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:

Answer 1

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. For this to work, ICommandHandler needs to be contravariant:

interface ICommand { }

class FooCommand : ICommand { }

class SpecialFooCommand : FooCommand { }

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

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

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

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