C# with types and options

imaginary.cs

//This is a weak point of C#: declaring classes is verbose.
//But it can be improved with C# 6 new syntax.
public class Age {
    public int Value { get; private set; }

    //Age is correct by construction, 
    //so if you have an Age then it is correct _by definition_ and you don't need to check anymore.
    public Age(int value) {
      if (!IsValidAge(value) throw new InvalidArgumentException("Age must be between 0 and 100");
      Value = value;
    }

    //A way to convert incorrect value to Age, but to Optional age!
    public static Option<Age> fromInt(int value) {
      if (IsValidAge(value)) return value.Some() else return value.None(); 
    }
  
    private static bool IsValidAge(int age) { return (value > 0 && value < 100); }
}

public static class Main {

    //Asking user for his age and trying to convert it to Option
    //Return Some(age) if the age is correct, or None if it was not correct.
    public static Option<Age> AskUsersAge() {
      Age.fromInt(int.Parse(Console.Readline()));
    }
    
    //Note that this method normally get's Age and not Option<Age>, 
    //and it doesn't need to check anyting because Age is correct by construction.
    public static void CalculateBio(Age age) {
       if (age.Value < 20)      return "You are young";
       else if (age.Value < 50) return "You are mid age";
       else                     return "You are going do die soon";
    }
    
    public static void Main(string[] args) {
      AskUserForAge()
        .Filter(x => f > 5) //we're not goint to print anything to babies 'cause they can't read.
        .Select(x => CalculateBio(x)) //call a function that takes "age" we have within the container if we have it
        .Do(x => Console.PrintLine(x)); //print the result if there is a result.
    }
}