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
      
       (); 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
      
       (); 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
      
        _partsQuery;     public FilteredCatalog(ComposablePartCatalog inner,                           Expression
       
        
         > expression)    {        _inner = inner;        _innerNotifyChange = inner as INotifyComposablePartCatalogChanged;        _partsQuery = inner.Parts.Where(expression);    }     public override IQueryable
         
           Parts    {        get        {            return _partsQuery;        }    }     public event EventHandler
          
            Changed    {        add        {            if (_innerNotifyChange != null)                _innerNotifyChange.Changed += value;        }        remove        {            if (_innerNotifyChange != null)                _innerNotifyChange.Changed -= value;        }    }     public event EventHandler
           
             Changing { add { if (_innerNotifyChange != null) _innerNotifyChange.Changing += value; } remove { if (_innerNotifyChange != null) _innerNotifyChange.Changing -= value; } }}
           
          
         
        
       
      

 

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