c# - Autofac:使用类型参数条件解析开放式泛型

Question

在使用 Autofac 的应用程序中作为它的 IoC 容器，我有一个带有两个类型参数的通用接口(interface):

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

和一个包装接口(interface)来动态选择适当的IMapper<TSource, TTarget>取决于其输入参数类型:

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

我希望我的 IDynamicMapper 实现在运行时找到合适的 IMapper<TSource, TTarget>组件，使用 Autofac，它有一个 TSource等于source.GetType()和 TTarget作为 T 的派生类型(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 容器作为它们在应用程序另一部分的服务接口(interface)(使用程序集扫描记录)。

---

更新 1

根据 Steven 的相关回答，我更新了我的界面以使用方差:

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) 时或 Resolve(typeof(IEnumerable<>).MakeGenericType(mapperType)) ，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> .第一次调用抛出异常，第二次调用返回一个空的可枚举对象。

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);
}