Like written so many times before, HttpUtility.UrlEncode is extremely outdated and buggy.
It doesn't encode the single quote ('), there's an open bug marked "Closed (Won't Fix)". So don't wait for a patch.
It doesn't encode the plus sign (+) either.

So please use the AntiXss library whenever it's remotely important to get it right.
Needless to say, same goes for the other encoding methods of HttpUtility, like HtmlEncode.

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