I don't think that can be done with pure wiki markup. From the lists help page on mediawiki.org:
For simplicity, list items in wiki markup cannot be longer than a paragraph. A following blank line will end the list and reset the counter on ordered lists. Separating unordered list items usually has no noticeable effects.
Paragraphs can be forced in lists by using HTML tags. Two line break symbols,
<br /><br />, will create the desired effect. So will enclosing all but the first paragraph with
This works for normal paragraphs, but not for multiline code blocks because they need actual newlines.
However, MediaWiki does support some HTML tags, including
<ul>, so you can obtain the desired affect using HTML markup, like so:
Line C Sharp