下面是我以前写的一个计算长数据的计算器的一部分，原则上，可以计算任意大的数据，只是时间可能会稍长，比如计算1024的1024次方开10个线程计算需要3分钟多的，当然我只给了你加法，其它的你自己按小学课本上的减乘除的方法计算就行了。
//这是一个长数据类
public class LongNumber
    {
        public NumberFlag Flag { get; set; }
        public string Number { get; set; }

        public LongNumber()
        {
            this.Flag = NumberFlag.Zero;
            this.Number = "0";
        }

        public LongNumber(string number)
        {
            if (!LongNumber.CheckNumber(number))
                throw new InvalidCastException("Input not is a number.");

            this.Flag = NumberFlag.Positive;
            this.Number = number;
        }

        public LongNumber(NumberFlag flag, string number)
        {
            if (!LongNumber.CheckNumber(number))
                throw new InvalidCastException("Input not is a number.");

            this.Flag = flag;
            this.Number = number;
        }

        public override string ToString()
        {
            if (LongNumber.EqualsZero(this))
                return "0";
            else
            {
                return string.Format("{0}{1}", this.Flag == NumberFlag.Positive ? string.Empty : "-", this.Number.TrimStart('0'));
            }
        }

        public static bool CheckNumber(string a)
        {
            if (string.IsNullOrWhiteSpace(a))
                return false;

            var content = a;
            foreach (var item in content)
            {
                if (!"0123456789.".Contains(item))
                {
                    return false;
                }
            }
            return true;
        }

        public static bool EqualsZero(LongNumber a)
        {
            foreach (var item in a.Number)
            {
                if (!"0.".Contains(item))
                {
                    return false;
                }
            }
            return true;
        }
    }

    public enum NumberFlag
    {
        Negative,
        Zero,
        Positive
    }
}

/// <summary>
        /// 比较两个长整数的大小
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <returns></returns>
        private int Compare(LongNumber a, LongNumber b)
        {
            //格式化数据
            var inputA = a.Number;
            var inputB = b.Number;
            FormatNumber(ref inputA, ref inputB);
            a.Number = inputA;
            b.Number = inputB;

            var flagA = NumberFlag.Zero;
            var flagB = NumberFlag.Zero;

            if (!LongNumber.EqualsZero(a))
                flagA = a.Flag;

            if (!LongNumber.EqualsZero(b))
                flagB = b.Flag;

            if (flagA == NumberFlag.Zero && flagB == NumberFlag.Zero)
            {
                return 0;
            }

            if (flagA > flagB)
                return 1;
            else if (flagA < flagB)
                return -1;
            else
            {
                for (var i = 0; i < a.Number.Length; i++)
                {
                    var itemA = int.Parse(a.Number[i].ToString());
                    var itemB = int.Parse(b.Number[i].ToString());

                    if (itemA == itemB)
                        continue;

                    if (itemA > itemB)
                        return 1;
                    else
                        return -1;
                }
                return 0;
            }
        }

        /// <summary>
        /// 获取小数位数
        /// </summary>
        /// <param name="a"></param>
        /// <returns></returns>
        private int getDecimalDigits(string a)
        {
            return a.IndexOf('.') >= 0 ? a.Length - a.IndexOf('.') - 1 : 0;
        }

/// <summary>
        /// 计算a加b的结果
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <returns></returns>
        private LongNumber Add(LongNumber a, LongNumber b)
        {
            //格式化数据
            var inputA = a.Number;
            var inputB = b.Number;
            FormatNumber(ref inputA, ref inputB);
            a.Number = inputA;
            b.Number = inputB;

            //定义进位值
            var carry = 0;
            var sbResult = new StringBuilder();
            for (var i = a.Number.Length - 1; i >= 0; i--)
            {
                var tempA = a.Number[i];
                var tempB = b.Number[i];

                if (tempA == '.')
                {
                    sbResult.Insert(0, tempA);
                    continue;
                }

                var tempResult = int.Parse(tempA.ToString()) + int.Parse(tempB.ToString()) + carry;
                if (tempResult > 9)
                {
                    carry = 1;
                    tempResult -= 10;
                }
                else
                {
                    carry = 0;
                }
                sbResult.Insert(0, tempResult.ToString());
            }

            if (carry > 0)
                sbResult.Insert(0, carry);

            return new LongNumber(sbResult.ToString());
        }