Implementing a deep clone method()

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C#

I need to implement a (deep) cloning method in .NET within my library information system.
Why is BinaryFormatter not being recognized?

using System.IO;
using System.Runtime.Serialization.Formatters;
public static Audit DeepClone<T>(Audit obj)
   {
       using (var ms = new MemoryStream())
       {
           var formatter = new BinaryFormatter();
           formatter.Serialize(ms, obj);
           ms.Position = 0;

           return (Audit)formatter.Deserialize(ms);
       }
   }

I am using Visual Studio 2010/.NET 4.5.

References:
1. Spec at http://www.scribd.com/doc/178540653/Software-Engineering-Project-IS3139-A-School-Library-v2?secret_password=127olku950mzko3glvep[^]
2. Code snippet sourced from http://stackoverflow.com/questions/129389/how-do-you-do-a-deep-copy-an-object-in-net-c-specifically[^]
3. BinaryFormatter Class documentation at http://msdn.microsoft.com/en-us/library/system.runtime.serialization.formatters.binary.binaryformatter(v=vs.110).aspx[^]