You've used StringDictionary as its datatype. However, there's a catch there: as per .NET API doc, "The key is handled in a case-insensitive manner; it is translated to lowercase before it is used with the string dictionary.". This is not OK for XML which is case sensitive. So if I am trying to lookup a namespace, say http://www.w3.org/2001/XMLSchema, it won't work unless I lower it. Which should not be the case. Thank you.