MEF和Unity有什么不同和目的？

问题描述

我刚刚开始学习DI（我正在使用WPF / Silverlight，但我有计划移动到ASP.NET）。从互联网上阅读了一些DI文章后，我有两个框架，我感兴趣，MEF和Unity。我想知道他们之间的真实世界是不同的，哪一个是好的。

I just start study DI (I'm working on WPF/Silverlight but I have a plan to move to ASP.NET). After I read some DI articles from internet there are two Frameworks that I'm interested in, MEF and Unity. I want to know what is the real-world different between them and which one is good to go.

推荐答案

主要的区别是统一你将明确地在组合中注册你想要使用的每个类：

The main difference is that with unity you will explicitly register each class you want to use in the composition:

var container = new UnityContainer();
container.RegisterType<IFoo,Foo>();
container.RegisterType<IBar,Bar>();
...
var program = container.Resolve<Program>();
program.Run();

p>

In MEF on the other hand, you mark classes with attributes instead of registering them somewhere else:

[Export(typeof(IFoo))]
public Foo
{
   ...
}

一见钟情看起来像一个小小的语法差异，但实际上更重要比起那个来说。 MEF旨在允许动态发现零件。例如，使用 DirectoryCatalog ，您可以通过简单地将应用程序文件夹中的新DLL删除来扩展应用程序。

At first sight this looks like a minor syntactic difference, but it is actually more important than that. MEF is designed to allow for the dynamic discovery of parts. For example, with a DirectoryCatalog you can design your application in such a way that you can extend it by simply dropping new DLLs in the application folder.

在此示例中，MEF将使用 [Export（typeof（IPlugin））] [/ code]属性，并将这些实例传递给程序构造函数：

In this example, MEF will find and instantiate all classes with an [Export(typeof(IPlugin))] attribute in the given directory and passes those instances to the Program constructor:

[Export]
public class Program
{
    private readonly IEnumerable<IPlugin> plugins;

    [ImportingConstructor]
    public Program(
       [ImportMany(typeof(IPlugin))] IEnumerable<IPlugin> plugins)
    {
        this.plugins = plugins;
    }

    public void Run()
    {
        // ...
    }
}

入口点：

public static void Main()
{
    using (var catalog = new DirectoryCatalog(".","*"))
    using (var container = new CompositionContainer(catalog))
    {
        var program = container.GetExportedValue<Program>();
        program.Run();
    }
}

为了适应这种动态组合场景，MEF有一个概念的稳定的组合，这意味着当它在某个地方遇到缺失依赖时，它将简单地将该部分标记为不可用，并且将继续组合。

To accommodate such dynamic composition scenarios, MEF has a concept of "stable composition", which means that when it runs into a missing dependency somewhere it will simply mark the part as unavailable and will continue the composition anyway.

，但它也使它。因此，如果您不需要动态发现零件和稳定组合，我将使用常规DI容器而不是MEF。与MEF不同，常规DI容器会在依赖关系丢失时给出明确的错误消息。

Stable composition can be quite useful, but it also makes it very difficult to debug a failed composition. So if you don't need dynamic discovery of parts and "stable composition", I would use a regular DI container instead of MEF. Unlike MEF, regular DI containers will give you clear error messages when a dependency is missing.

也可以通过使用DI容器获得最佳的两个世界它与MEF集成，如。使用Autofac构建核心应用程序，MEF可用于需要动态扩展的部件。

It might also be possible to get the best of both worlds by using a DI container which integrates with MEF, like Autofac. Use Autofac to compose the core application, and MEF for the parts which need to be dynamically extensible.

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