Autofac: resolve an open generic with conditions on its type parameters

Question

In an application using Autofac as its IoC container, I have a generic interface with two type parameters:

public interface IMapper<TSource, TTarget>
{
    TTarget GetTarget(TSource source);
}

and a wrapper interface to dynamically select the appropriate IMapper<TSource, TTarget> depending on its input parameter types:

public interface IDynamicMapper
{
    T GetTarget<T>(object source);
}

I want my implementation of IDynamicMapper to find at runtime the appropriate IMapper<TSource, TTarget> component, using Autofac, which has a TSource equal to source.GetType() and TTarget being a derived type of T (or T itself):

public class DynamicMapper : IDynamicMapper
{
    private readonly ILifetimeScope _scope;

    public DynamicMapper(ILifetimeScope scope)
    {
        this._scope = scope;
    }

    T IDynamicMapper.GetTarget<T>(object source)
    {
        Type sourceType = source.GetType();
        Type targetBaseType = typeof(T);

        //TODO: find an IMapper<TSource, TTarget> implementation where
        // 1) Condition on TSource: typeof(TSource) == sourceType
        // 2) Condition on TTarget: targetBaseType.IsAssignableFrom(typeof(TTarget))
        // Many implementations can potentially match these criterias,
        // choose the 1st one
        // (this should not happen if the application is designed correctly)

        if (mapper == null)
        {
            throw new ArgumentException(
                "Could not find an IMapper<TSource, TTarget> implementation" +
                " for the supplied parameters"
            );
        }

        // call mapper.GetTarget(source) and return the result
        // (mapper is probably dynamic, but its runtime type implements
        // TTarget IMapper<TSource, TTarget>.GetTarget(TSource source))
    }
}

All my components are registered to the Autofac container as their service interfaces in another part of the application (using assembly scanning for the record).

---

UPDATE 1

Based on Steven's pertinent answers I updated my interface as follow to use variance:

public interface IMapper<in TSource, out TTarget>
{
    TTarget GetTarget(TSource source);
}

My dynamic mapper's GetTarget() method looks like this:

T IDynamicMapper.GetTarget<T>(object source)
{
    Type sourceType = source.GetType();
    Type targetBaseType = typeof(TTarget);
    Type mapperType = typeof(IMapper<,>).MakeGenericType(sourceType, targetBaseType);

    // This fails with ComponentNotRegisteredException
    dynamic mapper = this._scope.Resolve(mapperType);

    // This also fails (mapper is null):
    // IEnumerable<object> mappers = (IEnumerable<object>)this._scope.Resolve(typeof(IEnumerable<>).MakeGenericType(mapperType));
    // dynamic mapper = mappers.FirstOrDefault();

    // Invoke method
    return mapper.GetTarget((dynamic)source);
}

However, when calling Resolve(mapperType) or Resolve(typeof(IEnumerable<>).MakeGenericType(mapperType)), the component is not resolved, although it is present in the container's registrations, mapped to the service IMapper<TSource, TTarget>. The 1st call throws an exception and the 2nd one returns an empty enumerable.

Answer 1

Autofac does not support covariant generic types (ISomething<out T>). Another IoC container like Simple Injector could do the trick in this case, but to make it work with Autofac I ended up using another interface:

Services:

public interface IMapper<TSource, out TTarget> : IMapperLocator<TSource>
{
    TTarget Extract(TSource source);
}
public interface IMapperLocator<TSource>
{
}
public interface IDynamicMapper
{
    T Extract<T>(object source);
}

Implementation:

public class DynamicMapper : IDynamicMapper
{
    private readonly ILifetimeScope _scope;

    public DynamicMapper(ILifetimeScope scope)
    {
        this._scope = scope;
    }

    T IDynamicMapper.Extract<T>(object source)
    {
        // Get useful types
        Type sourceType = source.GetType();
        Type targetBaseType = typeof(TTarget);
        Type mapperBaseType = typeof(IMapper<,>).MakeGenericType(sourceType, targetBaseType);
        Type locatorType = typeof(IMapperLocator<>).MakeGenericType(sourceType);
        Type enumType = typeof(IEnumerable<>).MakeGenericType(locatorType);

        // Get all mapper implementations that take a TSource with the
        // same type as the source object
        var mappers = (IEnumerable<object>)this._scope.Resolve(enumType);

        // Among all the mappers with the right TSource, select the one
        // with TTarget assignable to T (throws if there is 0 or more than
        // one mapper, as this would be an implementation error)
        dynamic mapper = mappers.Single(x => mapperBaseType.IsAssignableFrom(x.GetType()));

        // The method must implemented implicitly.
        // A workaround would be to use a wrapper (IMapperWrapper<TSource, out TTarget>)
        // that implements the method implicitly and invokes the mapper's method
        // without using dynamic
        return mapper.Extract((dynamic)source);
    }
}

Answer 2

This should do the trick:

T IDynamicMapper.GetTarget<T>(object source) {

    Type mapperType = typeof(IMapper<,>).MakeGenericType(source.GetType(), typeof(T));

    // Will throw when no registration exists.
    // Note the use of 'dynamic'.
    dynamic mapper = scope.Resolve(mapperType);

    return (T)mapper.GetTarget<T>((dynamic)source);
}