C# function to convert Numerals

I'm trying to write a practical function to convert an input with Roman Numerals into Decimal. I did it before already in 2 directions via Javascript, but in the C# version I have an issue with the while iteration in my loop, and I don´t know yet how to get this working.

class ToRoman
{
    public static int RomanToDecimal(string romanNums)
    {
        int result = 0; 

        int[] deci = new int[] {1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1};
        string[] roman = new string[] {"M", "CM", "D", "CD", "C", "XD", "L", "XL", "X", "IX", "V", "IV", "I"};

        for (int i = 0; i < deci.Length; i++)
        {
            while (romanNums.IndexOf(roman[i] == romanNums[i])
            {
                result += deci[i];
                romanNums = romanNums.Replace(roman[i]," ");
            }
        }
        return result;
    }

    static void Main()
    {
        Console.WriteLine(RomanToDecimal("V"));
        Console.WriteLine(RomanToDecimal("XIX"));
        Console.WriteLine(RomanToDecimal("MDXXVI"));
        Console.WriteLine(RomanToDecimal("MCCCXXXVII"));
    }
}