How to get QueryString from a href?

Question

I am trying to stop XSS attack so I am using html agility pack to make my whitelist and Microsoft Anti-Cross Site Scripting Library to deal with the rest.

Now I am looking at encoding all html hrefs. I get a big string of html code that can contain hrefs. Accours to MS Library they have an URL encode but if you encode the whole URl then it can't be used. So in the example they just encode the query string

UrlEncode Untrusted input is used in a URL (such as a value in a querystring) Click Here!

http://msdn.microsoft.com/en-us/library/aa973813.aspx

So now my questions is how do I parse through a href and find the query string. Is it always just "?" then query string or can it have spaces and be written in different ways?

Edit

This urls will not be written by me but the users who will share them. So that's why I need a way to make sure I get all query strings and not just ones in valid format. If it can work invalid format I have to grab these ones too. Hackers won't care if it is valid format or not as long as it still does what they want.

Answer 1

I believe it is always the part after the ? but you can easily use the Uri class for this:

Uri uri = new Uri("http://foo.com/page.html?query");
string query = uri.Query;

That will include the ? itself. Of course, you can fetch the other bits as well, which could be handy.