Binding singleton to multiple services in Ninject

Question

I have a problem which seems very similar to the one described in http://markmail.org/message/6rlrzkgyx3pspmnf which is about the singleton actually creating more than a single instance if you're accessing it using different service types.

I'm using the latest release of Ninject 2 for Compact Framework and the exact issue I'm having is that if I bind the same provider method to:

Func<Service> serviceCreator = () => new Service(false);
kernel.Bind<IService>().ToMethod(serviceCreator).InSingletonScope();
kernel.Bind<Service>().ToMethod(serviceCreator).InSingletonScope();

It seems to be creating 2 instances of Service if I resolve both as IService and Service.

This causes a circular dependency exception when resolving Service.

Is this by design, or is it a bug?

Answer 1

We used Ruben's method in our project, but found that it wasn't intuitive why you were going back to the Kernel in your binding. I created an extension method and helper class (below) so you can do this:

kernel.Bind<IService>().ToExisting().Singleton<Service>();

That seemed to express the intent more clearly to me.

public static class DIExtensions
{
    public static ToExistingSingletonSyntax<T> ToExisting<T>(this IBindingToSyntax<T> binding)
    {
        return new ToExistingSingletonSyntax<T>(binding);
    }
}

// Had to create this intermediate class because we have two type parameters -- the interface and the implementation,
// but we want the compiler to infer the interface type and we supply the implementation type.  C# can't do that.
public class ToExistingSingletonSyntax<T>
{
    internal ToExistingSingletonSyntax(IBindingToSyntax<T> binding)
    {
        _binding = binding;
    }

    public IBindingNamedWithOrOnSyntax<T> Singleton<TImplementation>() where TImplementation : T
    {
        return _binding.ToMethod(ctx => ctx.Kernel.Get<TImplementation>()).InSingletonScope();
    }


    private IBindingToSyntax<T> _binding;
}

Answer 2

By the way, Ninject 3 allows this syntax:

kernel.Bind<IService, Service>().ToMethod(serviceCreator).InSingletonScope();

Or, similarly:

kernel.Bind(typeof(IService), typeof(Service)).ToMethod(serviceCreator).InSingletonScope();

This latter approach works better if you have many services, or if you discovered the services dynamically at runtime (you can pass the params-style arguments as an array directly).

Answer 3

In V3, there is finally a solution for this in the shape of new overloads on Bind, see this related: question.

---

If you want the singleton to be shared, you need to change your second Bind to:

kernel.Bind<Service>().ToMethod(()=>kernel.Get<IService>()).InSingletonScope();

Re circular references and confusion etc. Internally the implicit self-binding will add an implicit binding registration for Service. You should post the exception.

EDIT: Re your comment. If you do it like this:

Func<Service> serviceCreator = () => new Service(false);
kernel.Bind<Service>().ToMethod(serviceCreator).InSingletonScope();
kernel.Bind<IService>().ToMethod(()=>kernel.Get<Service>()).InSingletonScope();

Then no implicit Class Self Binding gets generated when IService gets Resolved - it uses the existing one.

There was another Q here on SO in recent weeks someone was doing this type of thing but was running into an issue with IInitializable - that example would have the correct ordering but the one above makes sense based on my reading of the source and the way in which it generates the implicit class self-bindings.