Ninject - Managing Inconvariance of generic types?

Question

I am having trouble using Ninject to load my generic type implementations given the following simplified interface/class structure.

public interface IEntry {}

public class TestEntry : IEntry {}

public interface IDBConnection<T> {}

public class DBConnection<T> : IDBConnection<T> where T : IEntry {}

I am using binds within my loaded NinjectModule:

Bind<IEntry>().To<TestEntry>();
Bind(typeof(IDBConnection<>)).To(typeof(DBConnection<>));

I want to fetch an instance of DBConnection<TestEntry> with a call of:

Kernel.TryGet<IDBConnection<IEntry>>();

However this just returns an open instance type of DBConnection<IEntry> ; I have been able to return an instance of DBConnection<TestEntry> if I change my Kernel.Get call to:

Kernel.TryGet<IDBConnection<TestEntry>>();

I understand that generics are incovariant but it seems that we circumvent the entire purpose of DI/IOC if I need to stipulate the implementation of my generic class in order for Ninject to load it... So I must be either binding, fetching or understanding things incorrectly.

Furthermore I tried a different approach to binding/loading:

Bind<IEntry>().To<TestEntry>();
Bind(typeof(IDBConnection<IEntry>)).To(typeof(DBConnection<TestEntry>));

and

Kernel.TryGet<IDBConnection<IEntry>>();

However, this yields the exception:

System.InvalidCastException : Unable to cast object of type 'DBConnection1[TestEntry]' to type 'IDBConnection1[IEntry]'.

This is because the generic type IDBConnection<IEntry> is not covariant with DBConnection<TestEntry> right?

I want to be able to Ninject DBConnection<TestEntry> into my IDBConnection<IEntry> declaration for consumption; however incovariance of generics seems to disallow this. What's the solution?

Edit: Here is a unit Test to demonstrate/explain

    public interface IEntry { }

    public class TestEntry : IEntry { }

    public interface IDBConnection<T> where T : IEntry { }

    public class DBConnection<T> : IDBConnection<T> where T : IEntry { }

    class TestModule : NinjectModule
    {
        public override void Load()
        {
            Bind<IEntry>().To<TestEntry>();
            Bind(typeof(IDBConnection<IEntry>)).To(typeof(DBConnection<TestEntry>));
        }
    }

    [Test]
    public void NinjectGenericLoadTest()
    {
        /// this loads the expected type from interfaces however is useless
        /// since loaded against a "var" 
        ///(runtime casts knowing the impl would be required to use)
        StandardKernel kernel = new StandardKernel(new TestModule());
        var ninjected = kernel.TryGet(typeof(IDBConnection<IEntry>));
        Assert.IsInstanceOf<DBConnection<TestEntry>>(ninjected);

        /// The following is what I want but it won't compile 
        ///:"Cannot implicitly convert type 'object' to 
        ///'EasyMongo.Contract.IReader<EasyMongo.Contract.IEasyMongoEntry>'. 
        /// An explicit conversion exists (are you missing a cast?)"
        //kernel = new StandardKernel(new TestModule());
        //IDBConnection<IEntry> ninjectedInterface = kernel.TryGet(typeof(IDBConnection<IEntry>));
        //Assert.IsInstanceOf<DBConnection<Entry>>(ninjectedInterface);

        /// this throws System.InvalidCastException : Unable to cast object of type 
        /// 'DBConnection`1[EasyMongo.Test.Base.RandomTest+Entry]' 
        /// to type 'IDBConnection`1[EasyMongo.Test.Base.RandomTest+IEntry]'.
        /// this is due to incovariance of generic types such that DBConnection<Entry> 
        /// is not a IDBConnection<IEntry> 
        IDBConnection<IEntry> ninjectedInterface = (IDBConnection<IEntry>)kernel.TryGet(typeof(IDBConnection<IEntry>));
        Assert.IsInstanceOf<DBConnection<TestEntry>>(ninjectedInterface);
    }

Is Entry a domain object? Rarely do I see examples of people having their domain objects implement interfaces. — Simon Whitehead
Yes it is; I would assume you also don't see examples of templated domain objects? However if the model class doesn't implement an interface the problem of a child/parent class being noncovariant in a generic remains. C#/Ninject can't use covariance with generics (right?). It seems like generics and DI don't play well together since inevitably you need to hard-code an implementation class ala IKernel.TryGet<ISomeInterface<HeresTheBruteForceImpl>>() thereby negating the purpose of DI/IOC in the first place. — TheFastCat
The driving force behind this design is the MongoDB C# driver which is embedded and heavily relies on template domain objects - can see some examples towards the bottom here: [docs.mongodb.org/ecosystem/tutorial/use-csharp-driver/] — TheFastCat
Will you be having multiple implementations of IEntry? If so, it makes only sense that you specify the Entry Type when retrieving the IDBConnection. If not: Is IDBConnection<T> fix or can you change it's variance? If you can't, you are stuck. How about using a simple IDBConnection interface which has generic-typed methods Retrieve<TEntry> where TEntry : IEntry and so on? — BatteryBackupUnit
I added a unit test to demonstrate the issue. @BatteryBackupUnit thank you for the suggestion of favoring generic methods in favor of generic class; investigating this now. Though it does simplify injection, the downside is additional onus on consumer calls to stipulate a type (that shouldn't change after instantiation :/) — TheFastCat

Remo Gloor Remo Gloor · Accepted Answer · 2013-09-23T08:34:54

Ninject will always return the type you ask for. If you ask for IDBConnection<IEntry> then you will get that type if you ask for IDBConnection<TestEntry>. There is no super logic that will analyze your code and get you a different type then the one you are asking for.

But asking for things like IDBConnection directly is the wrong way to use Ninject anyway. You should inject it using constructor injection:

 public class NeedDbConnection<T> {
      public NeedDbConnection(IDBConnection<T> connection) { ... } 
 }

That way you get your specific db connection appropriate for that class.

Ninject - Managing Inconvariance of generic types?

1 Answers