asp.net-mvc – 控制器没有从不同的程序集加载？

我有两个项目：

> Web应用程序
>类库

在Web应用程序中,我定义了一个Route：

routes.MapRoute("Root","root/{Action}",new {Controller = "Root",Action = "Index" });

现在当我点击以下URL时,路由匹配,但是会引发404错误.任何人都可以告诉我为什么会这样吗？ (p.s.RootController位于类库中)

http://webapp/root

我试过添加一个自定义控制器工厂：

public class ControllerFactory : DefaultControllerFactory {
    protected override IController GetControllerInstance(RequestContext reqContext,Type controllerType)
    {
        // reqContext.Route is correct and has the "Root" route loaded
        // controllerType seems to be null ??

        // if I break execution here and manually set controllerType to
        // typeof(ClassLibrary.RootController) and continue from there,// then everything works as expected... 

        // So this method is being called without controllerType... but why??
    }
}

我还尝试在路由中添加名称空间属性：

routes.MapRoute("Root",new {
                Controller = "Root",Action = "Index",Namespaces = new[] { typeof(RootController).Namespace }                     
              });