I have the following structure:

Photo Collage is a page with radio buttons to select one of the products (4x4, 5x5...).

When I click my submit button, I need to go to 'Theme' page which is a direct child of Photo Collage (not in the photo), but the URL needs to change depending on the product selected (the selection influences the content of the Theme page).

This way, I don't have to duplicate the theme page for each product, but the Theme page is bookmarkable with the selected product.

My problem is that after messing for a while with the Umbraco pipeline and reading a lot of posts. I can't figure out how to change the URL. Even though I am sure the URL returned by the IUrlProvider is unique I still get an error on the back end.

public string GetUrl(UmbracoContext umbracoContext, int id, Uri current, UrlProviderMode mode)
    {
        //throw new NotImplementedException();
        // Take curent content item from cache by its id
        var content = umbracoContext.ContentCache.GetById(id);

        // Check if the content exists and it has alias companyItem
        if (content != null && (content.DocumentTypeAlias == "product"))
        {
            if (content.Parent.DocumentTypeAlias == "productGroup" && content.Parent.Parent.DocumentTypeAlias == "productDetail")
            {
                bool hasThemeAfter = false;

                foreach (var child in content.Parent.Parent.Children)
                    if (child.DocumentTypeAlias == "themes" && child.Children.Count() <= 0)
                        hasThemeAfter = true;

                if (hasThemeAfter)
                {
                    // Build path
                    string route = content.Parent.Url + "Themes/" + content.Name + "/";

                    //var domainUri = new Uri(current.GetLeftPart(UriPartial.Authority));

                    //return AssembleUrl(domainUri, route, current, mode).ToString();
                    return route;
                }
            }
        }

        return null;
}