MEF：如果将构造代码放入构造函数中，则导入不起作用，但如果在返回之前将其置于函数中，则无效

Question

我有以下服务。

[Export(typeof(IPluginsLoaderService))]
internal class PluginsLoaderService : IPluginsLoaderService
{
    public PluginsLoaderService()
    {
        AggregateCatalog catalog = new AggregateCatalog();
        catalog.Catalogs.Add(new DirectoryCatalog(@"..\..\Plugins\Shapes"));
        catalog.Catalogs.Add(new DirectoryCatalog(@"..\..\Plugins\Behaviours"));
        CompositionContainer container = new CompositionContainer(catalog);
        container.ComposeParts(this);
    }

    [ImportMany(typeof (IBehaviorPlugin))] 
    public IEnumerable<IBehaviorPlugin> BehaviorPlugins { get; set; }

    [ImportMany(typeof(IGeometryPlugin))]
    public IEnumerable<IGeometryPlugin> ShapePlugins { get; private set; }
}

在其他课程中，我将其导入到这样的属性：

    [Import]
    public IPluginsLoaderService PluginsLoaderService { get; set; }

PluginsLoaderService不为null，但BehaviorPlugins和ShapePlugins属性包含0个元素。

如果将零件代码组合到下面的函数中，则会加载插件。

[Export(typeof(IPluginsLoaderService))]
internal class PluginsLoaderService : IPluginsLoaderService
{
    [ImportMany(typeof (IBehaviorPlugin))] 
    public IEnumerable<IBehaviorPlugin> BehaviorPlugins { get; set; }

    [ImportMany(typeof(IGeometryPlugin))]
    public IEnumerable<IGeometryPlugin> ShapePlugins { get; private set; }

    public IEnumerable<IBehaviorPlugin> GetBehaviorPlugins()
    {
        AggregateCatalog catalog = new AggregateCatalog();
        catalog.Catalogs.Add(new DirectoryCatalog(@"..\..\Plugins\Shapes"));
        catalog.Catalogs.Add(new DirectoryCatalog(@"..\..\Plugins\Behaviours"));
        CompositionContainer container = new CompositionContainer(catalog);
        container.ComposeParts(this);
        return BehaviorPlugins;
    }

    public IEnumerable<IGeometryPlugin> GetShapePlugins()
    {
        AggregateCatalog catalog = new AggregateCatalog();
        catalog.Catalogs.Add(new DirectoryCatalog(@"..\..\Plugins\Shapes"));
        catalog.Catalogs.Add(new DirectoryCatalog(@"..\..\Plugins\Behaviours"));
        CompositionContainer container = new CompositionContainer(catalog);
        container.ComposeParts(this);
        return ShapePlugins;
    }
}

为什么会这样？如何以更好的方式组织它？是否可以在MefBootstrapper内部进行？这是我的引导程序代码。

    public class Bootstrapper : MefBootstrapper
{
    protected override DependencyObject CreateShell()
    {
        return Container.GetExportedValue<Shell>();
    }

    protected override void InitializeShell()
    {
        base.InitializeShell();
        Application.Current.MainWindow = (Window) this.Shell;
        Application.Current.MainWindow.Show();
    }


    protected override void ConfigureAggregateCatalog()
    {
        base.ConfigureAggregateCatalog();


        this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(GetType().Assembly));
        this.AggregateCatalog.Catalogs.Add(new DirectoryCatalog(@"..\..\Plugins\Shapes"));
        this.AggregateCatalog.Catalogs.Add(new DirectoryCatalog(@"..\..\Plugins\Behaviours"));            
        this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(ToolbarModule).Assembly));
        this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(ToolboxModule).Assembly));
        this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(CanvasModule).Assembly));
        this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(StatusBarModule).Assembly));
        this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(IPluginsLoaderService).Assembly));
    }
}