I have a whole web page source code in a string type variable.
These code is given below:-
If you use System.Windows.Controls.WebBrowser control then this should work, however it works on assumption that you only have one CODE tag:
var myTagValue = (wbBrowser.Document as mshtml.HTMLDocument)
.getElementsByTagName("CODE")
.OfType<mshtml.IHTMLElement>()
.First()
.innerText
You'll need to reference Internet Explorer's COM library: Microsoft HTML Object Library. getElementsByTagName returns you list of all matching tags, no matter where they are in HTML