C# How to create special instances of a class?

Question

For some classes, ideally, I'd like to create special named instances, similar to "null." As far as I know, that's not possible, so instead, I create static instances of the class, with a static constructor, similar to this:

public class Person
{
    public static Person Waldo;  // a special well-known instance of Person
    public string name;
    static Person()  // static constructor
    {
        Waldo = new Person("Waldo");
    }
    public Person(string name)
    {
        this.name = name;
    }
}

As you can see, Person.Waldo is a special instance of the Person class, which I created because in my program, there are a lot of other classes that might want to refer to this special well-known instance.

The downside of implementing this way is that I don't know any way to make all the properties of Person.Waldo immutable, while all the properties of a "normal" Person instance should be mutable. Whenever I accidentally have a Person object referring to Waldo, and I carelessly don't check to see if it's referring to Waldo, then I accidentally clobber Waldo's properties.

Is there a better way, or even some additional alternative ways, to define special well-known instances of a class?

The only solution I know right now, is to implement the get & set accessors, and check "if ( this == Waldo) throw new ..." on each and every set. While this works, I assume C# can do a better job than me of implementing it. If only I can find some C# way to make all the properties of Waldo readonly (except during static constructor.)

Answer 1

Make a private class inside the Person that inherits the Person, ImmutablePerson : Person.

Make all the property setters locked up: override them with NotImplementedException for instance.

Your static Person initialization becomes this: public static readonly Person Waldo = new ImmutablePerson("Waldo");

And static constructor may be removed, too.

Answer 2

Maybe you could have the following hierarchy:

class Person
{
    protected string _name;
    public virtual string Name{
        get{
            return _name;
        }
    }
}

class EditablePerson:Person
{
    public new string Name{
        get{
            return _name;
        }
        set{
            _name=value;
        }
    }
    public Person AsPerson()
    {
        //either return this (and simply constrain by interface)
        //or create an immutable copy
    }
}