Redirecting default.aspx to root virtual directory

I have a simple ASP.NET 3.5 application running under IIS7 under a virtual directory. So the URL of my app is like http://example.com/app. I want to 301-redirect the request to example.com/app/default.aspx to example.com/app for better SEO. I have to do this redirect through code only, not by any IIS settings. I am unable to do so via code mentioned in this article:

Mass 301 redirects in ASP.NET, including pages that need to redirect to a different place depending on the query string parameters

We have several pages of our site indexed using old non-SEO friendly URLS such as http://www.domain.com/DocumentDetails.aspx?id=555. Recently we implemented routing that uses slugs stored in the database and looks up the slug to forward you to the right page using routing, for example: http://www.domain.com/Documents/Title-of-the-Document