[NSString stringWithContentsOfURL:] is not linked in a single point.
You can call it manually as follows:
var url = new NSUrl ("http://www.google.com/");
var str = (NSString) Runtime.GetNSObject (Messaging.IntPtr_objc_msgSend_IntPtr (Class.GetHandle ("NSString"), Selector.GetHandle ("stringWithContentsOfURL:"), url.Handle));
source
share