Question

I’m using Munq as the DI container in a MVC3 project. I have a service layer that retrieves a DTO from a repository. Depending on a property in that DTO I need to use one of two strategies to perform calculations on the DTO. I can register a named type in the container e.g.

Container.Register<ICalculation>("Type1", c => new Type1Calculation);
Container.Register<ICalculation>("Type2", c => new Type2Calculation);

Then I can refer directly to the container when trying to instantiate the appropriate strategy e.g.

var calc = Container.Resolve<ICalculation>(dto.ServiceType);

But this leaves me with a dependency on the container itself with the associated coupling and testing issues. What do I need to pass into the service constructor that would allow me to get the correct calculation but without the dependency on the container? Should I pass in a factory instead so the dependency is in the factory and not in the service class?

Answer 1

I’ve added the Munq solution to this. First the factory which includes the IDependencyResolver interface to allow the factory to use the container to resolve dependencies in the factory method:

public class CalculationFactory
{
    private readonly IDependencyResolver _resolver;
    public CalculationFactory(IDependencyResolver resolver)
    {
        ThrowIfNullArgument(resolver, "resolver", typeof(IDependencyResolver));
        _resolver = resolver;
    }

    public static ICalculation CreateCalculator(int serviceType)
    {
        switch (serviceType)
        {
            case 1: return _resolver.Resolve<ICalculation>("Type1");
            case 2: return _resolver.Resolve<ICalculation>("Type2");
            default: return _resolver.Resolve<ICalculation>("InvalidType");
        }
    }
}

Then in Global.asax.cs register the appropriate interfaces/classes, passing in the container to the factory. So now I can set up my tests and the only extra dependency is IDependencyResolver within the factory:

ioc.Register(c => new CalculationFactory(c));
ioc.Register<ICalculation>("Type1", c => c.Resolve<Type1Calculation>());
ioc.Register<ICalculation>("Type2", c => c.Resolve<Type2Calculation>());
ioc.Register<ICalculation>("InvalidType", c => c.Resolve<InvalidCalculation>());