Autofac：解析具有类型参数条件的开放泛型

Question

在使用 Autofac 作为其 IoC 容器的应用程序中，我有一个带有两个类型参数的通用接口：

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

和一个包装器接口，用于根据其输入参数类型动态选择适当的IMapper<TSource, TTarget>：

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

我希望 IDynamicMapper 的实现在运行时使用 Autofac 找到适当的 IMapper<TSource, TTarget> 组件，该组件的 TSource 等于 source.GetType() 和 TTarget 是T（或T 本身）：

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))
    }
}

我的所有组件都注册到 Autofac 容器，作为应用程序另一部分中的服务接口（使用程序集扫描记录）。

---

更新 1

根据 Steven 的相关回答，我更新了如下界面以使用方差：

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

我的动态映射器的GetTarget() 方法如下所示：

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);
}

但是，当调用Resolve(mapperType) 或Resolve(typeof(IEnumerable<>).MakeGenericType(mapperType)) 时，组件未解析，尽管它存在于容器的注册中，映射到服务IMapper<TSource, TTarget>。第一次调用引发异常，第二次调用返回空枚举。

Answer 1

这应该可以解决问题：

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);
}

Answer 2

Autofac 不支持协变泛型类型 (ISomething<out T>)。在这种情况下，另一个 IoC 容器（如 Simple Injector）可以解决问题，但为了使其与 Autofac 一起使用，我最终使用了另一个接口：

服务：

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

实施：

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);
    }
}