你在哪里把非控制器，非模型代码放在ASP.Net MVC项目中？

Where do you put non-controller, non-model code, like util classes, extension methods and so on in a ASP.Net MVC project? Maybe there's not a specific place to put it, you just put it anywhere, if so, any recommendation? Any best practices?Where do you put non-controller, non-model code