为什么枚举MEF部分会导致容器初始化失败

以下代码引发InvalidOperationException，消息：全局容器已初始化

        var aggcat = new AggregateCatalog();
        aggcat.Catalogs.Add(new AssemblyCatalog(Assembly.GetExecutingAssembly().Location));
        // This line does something to the catalog
        aggcat.Parts.ToArray();
        _container = new CompositionContainer(aggcat, true);
        // Exception gets thrown here
        System.ComponentModel.Composition.Hosting.CompositionHost.Initialize(_container);

当我注释掉Parts.ToArray（）行时，它工作得很好。这里发生了什么？