using System;

class Program {
    // *** before: ***
    // a value type could not hold "no value", so a missing number was
    // encoded by a magic constant or carried a second flag next to it
    const int NoAge = -1;

    static int FindAgeOld(string name) {
        if (name == "Ann") return 31;
        return NoAge;                     // the caller has to know about -1
    }

    // *** in version 2.0: ***
    // int? is Nullable<int>: the same number plus the state "no value"
    static int? FindAgeNew(string name) {
        if (name == "Ann") return 31;
        return null;
    }

    static void Main() {
        int old = FindAgeOld("Bob");
        Console.WriteLine(old == NoAge ? "unknown" : old.ToString());

        int? age = FindAgeNew("Bob");
        Console.WriteLine(age.HasValue ? age.Value.ToString() : "unknown");

        int? found = FindAgeNew("Ann");
        Console.WriteLine(found.GetValueOrDefault());
        Console.WriteLine(found + 1);     // 32, arithmetic lifts to int?

        // a comparison with an empty value is false on BOTH sides
        Console.WriteLine(age > 0);       // False
        Console.WriteLine(age <= 0);      // False as well
    }
}