Double to Hex conversion Using C#

internal static string DoubleToHex(double value, int maxDecimals)
{
    string result = string.Empty;
    if (value < 0)
    {
        result += "-";
        value = -value;
    }
    if (value > ulong.MaxValue)
    {
        result += double.PositiveInfinity.ToString();
        return result;
    }
    ulong trunc = (ulong)value;
    result += trunc.ToString("X");
    value -= trunc;
    if (value == 0)
    {
        return result;
    }
    result += ".";
    byte hexdigit;
    while ((value != 0) && (maxDecimals != 0))
    {
        value *= 16;
        hexdigit = (byte)value;
        result += hexdigit.ToString("X");
        value -= hexdigit;
        maxDecimals--;
    }
    return result;
}

internal static ulong HexToUInt64(string hex)
{
    ulong result;
    if (ulong.TryParse(hex, NumberStyles.HexNumber, CultureInfo.InvariantCulture, out result))
    {
        return result;
    }
    throw new ArgumentException("Cannot parse hex string.", "hex");
}