Question

Given the class below:

public class ConcreteEmployeeRoleCreator<T, TConcrete, TSpec> 
    where TConcrete : class, T, new()
    where T : EmployeeRole
    where TSpec : EmployeeRoleSpecification
{
    public ConcreteEmployeeRoleCreator(TSpec spec) { Specification = spec; }

    public TSpec Specification { get; private set; }

    public T Create() { return new TConcrete(); }

}

I would like to have a dictionary of ‘creators’, but I haven’t been able to work out how to do it yet. If I try and define the dictionary using the lowest common denominator types, the compiler is not allowing an instance to be added

[Test]
public void Creator_CanCreateFromSpec() {
    var creators = new Dictionary<string, ConcreteEmployeeRoleCreator<EmployeeRole, EmployeeRole, EmployeeRoleSpecification>>();
    var spec = new SalesmanRoleSpecification();
    var creator = new ConcreteEmployeeRoleCreator<EmployeeRole, Salesman, SalesmanRoleSpecification>(spec);
    creators.Add("salesman", creator); <==== ** compile error **
}

Salesman is an EmployeeRole, and SalesmanRoleSpecification is an EmployeeRoleSpecification (or I wouldn’t be able to define the creator above without a compiler error as well).

SO I guess it is the way I am declaring the dictionary? What am I doing wrong?

Cheers,
Berryl

Answer 1

Generic covariance has to be specified at the declaration of the covariant type – and it can only be specified for interfaces and delegates.

So, given that you’re using a class, a

ConcreteEmployeeRoleCreator<EmployeeRole, Salesman, SalesmanRoleSpecification>

will never be a

ConcreteEmployeeRoleCreator<EmployeeRole, EmployeeRole, EmployeeRoleSpecification>

You’ll need to look for an alternative approach. To be honest, by the time you get to three type parameters and want two of them to be covariant, you’ve got a pretty hard-to-understand design to start with, I’m afraid 🙁