c# - 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

question from:https://stackoverflow.com/questions/619716/why-cast-to-an-interface

Answer 1

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();
}