c# - 带有托管扩展框架 (MEF) 的工厂模式

Question

我正在尝试使用 MEF 实现工厂模式。

这是我的解决方案

核心项目

IClass
ObjectFactory static Class(This is where the problem is)

项目A

[Export(typeof(IClass))]
[ExportMetadata("Type", "TypeA")]
public classA : IClass
{}

项目B

[Export(typeof(IClass))]
[ExportMetadata("Type", "TypeB")]
public classB : IClass
{}

当我尝试动态创建对象时遇到问题

这是工厂类:

public static class ObjectFactory
{
    private static readonly CompositionContainer _container;

    [ImportMany]
    public static IEnumerable<Lazy<IClass, IMetaData>> objectTypes;
    static ObjectFactory()
    {
        AggregateCatalog catalog = new AggregateCatalog();

        catalog.Catalogs.Add(new DirectoryCatalog(Environment.CurrentDirectory));
        _container = new CompositionContainer(catalog);

        try
        {
            objectTypes = _container.GetExports<IClass, IMetaData>();
        }
        catch (CompositionException compositionException)
        {
            Console.WriteLine(compositionException.ToString());
            Console.ReadLine();
        }
    }

    public static IClass CreateObject(ObectType objectType)
    {
        IClass outProvider;

        Type typeToLoad = objectTypes.Where(x => x.Metadata.Type == objectType.ToString()).FirstOrDefault().GetType();
        outProvider = (IClass)Activator.CreateInstance(typeToLoad);

        return outProvider;
    }
}

Answer 1

如果您希望在每次调用 CreateObject 时提供一个新的“NonShared”实例，那么我建议进行此重构。

private static readonly CompositionContainer _container;

static ObjectFactory()
{       
    var directoryCatalog = new DirectoryCatalog(Environment.CurrentDirectory)
    _container = new CompositionContainer(directoryCatalog);        
}

public static IClass CreateObject(ObectType objectType)
{       
    var objectTypes objectTypes = new List<Lazy<IClass, IMetaData>>();
    try
    {
       objectTypes.AddRange(_container.GetExports<IClass, IMetaData>());
    }
    catch (CompositionException compositionException)
    {
        Console.WriteLine(compositionException.ToString());
        Console.ReadLine();
    }

    return objectTypes.FirstOrDefault(x => x.Metadata.Type == objectType.ToString());
}

您会看到 MEF 每次组合类型或调用 GetExports(以及此函数的所有其他重载)时都会解析新实例(即非共享实例)。或者，您可以导出 IClass 工厂，然后您将拥有这些提供程序的集合。

附注您示例中的 objectTypes 成员上的 [ImportMany] 是多余的，因为您没有编写这种类型(我不相信您甚至可以，因为它是静态的)，您只需从 GetExports 的输出以编程方式设置它