As a quick shot I'd say you have a mistake here:
Func test= t => partial.ToString();
since it references the method partial
instead of its result.
I think what you probably meant/wanted was:
Func test= t => partial(t).ToString();
BTW: If MvcHtmlString
would derive from String, your code would work, since the result type is covariant. But unfortunately this is actually not the case, it was just my 2 cents as a side note.