Why cast to an interface?

Question

In Jesse Liberty's Programming C# (p.142) he provides an example where he casts an object to an interface.

 interface IStorable  {     ...  }   public class Document : IStorable  {     ...  }   ...  IStorable isDoc = (IStorable) doc;    ... 

What is the point of this, particularly if the object's class implements the inteface anyway?

EDIT1: To clarify, I'm interested in the reason for the cast (if any), not the reason for implementing interfaces. Also, the book is his 2001 First Edition (based on C#1 so the example may not be germane for later versions of C#).

EDIT2: I added some context to the code

Answer 1

Because you want to restrict yourself to only methods provided by the interface. If you use the class, you run the risk of calling a method (inadvertently) that's not part of the interface.

Answer 2

There is only one reason when you actually need a cast: When doc is of a base type of an actual object that implements IStorable. Let me explain:

public class DocBase {   public virtual void DoSomething()   {    } }  public class Document : DocBase, IStorable {   public override void DoSomething()   {     // Some implementation     base.DoSomething();   }    #region IStorable Members    public void Store()   {     // Implement this one aswell..     throw new NotImplementedException();   }    #endregion }  public class Program {   static void Main()   {     DocBase doc = new Document();     // Now you will need a cast to reach IStorable members     IStorable storable = (IStorable)doc;   } }  public interface IStorable {   void Store(); }