Tidy + XHTML from .NET

I've written a simple c#.Net wrapper for the Tidy API binary as provided by Cory Nelson. It all works fine and dandy, but Xhtml output seems to be malfunctioning (single quotes not being converted to double quotes, single tags not being internally terminated - basically it's outputting well formed HTML but not XHTML).

It appears my method for setting options is somehow wrong. I've imported the TidyOptGetIdForName method, passed in 'TidyXhtmlOut', which returns an integer value of 83. I then pass this value into an import of TidyOptSetBool. I've tried using .Net marshalling to pass the boolean value, and have tried a variety of integer values (0, 1, -1) but all seem to do nothing, despite the return code being 0.

Had anybody got any hints?