Number base conversion class in C#

public static string Convert(string value, int sourceRadix, int targetRadix)
{
    const string digits = "0123456789abcdefghijklmnopqrstuvwxyz";

    System.Numerics.BigInteger bigint = 0;

    for (int index = value.Length - 1; index >= 0; index--)
    {
        bigint += digits.IndexOf(value[index].ToString(System.Globalization.CultureInfo.InvariantCulture), StringComparison.OrdinalIgnoreCase) * System.Numerics.BigInteger.Pow(sourceRadix, value.Length - 1 - index);
    }

    System.Text.StringBuilder result = new StringBuilder();

    System.Numerics.BigInteger workingValue = bigint;
    while (workingValue > 0)
    {
        int digitValue = (int)(workingValue % targetRadix);
        result.Insert(0, digits[digitValue]);
        workingValue = (workingValue - digitValue) / targetRadix;
    }
    return result.ToString();
}

private static List<NumberCharacter> populateCharacterList(int baseNumber)
{
    //this characterlist is a local list and is returned at the end of the method
    List<NumberCharacter> CharacterList2 = new List<NumberCharacter>();

    //This allows for the program to select from this array ... up to 26 characters beyond base-10 or... base-36
    char[] arrayChars = new char[] { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z' };

    char[] tempArray = arrayChars;
    int tempNumber = baseNumber;

    //this int will control which character is selected from the arraryChars Array
    int iterations = 0;
    int value = 0;
    do
    {
        if (value > 9)
        {
            NumberCharacter nctemp = new NumberCharacter();
            nctemp.DecNumber = value;
            nctemp.Character = tempArray[iterations];
            iterations++;
            CharacterList2.Add(nctemp);
            value++;
            tempNumber--;
        }
        else
        {
            NumberCharacter nctemp = new NumberCharacter();
            nctemp.DecNumber = value;
            nctemp.Character = (char)value;
            CharacterList2.Add(nctemp);
            value++;
            tempNumber--;
        }
    }
    while (tempNumber != 0);
    return CharacterList2;
}

public class NumberCharacter
{
    public long DecNumber { get; set; }
    public char Character { get; set; }
}

private static string getAssociatedDecimalNumberForCharacter(string passedString)
{
    foreach (NumberCharacter x in CharacterList)
    {
        if (x.Character.ToString() == passedString)
        {
            return x.DecNumber.ToString();
            break;
        }
    }
    return "^";
}

private static string getAssociatedCharacterForDecimalNumber(long tempVal2)
{
    foreach (NumberCharacter x in CharacterList)
    {
        if(x.DecNumber == tempVal2)
        {
            return x.Character.ToString();
            break;
        }
    }

    return "^";
}

public static string getDecimalNumberForPassedValue(string passedNumber, int fromBase)
{
    List<double> doubleList = new List<double>();

    List<string> characterList = new List<string>();
    List<string> resultsList = new List<string>();
    //intList.Add(1);
    double tempVal = 1;
    //We will add the Decimal Values of Each Place Value to a List
    for (int i = 0; i < passedNumber.Length; i++ )
    {
        doubleList.Add(tempVal);
        tempVal = tempVal * fromBase;
        string subString = passedNumber[passedNumber.Length - (i + 1)].ToString();
        //substring is an iteration of each character that is passed in... we can check to make sure the character is
        //not greater than the value of the fromBase...
        characterList.Add(subString);
    }

    double tempNumber2 = 0;
    for (int j = doubleList.Count - 1; j >= 0; j--)
    {
        double Num;
        bool isNum = double.TryParse(characterList[j], out Num);
        if (isNum)
        {
            if(double.Parse(characterList[j]) >= fromBase)
            {
                MessageBox.Show("Be Sure the Characters you are using Do not go Beyond the bounds of the Selected Numbering System... For Example, Do not use the letter A in a Decimal Number...");
                return "null";
            }
            else
            {
                double tempVal3 = doubleList[j] * double.Parse(characterList[j]);
                tempNumber2 = tempNumber2 + tempVal3;
            }

        }
        else
        {
            try
            {
                string tempString = getAssociatedDecimalNumberForCharacter(characterList[j]);
                if (double.Parse(tempString) >= fromBase)
                {
                    MessageBox.Show("Be Sure the Characters you are using Do not go Beyond the bounds of the Selected Numbering System... For Example, Do not use the letter A in a Decimal Number...");
                    return "null";
                }
                else
                {
                    //string tempString = getAssociatedDecimalNumberForCharacter(characterList[j]);
                    double tempVal3 = doubleList[j] * double.Parse(tempString);
                    tempNumber2 = tempNumber2 + tempVal3;
                }
            }
            catch (Exception e)
            {
                MessageBox.Show("Be Sure the Characters you are using Do not go Beyond the bounds of the Selected Numbering System... For Example, Do not use the letter A in a Decimal Number..." + e.ToString());
                return "null";
                //break;
            }
        }
    }
    string resultsString = tempNumber2.ToString();
    return resultsString;
}

public static string CreateSelectedBaseNumberFromDecimal(double DecNumber, int baseNumber)
{
    //We will add the Decimal Values of Each Place Value's Exponential Value to a List
    //For instance, for decimal we would add 1, 10, 100, 1000, etc... These numbers represent the associated exponential value of the base
    //10^0 = 1; 10^1 = 10; 10^2 = 100... etc... because each value added to the list also has an index location... index location 0 for the value 1;
    // index location 1 for the value 10; index value 2 for the value 100... these index values also correspond to the associated exponential
    // power for that particular place value
    List<double> doubleList = new List<double>();

    List<string> resultsList = new List<string>();

    double tempVal = 1;
    double tempNumber = DecNumber;
    do
    {
        doubleList.Add(tempVal);
        tempVal = tempVal * baseNumber;
        if(tempVal > tempNumber)
        {
            tempNumber = 0;
        }
    }
    while (tempNumber != 0);
    double tempNumber2 = DecNumber;
    for (int i = doubleList.Count - 1; i >= 0; i--)
    {
        double tempVal2 = tempNumber2 / doubleList[i];
        double remainder = tempNumber2 % doubleList[i];
        if (tempVal2 > 0)
        {
            string tempString5 = tempVal2.ToString()[0].ToString();
            if (tempVal2 > 10)
            {
                tempString5 = getAssociatedCharacterForDecimalNumber(Int64.Parse(tempVal2.ToString().Substring(0, 2)));
                double subtractor = double.Parse(getAssociatedDecimalNumberForCharacter(tempString5)) * doubleList[i];
                resultsList.Add(tempString5);
                tempNumber2 = tempNumber2 - subtractor;
            }
            else
            {
                double subtractor = double.Parse(tempString5) * doubleList[i];
                resultsList.Add(tempVal2.ToString()[0].ToString());
                tempNumber2 = tempNumber2 - subtractor;
            }
        }
        else
        {
            resultsList.Add("0");
        }
    }
    //return tempNumber2.ToString();
    string resultsString = "";
    foreach(string x in resultsList)
    {
        resultsString = resultsString + x;
    }
    return resultsString;
}