在c＃安装项目中强制回滚安装程序

I have created a custom action dll.I just want to check if a product with same name exists(Done).If yes tell the user to uninstall the product by throwing a InstallException.However if the products are installed in same directory the Install state of the prev product is removed & the install state of new product is copied in the base directory,due to which the prev product cannot be uninstalled.Is there any procedure to achieve a solution to this problem.I have created a custom action dll.I just want