Customizing Autofac's component resolution / Issue with generic co-/contravariance

Question

First, sorry for the vague question title. I couldn't come up with a more precise one.

Given these types:

                                                     { TCommand : ICommand }
       «interface»                   «interface»    /
      +-----------+         +----------------------/----+
      | ICommand  |         | ICommandHandler<TCommand> |
      +-----------+         +---------------------------+
            ^               | Handle(command: TCommand) |
            |               +---------------------------+
            |                              ^
            |                              |
      +------------+            +-------------------+
      | FooCommand |            | FooCommandHandler |
      +------------+            +-------------------+
            ^
            |
   +-------------------+
   | SpecialFooCommand |
   +-------------------+

I would like to write a method Dispatch that accepts any command and sends it to an appropriate ICommandHandler<>. I thought that using a DI container (Autofac) might greatly simplify the mapping from a command's type to a command handler:

void Dispatch<TCommand>(TCommand command) where TCommand : ICommand
{
    var handler = autofacContainer.Resolve<ICommandHandler<TCommand>>();
    handler.Handle(command);
}

Let's say the DI container knows about all the types shown above. Now I'm calling:

Dispatch(new SpecialFooCommand(…));

In reality, this will result in Autofac throwing a ComponentNotRegisteredException, since there is no ICommandHandler<SpecialFooCommand> available.

Ideally however, I would still want a SpecialFooCommand to be handled by the closest-matching command handler available, ie. by a FooCommandHandler in the above example.

Can Autofac be customized towards that end, perhaps with a custom registration source?

---

P.S.: I understand that there might be the fundamental problem of co-/contravariance getting in the way (as in the following example), and that the only solution might be one that doesn't use generics at all... but I would want to stick to generic types, if possible.

ICommandHandler<FooCommand> fooHandler = new FooCommandHandler(…);
ICommandHandler<ICommand> handler = fooHandler;
//                                ^
//              doesn't work, types are incompatible

Answer 1

Not really a fair answer, as I've extended Autofac since you posted the question... :)

As per Daniel's answer, you'll need to add the in modifier to the TCommand parameter of ICommandHandler:

interface ICommandHandler<in TCommand>
{
    void Handle(TCommand command);
}

Autofac 2.5.2 now includes an IRegistrationSource to enable contravariant Resolve() operations:

using Autofac.Features.Variance;

var builder = new ContainerBuilder();
builder.RegisterSource(new ContravariantRegistrationSource());

With this source registered, services represented by a generic interface with a single in parameter will be looked up taking variant implementations into account:

builder.RegisterType<FooCommandHandler>()
   .As<ICommandHandler<FooCommand>>();

var container = builder.Build();
container.Resolve<ICommandHandler<FooCommand>>();
container.Resolve<ICommandHandler<SpecialFooCommand>>();

Both calls to Resolve() will successfully retrieve the FooCommandHandler.

If you can't upgrade to the latest Autofac package, grab the ContravariantRegistrationSource from http://code.google.com/p/autofac/source/browse/src/Source/Autofac/Features/Variance/ContravariantRegistrationSource.cs - it should compile against any recent Autofac build.

Answer 2

What you are asking is not possible without own coding. Basically, you are asking the following: If the type I tried to resolve isn't found, return another type that can be converted to it, e.g. if you try to resolve IEnumerable return a type that is registered for ICollection. This is not supported. One simple solution would be the following: Register FooCommandHandler as a handler for ICommandHandler<SpecialFooCommand>. For this to work, ICommandHandler needs to be contravariant:

interface ICommand { }

class FooCommand : ICommand { }

class SpecialFooCommand : FooCommand { }

interface ICommandHandler<in T> where T : ICommand
{
    void Handle(T command);
}

class FooCommandHandler : ICommandHandler<FooCommand>
{
    public void Handle(FooCommand command)
    {
        // ...
    }
}

var builder = new ContainerBuilder();
builder.RegisterType<FooCommandHandler>()
       .As<ICommandHandler<SpecialFooCommand>>()
       .As<ICommandHandler<FooCommand>>();
var container = builder.Build();
var fooCommand = new FooCommand();
var specialCommand = new SpecialFooCommand();
container.Resolve<ICommandHandler<FooCommand>>().Handle(fooCommand);
container.Resolve<ICommandHandler<FooCommand>>().Handle(specialCommand);
container.Resolve<ICommandHandler<SpecialFooCommand>>().Handle(specialCommand);

BTW: The way you are using the container, you apply the Service locator anti-pattern. This should be avoided.

Answer 3

I like to add an alternative approach, which also works without C# 4.0 variance support.

You can create a special decorator / wrapper that allows executing a command as its base type:

public class VarianceHandler<TSubCommand, TBaseCommand> 
    : ICommandHandler<TSubCommand>
    where TSubCommand : TBaseCommand
{
    private readonly ICommandHandler<TBaseCommand> handler;

    public VarianceHandler(ICommandHandler<TBaseCommand> handler)
    {
        this.handler = handler;
    }

    public void Handle(TSubCommand command)
    {
        this.handler.Handle(command);
    }
}

With this in place, the following line of code would allow you to handle SpecialFooCommand as its base type:

builder.Register<FooCommandHandler>()
    .As<ICommandHandler<FooCommand>>();

builder.Register<VarianceHandler<SpecialFooCommand, FooCommand>>()
    .As<ICommandHandler<SpecialFooCommand>>();

Note that the use of such VarianceHandler works for most DI containers.