public class NewUrl
{
private string _key;
private string _url;

public string Key
{
get { return _key; }
set { _key = value; }
}

public string Url
{
get { return _url; }
set { _url = value; }
}
}


protected void Application_BeginRequest(object sender, EventArgs e)
{
List<NewUrl> routes = new List<NewUrl>()
{ new NewUrl() { Key = "Categories", Url = "~/CodeSubmissionsByCategory.aspx?id=" },
new NewUrl() { Key = "Refactorings", Url = "~/SubmissionDetails.aspx?id=" }
};

HttpContext context = HttpContext.Current;
string oldPath = context.Request.Url.ToString();

var newRoute = (from route in routes
where oldPath.IndexOf(route.Key) > -1
select route).SingleOrDefault();

if (newRoute == null) return;

int i = oldPath.IndexOf(newRoute.Key);

if (i > -1)
{
int underScoreIndex = oldPath.IndexOf("_");
int startIndex = (i + newRoute.Key.Length + 1);
int numberOfCharacters = underScoreIndex - startIndex;

int id = Int32.Parse(oldPath.Substring(startIndex, numberOfCharacters));
context.RewritePath(newRoute.Url + id, false);

}
}