C#大数据运算

C#整数的加减乘除：
class Comput
{
    //64位机字长表示的最大十进制数长度为19
    private readonly int MAX_LEN = 19;

    /// <summary>
    /// 大数(整型)相加
    /// </summary>
    /// <param name="strNum1">被加数</param>
    /// <param name="strNum2">加数</param>
    /// <returns>返回相加结果</returns>
    public string BigNumAdd(string strNum1, string strNum2)
    {
        if (null == strNum1 || null == strNum2 )
        {
            return string.Empty;
        }

        bool _isNum1 = Regex.IsMatch(strNum1, @"^[0-9]|^-[0-9]");
        bool _isNum2 = Regex.IsMatch(strNum2, @"^[0-9]|^-[0-9]");
        if (!_isNum1 || !_isNum2)
        {
            return "输入的参数不全是数字";
        }

        bool _is_Negative_Add = false;
        if (0 == string.Compare(strNum1.Substring(0, 1), "-") &&
            0 == string.Compare(strNum2.Substring(0, 1), "-"))
        {

            //strnum1和strnum2都是负数
            _is_Negative_Add = true;
            strNum1 = strNum1.Substring(1, strNum1.Length - 1);
            strNum2 = strNum2.Substring(1, strNum2.Length - 1);
        }
        else if (0 == string.Compare(strNum1.Substring(0, 1), "-") &&
                 0 != string.Compare(strNum2.Substring(0, 1), "-"))
        {
            //strnum1是负数,strnum2是正数
            return BigNumSub(strNum2, strNum1.Substring(1, strNum1.Length - 1));
        }
        if (0 != string.Compare(strNum1.Substring(0, 1), "-") &&
            0 == string.Compare(strNum2.Substring(0, 1), "-"))
        {
            //strnum1是正数,strnum2是负数
            return BigNumSub(strNum1, strNum2.Substring(1, strNum2.Length - 1));
        }

        if (1 < strNum1.Length)
        {
            strNum1 = strNum1.TrimStart('0');
        }
        if (1 < strNum2.Length)
        {
            strNum2 = strNum2.TrimStart('0');
        }

        long _lngNum1 = 0;
        long _lngNum2 = 0;
        int _intlen1 = strNum1.Length;