Convert to Roman number

RomanNumerals.cs

using System;
using System.Collections.Generic;
using System.Linq;

public class RomanNumerals
{
    struct NumberInfo
    {
        public int Value;
        public int RangeBegin;
        public int RangeEnd;
        public int PowLen => RangeBegin.ToString().Length - 1;

        public NumberInfo(int value, int rangeBegin, int rangeEnd)
        {
            Value = value;
            RangeBegin = rangeBegin;
            RangeEnd = rangeEnd;
        }

        public bool InRange(int n)
        {
            return (n >= RangeBegin) && (n <= RangeEnd);
        }
    }
    static Dictionary<char, NumberInfo> numbers = new Dictionary<char, NumberInfo>(){
        {'I', new NumberInfo(1, 1, 3)},
        {'V', new NumberInfo(5, 4, 8)},
        {'X', new NumberInfo(10, 9, 39)},
        {'L', new NumberInfo(50, 40, 89)},
        {'C', new NumberInfo(100, 90, 399)},
        {'D', new NumberInfo(500, 400, 899)},
        {'M', new NumberInfo(1000, 900, 4000)}
    };

    public static string ToRoman(int n)
    {
        if (n <= 3)
            return new String(numbers.ElementAt(0).Key, n);

        foreach (var i in numbers)
            if (i.Value.InRange(n))
                if (n % (int)Math.Pow(10, i.Value.PowLen) == 0 || n.ToString().Length == 1)
                {
                    if (n < i.Value.Value)
                        return ToRoman(i.Value.Value - n) + i.Key;
                    else if (n == i.Value.Value)
                        return i.Key.ToString();
                    else if (n > i.Value.Value)
                        return i.Key + ToRoman(n - i.Value.Value);
                }
                else
                    return ToRoman(n - (n % (int)Math.Pow(10, i.Value.PowLen))) + ToRoman(n % (int)Math.Pow(10, i.Value.PowLen));

        return "";
    }

    public static int FromRoman(string romanNumeral)
    {
        if (romanNumeral.Length == 1)
            return numbers[romanNumeral[0]].Value;
        if (string.IsNullOrWhiteSpace(romanNumeral))
            return 0;
        var bni = BiggestNumberIndex(romanNumeral);
        var BiggestNumber = FromRoman(romanNumeral[bni].ToString());
        var Before = FromRoman(romanNumeral.Substring(0, bni));
        var After = bni == romanNumeral.Length - 1 ? 0 : FromRoman(romanNumeral.Substring(bni + 1));
        return BiggestNumber - Before + After;
    }

    static int BiggestNumberIndex(string str)
    {
        int index = 0, number = 0;
        for (int i = 0; i < str.Length; i++)
            if (numbers[str[i]].Value > number)
            {
                index = i;
                number = numbers[str[i]].Value;
            }
        return index;
    }
}