ExportAttribute, ImportAttribute, CompositionContainer and MEF in ASP.NET MVC 3

Introduction

本文阐述了托管扩展框架（Managed
Extensibility Framework）在ASP.NET MVC 3的应用。本文不处理MEF或ASP.NET MVC
3系统的复杂性。

Section 1 MEF Basics

以微软观点，MEF不是控制系统的一个反演。然而，MEF提供控制系统的反演能力。

MEF中三个基本结构是exportattribute ，importattribute和compositioncontainer。

你在使用ExportAttribute特性标记以下的代码：

class
field
property
indexer
method
你在使用ImportAttribute特性标记以下的代码：

field
property
indexer
argument.
例1 — 使用ExportAttribute标记类:

[ExportAttribute]
public class A
{
public void ShowMessage()
{
Console.WriteLine("this is class A");
}
}

例2 —
使用

ImportAttribute标记属性：

public class B
{
[ImportAttribute]
public A PropertyA { get; set; }
}

例3 — 使用ExportAttribute标记方法:

public class C
{
[ExportAttribute]
public void DoSomething()
{
}
}

另一个MEF 构造是CompositionContainer 类. 把 CompositionContainer应用在 标记了 ExportAttribute 或ImportAttribute特性的代码上. CompositionContainer 类尝试把exports 匹配
imports.
例4演示了一个完整的程序，定义标记了exportattribute的A类或importattribute的B类。compositioncontainer接收A和B类的实例。该compositioncontainer返回一个组成部分。我选择用控制台应用程序实现。不像一个ASP.NET
MVC 3应用程序，控制台应用程序只需要一个文件。这允许你容易看到三MEF结构的作用。

namespace MefExample4
{
using System;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;

[ExportAttribute]
public class A
{
public void ShowMessage()
{
Console.WriteLine("this is class A");
}
}

[ExportAttribute]
public class B
{
[ImportAttribute]
public A PropertyA { get; set; }
}

class Program
{
static void Main(string[] args)
{
// Declare a composition container.
CompositionContainer compositionContainer = new CompositionContainer();

// Feed the co
bbbb
ntainer instances of A and B.
compositionContainer.ComposeParts(new A(), new B());

// Retrieve the composed part.
B b = compositionContainer.GetExportedValueOrDefault<B>();

// Use the imported construct of B.
b.PropertyA.ShowMessage();
}
}
}

Section 2- MEF in ASP.NET MVC 3

1. You mark pieces of code that MEF should take care of with ExportAttributes and ImportAttributes.

Example 8 —

MessageSource类 

标记 

ExportAttribute

namespace MefMvc01
{
using System.ComponentModel.Composition;

[ExportAttribute]
public class MessageSource
{
public MessageSource()
{
this.Message = "this message is from MessageSource";
}

public string Message { get; private set; }
}
}

Example 9 —  

HomeController类标记

 

ExportAttribute

namespace MefMvc01.Controllers
{
using System.ComponentModel.Composition;
using System.Web.Mvc;

[ExportAttribute]
public class HomeController : Controller
{
[ImportAttribute]
private MessageSource messageSource;

public ActionResult Index()
{
return View(this.messageSource);
}
}
}

2. You create an instance of a CompositionContainer. You supply it with your marked code.

Example 11 — 定义

CompositionContainer

protected void Application_Start()
{
AreaRegistration.RegisterAllAreas();

RegisterGlobalFilters(GlobalFilters.Filters);
RegisterRoutes(RouteTable.Routes);

CompositionContainer compositionContainer = new CompositionContainer();
compositionContainer.ComposeParts(new HomeController(),
new MessageSource());
}

3. You implement the IDependencyResolver interface.

在例12, 在工程增加 实现了IDependencyResolver接口的

MefDependencySolver 

类.

IDependencyResolver 

接口定义了两个方法
: 

GetService 

and 

GetServices

.

namespace MefMvc01
{
using System;
using System.Collections.Generic;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.Web.Mvc;

public class MefDependencySolver : IDependencyResolver
{
public MefDependencySolver(CompositionContainer compositionContainer)
{
this.compositionContainer = compositionContainer;
}

private CompositionContainer compositionContainer;

public object GetService(Type serviceType)
{
string name = AttributedModelServices.GetContractName(serviceType);
return compositionContainer.GetExportedValueOrDefault<object>(name);
}

public IEnumerable<object> GetServices(Type serviceType)
{
return this.compositionContainer
.GetExportedValues<object>(serviceType.FullName);
}
}
}

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