Roman Numerals converter in C#. Run as a console application from Visual Studio. The 'Romanise' function is where the magic happens.

Program.cs

using System;

namespace RomanNumerals
{
    class Program
    {
        // Create rules in descending order
        static Rule[] Rules = new Rule[]
        {
            new Rule(1000, "M"),
            new Rule(900, "CM"),
            new Rule(500, "D"),
            new Rule(400, "CD"),
            new Rule(100, "C"),
            new Rule(90, "XC"),
            new Rule(50, "L"),
            new Rule(40, "XL"),
            new Rule(10, "X"),
            new Rule(9, "IX"),
            new Rule(5, "V"),
            new Rule(4, "IV"),
            new Rule(1, "I"),
        };

        static void Main(string[] args)
        {
            // Let the user convert numbers until they close the program
            while (true)
            {
                Console.WriteLine("Enter a positive integer:");
                int n;
                if (Int32.TryParse(Console.ReadLine(), out n) && n > 0)
                    Console.WriteLine("{0} in roman numerals is:\t {1}", n, Romanise(n)); // Write out the result
                else
                    Console.WriteLine("That's not a positive integer");
            }
        }

        private static string Romanise(int n)
        {
            if (n == 0) return ""; // Recursion termination

            foreach (var rule in Rules) // Rules are in descending order
                if (rule.N <= n)
                    return rule.Symbol + Romanise(n - rule.N); // Recurse 

            // If this line is reached then n < 0
            throw new ArgumentOutOfRangeException("n must be greater than or equal to 0");
        }

        // Represents a substitution rule for a roman-numerals like numerical system
        // Number 'N' is equivilant to string 'Symbol' in the system.
        class Rule
        {
            public int N { get; set; }
            public string Symbol { get; set; }
            public Rule(int n, string symbol)
            {
                N = n;
                Symbol = symbol;
            }
        }
    }
}