model can be improved. Thanks

Edit (as requested here’s the GetUser method):

public T GetUser<T>(int id) where T : User {
    return _session.Get<T>(id);
}

Note: _session is an NHibernate session.

Answer 1

The problem here is that you are passing a generic type as a type parameter to your GetUser() method, that may not satisfy the condition of inheritance of the generic type accepted by your GetUser() method.

The compiler ensures that the type passing as parameters always satisfy the type conditions.
Indeed, you could write a class NotUser that implements IUser while not being a User.
Your User<T>() method could then be called with NotUser as the type parameter, and that would result in a call to GetUser() with a type parameter that does not inherit from User.

Your GetUser() method only accept a type that inherits from User.

You need to either change GetUser() prototype to accept a generic type that implements IUser, or the User<T> prototype to accept only generic types that inherits from User.

Either this:

public T GetUser<T>(int id) where T : IUser {
    return _session.Get<T>(id);
}

or this:

public T User<T>() where T : User {
    return _usersService.GetUser<T>(1));
}

EDIT:
Whether you should pick the first or the second method depends on what you want to do.

1) You want your IWorkContext to work with the IUser interface, and you accept that the GetUser() method returns a IUser => pick option 1

2) You want the GetUser() method to take a type that inherits from User, and you accept that the IWorkContext interface works with User instead of IUser => pick option 2, and modify the IWorkContext interface.

To me, option 1) is better, as you don’t need to modify the interface and you don’t break the genericity. But it depends on what you want to do exactly.