GetExports<T，MEF中的TMetaDataView>不返回AllowMultiple = True的元素

Question

我不确定MEF是如何工作的。我用AllowMultiple=true创建了一个自定义属性。这就是它：

public interface IQuestionFactoryMetadataView
{
    IQuestionFactoryMetadata[] Metadatas { get; }
}

public interface IQuestionFactoryMetadata
{
    Levels Difficulty { get; }
    int Quantity { get; }
}

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
public class ExportQuestionFactoryAttribute : ExportAttribute, IQuestionFactoryMetadata
{
    public ExportQuestionFactoryAttribute(Type type, Levels difficulty, int quantity)
        : base(type.Namespace, typeof(IQuestionFactory))
    {
        this.Difficulty = difficulty;
        this.Quantity = quantity;
    }

    public Levels Difficulty { get; private set; }
    public int Quantity { get; private set; }
}

// 
[ExportQuestionFactory(typeof(NonConcreteQuestionFactory), Levels.Beginner, 10)]
[ExportQuestionFactory(typeof(NonConcreteQuestionFactory), Levels.Medium, 20)]
[ExportQuestionFactory(typeof(NonConcreteQuestionFactory), Levels.Expert, 30)]
[ExportQuestionFactory(typeof(NonConcreteQuestionFactory), Levels.Master, 40)]
public class NonConcreteQuestionFactory : QuestionTemplateFactory
{
    ...
}

如果我使用弱类型的元数据，我的导出会突然满足：

        var exports = container.GetExports<IQuestionFactory, IDictionary<string, object>>(typeof(PEMDAS.Core.TimesTables.WorksheetTemplate).Namespace);

但是如果我使用强类型，我的exports变量是空的。

        var exports = container.GetExports<IQuestionFactory, IQuestionFactoryMetadataView>(typeof(PEMDAS.Core.TimesTables.WorksheetTemplate).Namespace);

Answer 1

我看不到QuestionFactoryMetadataView，你能展示一下那种类型吗？

有吗？

var exports = container.GetExports<IQuestionFactory, IQuestionFactoryMetadata>(...)

工作?