hiding property from derived class

Question

Jon Skeet brought up this issue once in his videos (though didn't provide with an answer).

Let's say we have a Class named Person and the Person class has Name property

Then we have another class, Spy. Of course a Spy is a Person so we will derive from the Person class.

public class Person
{
    public string Name { get; set; }
}

public class Spy : Person
{

}

We don't want people to know the Spy's name so we'd want this to give a compilation error:

static void ReportSpy(Spy spy) {
  string name = spy.Name;
}

or either:

static void ReportSpy(Spy spy)
{
   Person spyAsPerson = spy;
   string name = spyAsPerson.Name;
}

How could we prevent this kind of things from happening?

Answer 1

Make the Name property virtual in the base Person class. In derived Spy class, override the property and throw Exception in getter.

public class Person
{
    public virtual string Name { get; set; }
}

public class Spy : Person
{
    public override string Name
    {
        get
        {
            throw new Exception("You never ask for a spy's name!");
        }
        set
        {
            base.Name = value;
        }
    }
}

But, rather than throwing exception, I'd suggest something like

get
{
    return "**********";
}

Because, it breaks LSP (mentioned in another answer). What that means here (just an example) is, I can always do like

Person x = new Spy();

and pass it to some other method, which might be like

void RegisterForNextBallGame(Person p)
{
    playerList.Add(p.Name);
}

This method being unaware of the some spy roaming around the stadium, crashes while doing a simple honest duty!

Edit

Just to make it clear, this name=********** is still not a right solution. It will just save from the exceptions! Later, one might find lot of Persons walking down the code with name ********** which will cause later surprises and other issues.

A better solution would be a better design. Check Nathan's answer to get some hint.