我在 .Net 4.0 中使用 MEF 为我节省了大量的抽象工厂代码和配置 gubbins。无法移动到 .net 4.5，因为它未部署。
班上
/// <summary>
/// Factory relies upon the use of the .net 4.0 MEF framework
/// All processors need to export themselves to make themselves visible to the 'Processors' import property auto MEF population
/// This class is implemented as a singleton
/// </summary>
public class MessageProsessorFactory
{
private static readonly string pluginFilenameFilter = "Connectors.*.dll";
private static CompositionContainer _container;
private static MessageProsessorFactory _instance;
private static object MessageProsessorFactoryLock = new object();
/// <summary>
/// Initializes the <see cref="MessageProsessorFactory" /> class.
/// Loads all MEF imports
/// </summary>
/// <exception cref="System.NotSupportedException"></exception>
private MessageProsessorFactory()
{
lock (MessageProsessorFactoryLock)
{
if (_container == null)
{
RemoveDllSecurityZoneRestrictions();
//Create a thread safe composition container
_container = new CompositionContainer(new DirectoryCatalog(".", pluginFilenameFilter), true, null);
_container.ComposeParts(this);
}
}
}
/// <summary>
/// A list of detected class instances that support IMessageProcessor
/// </summary>
[ImportMany(typeof(IMessageProcessor), RequiredCreationPolicy = CreationPolicy.NonShared)]
private List<Lazy<IMessageProcessor, IMessageProccessorExportMetadata>> Processors { get; set; }
/// <summary>
/// Gets the message factory.
/// </summary>
/// <param name="messageEnvelope">The message envelope.</param>
/// <returns><see cref="IMessageProcessor"/></returns>
/// <exception cref="System.NotSupportedException">The supplied target is not supported: + target</exception>
public static IMessageProcessor GetMessageProcessor(MessageEnvelope messageEnvelope)
{
if (_instance == null)
_instance = new MessageProsessorFactory();
var p = _instance.Processors.FirstOrDefault(
s => s.Metadata.ExpectedType.AssemblyQualifiedName == messageEnvelope.AssemblyQualifiedName);
if (p == null)
throw new NotSupportedException(
"The supplied type is not supported: " + messageEnvelope.AssemblyQualifiedName);
return p.Value;
}
/// <summary>
/// Removes any zone flags otherwise MEF wont load files with
/// a URL zone flag set to anything other than 'MyComputer', we are trusting all pluggins here.
/// http://msdn.microsoft.com/en-us/library/ms537183(v=vs.85).aspx
/// </summary>
private static void RemoveDllSecurityZoneRestrictions()
{
string path = System.IO.Path.GetDirectoryName(
System.Reflection.Assembly.GetExecutingAssembly().Location);
foreach (var filePath in Directory.EnumerateFiles(path, pluginFilenameFilter))
{
var zone = Zone.CreateFromUrl(filePath);
if (zone.SecurityZone != SecurityZone.MyComputer)
{
var fileInfo = new FileInfo(filePath);
fileInfo.DeleteAlternateDataStream("Zone.Identifier");
}
}
}
}
调用之后_container.ComposeParts(this);，处理器会填充所有找到的 IMessageProcessor 实现。伟大的。
笔记
- GetMessageProcessor 被许多线程调用。
- 我们无法控制开发人员如何构建 IMessageProcessor 的类实现，因此我们不能保证它们是线程安全的 - 可重入。但是，该类必须使用 Export 属性进行检测。
导出属性
[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
public class MessageProccessorExportAttribute : ExportAttribute
{
public MessageProccessorExportAttribute()
: base(typeof(IMessageProcessor))
{
}
public Type ExpectedType { get; set; }
}
- ExpectedType 只是元数据，它记录了 IMessageProcessor.ProcessMessage() 期望处理的内容，纯粹是实现细节。
我的问题是，我到处都读到，每个 Imported 实例都是一个 Singleton，无论其通过 Lazy<> 引用构造时的激活策略如何。
因此，我们不能允许从 MEF 中的实例从 GetMessageProcessor 返回，因为多个线程将获取相同的实例，这是不希望的。啊！我想知道以下“解决方法”是否是最好的方法，或者我是否把 MEF 坚持概念弄错了。
我的解决方法是将看似毫无意义的RequiredCreationPolicy = CreationPolicy.NonShared属性设置更改为CreationPolicy.Shared.
然后更改函数 GetMessageProcessor 以手动创建一个新实例，与 MEF 完全分离。使用 MEF 共享实例 plurry 作为类型列表。
IMessageProcessor newInstance = (IMessageProcessor)Activator.CreateInstance(p.Value.GetType());
完整的方法；
public static IMessageProcessor GetMessageProcessor(MessageEnvelope messageEnvelope)
{
if (_instance == null)
_instance = new MessageProsessorFactory();
var p = _instance.Processors.FirstOrDefault(
s => s.Metadata.ExpectedType.AssemblyQualifiedName == messageEnvelope.AssemblyQualifiedName);
if (p == null)
throw new NotSupportedException(
"The supplied type is not supported: " + messageEnvelope.AssemblyQualifiedName);
// we need to create a new instance from the singleton instance provided by MEF to assure we get a instance un-associated with the MEF container for
// currently as of .net 4.0 it wants to keep references on objects which may impact memory consumption.
// As we have no control over how a developer will organise there class that exposes an Export,
// this could lead to multithreading issues as an imported lazy instance is a singleton regardless
// of the RequiredCreationPolicy.
// MEF is still invaluable in avoided a tone of abstract factory code and its dynamic detection of all supporting
// Exports conforming to IMessageProcessor means there is no factory config for us to maintain.
IMessageProcessor newInstance = (IMessageProcessor)Activator.CreateInstance(p.Value.GetType());
return newInstance;
}