在MEF2强类型的元数据（System.Composition）

问题描述

我使用的是从在MEF2不再使用懒＆LT; IExtension，IExtensionMetadata＆GT; ，但现在你必须提供元数据视图具体类型（也可能使用ExportFactory＆LT;>而不是懒惰＆LT;？>）。

I've read that in MEF2 you no longer use Lazy<IExtension, IExtensionMetadata>, but now you must provide a concrete type for the metadata view (and possibly use ExportFactory<> instead of Lazy<> ?).

不过，我无法找到如何这应该一切工作的例子 - 短短提到用一个具体类型，而不是一个接口的

However, I can't find any examples of how this should all work - just a few mentions of using a concrete type instead of an interface.

我已经尝试了一些东西，但不断收到以下错误 - 为帐户ID缺少，并没有提供缺省值导出的元数据的

I've tried a few things, but keep getting the following error - "Export metadata for 'AccountID' is missing and no default value was supplied".

我的code ...

My code...

创建容器（在Global.asax中或App_Start文件夹）：

Creating the container (in Global.asax or App_Start folder):

// Get assemblies that will be providing imports and exports
var assemblies = GetAssemblies();

// Get conventions that will be used to find imports and exports
var conventions = GetConventions();

var container = new ContainerConfiguration().WithAssemblies(assemblies, conventions).CreateContainer();

// Create and apply a MefControllerFactory so controllers can be composed
ControllerBuilder.Current.SetControllerFactory(new MefControllerFactory(container));

GetConventions（）方法：

GetConventions() method:

private static ConventionBuilder GetConventions()
{
    var conventionBuilder = new ConventionBuilder();
    conventionBuilder.ForTypesDerivedFrom<IController>().Export();

    conventionBuilder.ForTypesDerivedFrom<IExtension>().Export<IExtension>();
    conventionBuilder.ForTypesMatching(t => t.Namespace != null && t.Namespace.EndsWith(".Parts")).Export().ExportInterfaces();

    return conventionBuilder;
}

IExtension.cs：

IExtension.cs:

public interface IExtension
{
    void DoWork();
}

ExtensionMetadata.cs：

ExtensionMetadata.cs:

public class ExtensionMetadata
{
    public int AccountID { get; set; }
}

ExtensionA.cs（同ExtensionB.cs）：

ExtensionA.cs (same as ExtensionB.cs):

public void DoWork()
{
    System.Diagnostics.Debug.WriteLine("ExtensionA doing work..");
}

ExtensionManager.cs：

ExtensionManager.cs:

public class ExtensionManager
{       
    private IEnumerable<ExportFactory<IExtension, ExtensionMetadata>> _extensions;

    public ExtensionManager(IEnumerable<ExportFactory<IExtension, ExtensionMetadata>> extensions)
    {
        _extensions = extensions;
    }

    public void DoWork(int accountID)
    {
        foreach (var extension in _extensions)
        {
            if (extension.Metadata.AccountID == accountID)
            {
                extension.DoWork();
            }                   
        }           
    }
}

我想我失去了一些东西很重要的位置。基本上我想懒洋洋地导入所有扩展，检查他们的元数据，并且如果条件满足有扩展做一些事情。

I think I'm missing something quite major here. Basically I want to lazily import all Extensions, check their metadata and if a condition is fulfilled have that extension do something.

真的AP preciate您的意见或来样code /教程，涵盖我的方案的任何链接。

Would really appreciate your feedback or any links to sample code / tutorials that cover my scenario.

非常感谢！

推荐答案

我想我已经看完this SO质疑。

我创建了一个元数据属性：

I created a Metadata Attribute:

[MetadataAttribute]
public class ExtensionMetadataAttribute : ExportAttribute, IExtensionMetadata
{
    public int AccountID { get; set; }

    public ExtensionMetadataAttribute(int accountID) : base(typeof (IExtension))
    {
        AccountID = accountID;
    }
}

然后修改ExtensionA.cs：

Then modified ExtensionA.cs:

[ExtensionMetadata(1)]
public class ExtensionA : IExtension
{
    public void DoWork()
    {
        System.Diagnostics.Debug.WriteLine("ExtensionA doing work..");
    }
}

现在ExtensionManager.cs看起来是这样的：

And now ExtensionManager.cs looks like this:

public class ExtensionManager : IExtensionManager
{
    private readonly IEnumerable<ExportFactory<IExtension, ExtensionMetadata>> _extensions;

    public ExtensionManager(IEnumerable<ExportFactory<IExtension, ExtensionMetadata>> extensions)
    {
        _extensions = extensions;
    }

    public void DoWork(int accountID)
    {
        foreach (var extension in _extensions)
        {
            if (extension.Metadata.AccountID == accountID)
            {
                using (var foo = extension.CreateExport())
                {
                    foo.Value.DoWork();
                }
            }
        }
    }
}

这似乎这样的伎俩，但我仍然有兴趣在任何反馈重的最佳实践，性能问题等。

This seems to do the trick, but I would still be interested in any feedback re best practices, performance issues etc.

谢谢！

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