MEF 编程指南（八）：过滤目录

当使用子容器的时候，基于特定的标准（Specific Criteria）过滤目录是很必要的。比如，基于部件构造策略的过滤器是很常见的。下面的代码片段演示了如何构建的特殊途径（Particular Approach）:

var catalog = new AssemblyCatalog(typeof(Program).Assembly);
var parent = new CompositionContainer(catalog);

var filteredCat = new FilteredCatalog(catalog,
def => def.Metadata.ContainsKey(CompositionConstants.PartCreationPolicyMetadataName) &&
((CreationPolicy)def.Metadata[CompositionConstants.PartCreationPolicyMetadataName]) == CreationPolicy.NonShared);
var child = new CompositionContainer(filteredCat, parent);

var root = child.GetExportedObject<Root>();
child.Dispose();

如果 CreationPolicy 作为标准选择部件不能满足需求，你可能想到使用 [System.ComponentModel.Composition.PartMetadataAttribute] 替代。它允许为部件附加元数据，因此可以使用它构建过滤表达式。下面的示例就是 PartMetadata 特性的应用：

[PartMetadata("scope", "webrequest"), Export]
public class HomeController : Controller
{
}

这允许你创建局限于 Web 请求（逻辑）部件的子容器。请注意：范围边界取决于你的定义，换句话说，MEF 不知道 "webrequest"是什么，所以你不得不创建一些基础代码来在每次Web 请求时创建/回收（Create/Dispose）容器。

var catalog = new AssemblyCatalog(typeof(Program).Assembly);
var parent = new CompositionContainer(catalog);

var filteredCat = new FilteredCatalog(catalog,
def => def.Metadata.ContainsKey("scope") &&
def.Metadata["scope"].ToString() == "webrequest");
var perRequest = new CompositionContainer(filteredCat, parent);

var controller = perRequest.GetExportedObject<HomeController>();
perRequest.Dispose();

注意：我们并没有提供 FilteredCatalog 类。下面演示了如何构建一个简单的实现。

using System;
using System.ComponentModel.Composition.Primitives;
using System.ComponentModel.Composition.Hosting;
using System.Linq;
using System.Linq.Expressions;

public class FilteredCatalog : ComposablePartCatalog, INotifyComposablePartCatalogChanged
{
private readonly ComposablePartCatalog _inner;
private readonly INotifyComposablePartCatalogChanged _innerNotifyChange;
private readonly IQueryable<ComposablePartDefinition> _partsQuery;

public FilteredCatalog(ComposablePartCatalog inner,
Expression<Func<ComposablePartDefinition, bool>> expression)
{
_inner = inner;
_innerNotifyChange = inner as INotifyComposablePartCatalogChanged;
_partsQuery = inner.Parts.Where(expression);
}

public override IQueryable<ComposablePartDefinition> Parts
{
get
{
return _partsQuery;
}
}

public event EventHandler<ComposablePartCatalogChangeEventArgs> Changed
{
add
{
if (_innerNotifyChange != null)
_innerNotifyChange.Changed += value;
}
remove
{
if (_innerNotifyChange != null)
_innerNotifyChange.Changed -= value;
}
}

public event EventHandler<ComposablePartCatalogChangeEventArgs> Changing
{
add
{
if (_innerNotifyChange != null)
_innerNotifyChange.Changing += value;
}
remove
{
if (_innerNotifyChange != null)
_innerNotifyChange.Changing -= value;
}
}
}

原文地址：
http://mef.codeplex.com/wikipage?title=Filtering%20Catalogs&referringTitle=Guide