The answer is not simple, but is
very simple. If you have something without a root node (which is properly called
document element), this "something" is not XML at all, because this is not a well-formed XML text. You cannot work with such thing using anything related to XML. So, either use real XML, or forget about any .NET XML classes and create your own — from scratch (good luck with that :-)).
Let's see. Suppose you have the simplest example:
<!--
<a>First node</a>
<b>Second node</b>
Load it in any Web browser, for a simple test. This is a typical result:
XML Parsing Error: junk after document element
Location: [...]
Line Number 2, Column 1:<b>Second node</b>
^
That's it. Forget it.
—SA