Prevent Ninject from calling Initialize multiple times when binding to several interfaces

Question

We have a concrete singleton service which implements Ninject.IInitializable and 2 interfaces. Problem is that services Initialize-methdod is called 2 times, when only one is desired. We are using .NET 3.5 and Ninject 2.0.0.0.

Is there a pattern in Ninject prevent this from happening. Neither of the interfaces implement Ninject.IInitializable. the service class is:

public class ConcreteService : IService1, IService2, Ninject.IInitializable
{
    public void Initialize()
    {
        // This is called twice!
    }
}

And module looks like this:

public class ServiceModule : NinjectModule
{
    public override void Load()
    {
        this.Singleton<Iservice1, Iservice2, ConcreteService>();
    }
}

where Singleton is an extension method defined like this:

    public static void Singleton<K, T>(this NinjectModule module) where T : K
    {
        module.Bind<K>().To<T>().InSingletonScope();
    }

    public static void Singleton<K, L, T>(this NinjectModule module) 
        where T : K, L
    {
        Singleton<K, T>(module);
        module.Bind<L>().ToMethod(n => n.Kernel.Get<T>());
    }

Of course we could add bool initialized-member to ConcreteService and initialize only when it is false, but it seems quite a bit of a hack. And it would require repeating the same logic in every service that implements two or more interfaces.

---

Thanks for all the answers! I learned something from all of them! (I am having a hard time to decide which one mark correct).

We ended up creating IActivable interface and extending ninject kernel (it also removed nicely code level dependencies to ninject, allthough attributes still remain).

Answer 1

Ninject 3

Ninject 3.0 now supports multiple generic types in the call to bind, what you are trying to do can be easily accomplished in a single chained statement.

kernel.Bind<IService1, IService2>()
      .To<ConcreteService>()
      .InSingletonScope();

Ninject 2

You are setting up two different bindings K=>T and L=>T. Requesting instances of L will return transient instances of T. Requesting K will return a singleton instance of T.

In Ninject 2.0, an objects scope is per service interface bound to a scope callback.

When you have

Bind<IFoo>...InSingletonScope();
Bind<IBar>...InSingletonScope();

you are creating two different scopes.

You are saying "Binding to IFoo will resolve to the same object that was returned when .Get was called." and "Binding to IBar will resolve to the same object that was returned when .Get was called."

you can chain the bindings together, but you will need to remove IInitializable as it will cause duplicate initialization when the instance is activated:

kernel.Bind<IBoo>()
      .To<Foo>()
      .InSingletonScope();
      .OnActivation(instance=>instance.Initialize());

kernel.Bind<IBaz>()
      .ToMethod( ctx => (IBaz) ctx.Kernel.Get<IBoo>() );

or

kernel.Bind<Foo>().ToSelf().InSingletonScope()
    .OnActivation(instance=>instance.Initialize());
kernel.Bind<IBaz>().ToMethod( ctx => ctx.Kernel.Get<Foo>() );
kernel.Bind<IBoo>().ToMethod( ctx => ctx.Kernel.Get<Foo>() );

in order to get multiple interfaces to resolve to the same singleton instance. When I see situations like this, I always have to ask, is your object doing too much if you have a singleton with two responsibilities?

Answer 2

Update : Pretty sure using V3's multiple Bind overloads will address this; See this Q/A

---

Good question.

From looking at the source, the initialize bit happens after each Activate. Your Bind...ToMethod counts as one too. The strategy is pretty uniformly applied - there's no way to opt out in particular cases.

Your workaround options are to use an explicit OnActivation in your Bind which will do it conditionally (but to do that in a general way would require maintaining a Set of initialized objects (havent looked to see if there is a mechanism to stash a flag against an activated object)), or to make your Initialize idempotent through whatever means is cleanest for you.

EDIT:

    internal interface IService1
    {
    }

    internal interface IService2
    {
    }

    public class ConcreteService : IService1, IService2, Ninject.IInitializable
    {
        public int CallCount { get; private set; }
        public void Initialize()
        {
            ++CallCount;
        }
    }

    public class ServiceModule : NinjectModule
    {
        public override void Load()
        {
            this.Singleton<IService1, IService2, ConcreteService>();
        }
    }

Given the following helpers:

static class Helpers
{
    public static void Singleton<K, T>( this NinjectModule module ) where T : K
    {
        module.Bind<K>().To<T>().InSingletonScope();
    }

    public static void Singleton<K, L, T>( this NinjectModule module )
        where T : K, L
    {
        Singleton<T, T>( module );
        module.Bind<K>().ToMethod( n => n.Kernel.Get<T>() );
        module.Bind<L>().ToMethod( n => n.Kernel.Get<T>() );
    }
}

@Ian Davis et al. The problem is that:

    class Problem
    {
        [Fact]
        static void x()
        {
            var kernel = new StandardKernel( new ServiceModule() );
            var v1 = kernel.Get<IService1>();
            var v2 = kernel.Get<IService2>();
            var service = kernel.Get<ConcreteService>();
            Console.WriteLine( service.CallCount ); // 3
            Assert.AreEqual( 1, service.CallCount ); // FAILS
        }
    }

Because each activation (per Bind) initialises each time.

EDIT 2: Same when you use the following slightly more stripped down version:

static class Helpers
{
    public static void Singleton<K, L, T>( this NinjectModule module )
        where T : K, L
    {
        module.Bind<T>().ToSelf().InSingletonScope();
        module.Bind<K>().ToMethod( n => n.Kernel.Get<T>() );
        module.Bind<L>().ToMethod( n => n.Kernel.Get<T>() );
    }
}