Unity equivalent for Ninject's Bind.ToMethod Using CQRS

Question

I am following this CQS tuturiol https://github.com/Code-First/CQS-Sample and it uses Ninject.

I currently am using Unity and trying to convert this:

    Bind<IQueryFactory>().ToMethod(t => new QueryFactory(x => Container.Current.Resolve(x))).InTransientScope();

MY query factory is shown below:

public class QueryFactory : IQueryFactory
{
    private readonly Func<Type, object> _resolveCallback;

    public QueryFactory(Func<Type, object> resolveCallback)
    {
        _resolveCallback = resolveCallback;
    }

    public T ResolveQuery<T>()
        where T : class, IQuery
    {
        return _resolveCallback(typeof (T)) as T;
    }
}

I have tried container.RegisterType<IQueryFactory>(t => new InjectionFactory(u => u.Resolve(u))); but that doesnt work.

The query model given by that article is limiting, because the `IQuery` interface isn't generic. I would advice a model as explained [here](https://www.cuttingedge.it/blogs/steven/pivot/entry.php?id=92). — Steven, Jul 14 '15 at 11:36

Randy Levy · Accepted Answer · 2015-07-13T22:07:23.260

You'd think that an InjectionFactory would be the way to go. This compiles:

container.RegisterType<IQueryFactory>(new ContainerControlledLifetimeManager(),
    new InjectionFactory((c, x, o) => new QueryFactory(t => c.Resolve(x))));

The problem is that the type x is always IQueryFactory so that doesn't get the results you want which is to return an IQuery.

Unfortunately, I don't think the current design fits with Unity very well.

However, with some slight modifications you can get the factory working. There are a few different ways of doing this (e.g. you could inject the container in the factory and resolve from there) however in this example I'll use a dictionary.

First we change the factory definition:

public class QueryFactory : IQueryFactory
{
    private readonly IDictionary<Type, Func<IQuery>> _factoryQueries;

    public QueryFactory(IDictionary<Type, Func<IQuery>> factoryQueries)
    {
        this._factoryQueries = factoryQueries;
    }

    public T ResolveQuery<T>() where T : class, IQuery
    {
        return this._factoryQueries[typeof(T)]() as T;
    }

Basically, the Func<Type, object> resolveCallback is swapped for an IDictionary containing Funcs for creating all supported IQuery types.

Next register the IDictionary and the IQueryFactory:

IUnityContainer container = new UnityContainer();

// Register all IQuery's
container.RegisterType<IActiveUsersQuery, ActiveUsersQuery>();
container.RegisterType<IUserByEmailQuery, UserByEmailQuery>();

IDictionary<Type, Func<IQuery>> factoryQueries = new Dictionary<Type, Func<IQuery>>()
{
    { typeof(IActiveUsersQuery), () => container.Resolve<IActiveUsersQuery>() },
    { typeof(IUserByEmailQuery), () => container.Resolve<IUserByEmailQuery>() },
};

// Register mapping
container.RegisterInstance<IDictionary<Type, Func<IQuery>>>(factoryQueries);
container.RegisterType<IQueryFactory, QueryFactory>();

var factory = container.Resolve<IQueryFactory>();

factory.ResolveQuery<IActiveUsersQuery>().Execute();
factory.ResolveQuery<IActiveUsersQuery>().Execute();

As mentioned, a simpler QueryFactory approach that takes a dependency on the container is:

public class QueryFactory : IQueryFactory
{
    private readonly IUnityContainer container;

    public QueryFactory(IUnityContainer container)
    {
        this.container = container;
    }

    public T ResolveQuery<T>()
        where T : class, IQuery
    {
        return this.container.Resolve(typeof(T)) as T;
    }
}

However, some would object to having the factory depend on the container implementation.

Another approach would be to get rid of the factory approach and inject the queries directly; this would make the dependencies obvious but I can see the scenario where you would want to dynamically get a query based on the command type. Also, I've seen relations between commands and queries which could be handled with open generics resolved in the composition root.

Unity equivalent for Ninject's Bind.ToMethod Using CQRS

1 Answers1