Cannot use Xml classes in a Coded UI Test project in Visual Studio 2013 premium

Hi, I am working on a Coded UI Test project using Visual Studio 2013 Premium with .NET4.5. However, even I added "using System.Xml;" namespace, in the source code, I cannot access any Xml class, such as XmlElement, XmlDocument etc. Here I attached my screen shot. Would you please help? Thanks.