Web Forms :: Find Current URL Of Page During Code Behind Submission (NOT Current Web App URL)

I am looking for a way to figure out the current URL that the page is currently on (NOT what the asp.net page currently is, but where the CODE is at). ie. My web app is located at: [URL] my code is: String page = [URL]

String response = GetResponse(page); //basically the above code goes to the website [URL] and parses the HTML within it and brings it back and populates the variable string "response". But, sometimes the [URL] throws me a curve ball and redirects me to: [URL] I want to be able to use a try/catch to be able to "catch" the error of a different page: ie validateUser.aspx. So, I need to do to this: try

{
String page = [URL];
String response = GetResponse(page);
}
catch
{
//code to check the behind URL to see if [URL] is the URL OR IF [URL] is the current URL
}

understand I know how to find the URL of the current page the web app is on. I need to find the current page that threw the exception during the execution of the code behind.