What is the namespace for XmlTextReader

Dear All,
 
A Silverlight project with reference to System.Xml namespace. Following code not working:

XmlReader xmlReader = new XmlTextReader(System.IO.File.OpenRead(MyFile));
 
The error said:

Error   18   The type or namespace name 'XmlTextReader' could not be found (are you missing a using directive or an assembly reference?) 

How to resolve this problem?