Did you compile the WSDL yourself and incorporate it into your application? If so, I'd recommend going with the iControl Assembly. Just plug the .Net assembly into your project and you don't have to muck around with all the lower level bindings. You can find that download in the iControl Assembly Labs project under the Download menu.
With that being said, I don't see anywhere in your class where you actually make the call. The "error in xml document" usually implies that the SOAP response is not well formed and I'm not sure of a case where that could happen unless there was an exception on the server and in that case you'd get a 500 error back.
Pass along the code you are using to call the methods and I'll try to reproduce your issue and/or provide you some sample code.
If it helps, I wrote a tech tip a while back on the performance graph interfaces and some sample code with PowerShell. The syntax is somewhat close to C so that may help you as well.
http://devcentral.f5.com/Tutorials/TechTips/tabid/63/articleType/ArticleView/articleId/194/iControl-101--11--Performance-Graphs.aspx
-Joe