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MEF：如果我有多个出口但只需要一次导入怎么办？

[英]MEF: What if I have multiple exports but need only one import?

原文 2009-12-09 19:00:11 6 2 c#/ .net/ mef

I'm trying to wrap my mind around MEF. 我想把我的思绪包裹在MEF周围。 There is one thing I don't understand. 有一点我不明白。

Assume that I have an interface, named ISomething, which is a contract, and I have more than one assemblies in a folder that contains my application, and I have no idea how many ISomething implementations are in them, or which one contains one. 假设我有一个名为ISomething的接口，它是一个契约，我在包含我的应用程序的文件夹中有多个程序集，我不知道它们中有多少ISomething实现，或者哪一个包含一个。

If I create a host application in which I use MEF, and would like to get an ISomething, but only one, how does MEF decide which implementation it will give me? 如果我创建一个我使用MEF的主机应用程序，并希望得到一个ISomething，但只有一个，MEF如何决定它将给我哪个实现？

Or, similarly to a common IoC container, how do I tell it which implementation should it use? 或者，与常见的IoC容器类似，如何告诉它应该使用哪个实现？

Thanks in advance for your answers. 提前感谢您的回答。

2 个解决方案

See this blog post for a discussion of this issue and some of the options you have. 有关此问题的讨论以及您拥有的一些选项，请参阅此博客文章。 Also, Glenn Block has a blog post describing how to customize the container behavior with defaults . 此外，Glenn Block有一篇博客文章描述了如何使用默认值自定义容器行为。

In the case of MEF, if you have many Exports that will satisfy an Import, you have two options: 对于MEF，如果您有许多将满足Import的Export，您有两个选择：

Change your Import around to use [ImportMany] . 更改您的导入以使用[ImportMany] 。 Decide, at runtime, which of the Imports to use for your contract, potentially just picking the first, or one at random. 在运行时确定要用于合同的哪个Imports，可能只是选择第一个，或随机选择一个。
Use [ImportMany] in conjunction with Metadata in order to decide which Import to use. 将[ImportMany]与元数据结合使用以确定要使用的Import。