send xml file to web service

i want to send an xml file to a web service. the web service will accept the
xml file, process it and perhaps return some info.

how do i set up the web service and the client so that i can send the xml
file from client to web service?

currently it looks like this

[WebMethod]
public void GetScanFile(XmlDocument sf)
{
XmlTextWriter myXmlTextWriter = new XmlTextWriter ("c:\\WCCS-CS\\"+sf, null);
myXmlTextWriter.Close();
}

and i call it like this
WebRef.MyService a=new WebRef.MyService

XmlDocument x=new XmlDocument();

x.LoadDoc("C:\\test.xml");
a.GetScanFile(x);

ALL OF THE SYNTAX AND connections to the web serice work fine
when i hit the last line i get a System.xml system error, so obviously sending
an xml like this doesnt work. what will?