WebFormsRouting.cs

//global.asax

RouteTable.Routes.Add("ProductsRoute", new Route
    (
       "products/apparel", 
       new CustomRouteHandler("~/Products/ProductsByCategory.aspx",
           "category=18")
    ));


//CustomRouteHandler.cs
public class CustomRouteHandler : IRouteHandler
{
    public CustomRouteHandler(string virtualPath, string queryString)
    {
        this.VirtualPath = virtualPath;
        this.QueryString = queryString;
    }

    public string VirtualPath { get; private set; }
    public string QueryString { get; private set; }

    public IHttpHandler GetHttpHandler(RequestContext           |#A
          requestContext)                                       |#A
    {
        requestContext.HttpContext.RewritePath(
            String.Format("{0}?{1}", VirtualPath, QueryString));

        var page = BuildManager.CreateInstanceFromVirtualPath
             (VirtualPath, typeof(Page)) as IHttpHandler;
        return page;
    }
}