ASP.NET

Response.Redirect("Login.aspx?PreviousPage=" + Request.AppRelativeCurrentExecutionFilePath.ToString());

if(Request.QueryString["PreviousPage"] != null)
  Response.Redirect(Request.QueryString["PreviousPage"].ToString());
else
  Response.Redirect("Some where else, might be your products page etc...");

//get the current page url
String url = Page.Request.RawUrl;

//redirect the page to your new one
Response.Redirect(String.Format(CultureInfo.InvariantCulture, "~/login.aspx?pp={0}", url));

//get the url of the previous page.
if(Request.RawUrl.Contains("?pp="))
{
string previousPage = Request.QueryString["pp"]);
}