In C#:

using System;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace RomanNumerConverter
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

    private void button1\_Click(object sender, EventArgs e)
    {
        Hashtable ht = new Hashtable(); // Create a look-up table to look up Roman numerals and get there associated values.
        ht.Add('M', 1000);
        ht.Add('D', 500);
        ht.Add('C', 100);
        ht.Add('L', 50);
        ht.Add('X', 10);
        ht.Add('V', 5);
        ht.Add('I', 1);

        String RomanNumber = textBox1.Text;
        int value = 0;
        for (int index = 0; index < RomanNumber.Length; index++)  // Go through the entered Roman numeral a character at a time
        {
            int nextVal;
            int currentVal = Convert.ToInt32(ht\[RomanNumber\[index\]\]); // Get the value of the current character.
            if (index + 1 < RomanNumber.Length) // if you are not at the end of the string
            {
                nextVal = Convert.ToInt32(ht\[RomanNumber\[index + 1\]\]);// Peek at the next number to see you need to use it to combine characters for a single value(e.g. IV equates to 4)
            }
            else
            {
                nextVal = 0; // No more characters so just set this to 0 to simplfy the algorithm.
            }
            if (nextVal > currentVal)// If the value of nextVal is greater than currentVal, we have a subtractive situation (e.g. IV)
            {
                currentVal = nextVal - currentVal;  // To determine the value you have to subtract the value of the first number of the pair from the second number(e.g. IV = 5 - 1 = 4)
                index++;// Used nextVal to help determine the number so increment the index to skip over that character in the next iteration.
            }
            value += currentVal; // Add the values of the Roman numerals up.
        }
        label1.Text = value.ToString();  //Convert the sum to string to use as a text label.  This is the converted number in Arabic numerals.
    }
}

}