1 BigInteger num1 = BigInteger.Parse("乘数一");
2 BigInteger num2 = BigInteger.Parse("乘数二");
3 Console.WriteLine(num1* num2);
4 Console.ReadKey();

 1             static string getbackresult(string s1, string s2)
 2             {
 3                 string lastResult = "";
 4                 int ten1 = -1;
 5                 int[] result = new int[s1.Length + s2.Length];//用以记录计算结果
 6                 for (int i = s1.Length - 1; i >=0; i--)
 7                 {
 8                     int c1 = Convert.ToInt16(s1[i].ToString());//从s1中从后往a前取一个数并记录这个数的位置
 9                     ten1++;
10                     int resultindex = result.Length - 1 - ten1;
11                     for (int j = s2.Length - 1; j >=0; j--)
12                     {
13                         int c2 = Convert.ToInt16(s2[j].ToString());//从?s2中从后往前取一个数
14                         int cc = c1 * c2 + result[resultindex];
15                         if (cc > 10)
16                         {
17                             result[resultindex] = cc % 10;
18                             int lastindex = resultindex - 1;  //往前移动一位
19                             int lastvalue = result[lastindex] + cc / 10;  //把刚刚得到的十位的数字赋值到前面
20                             while (cc > 10)
21                             {
22                                 cc = result[lastindex] + cc / 10;
23                                 result[lastindex] = cc % 10;
24                                 lastindex--;//进位
25                             }
26                         }
27                         else
28                         {
29                             result[resultindex] = cc;
30                         }
31                         resultindex--;//进位
32                     }
33                 }
34                 StringBuilder sb = new StringBuilder();
35                 foreach (int item in result)
36                 {
37                     sb.Append(item);
38                 }
39                 lastResult = sb.ToString();
40                 if (lastResult[0]=='0')
41                 {
42                     lastResult = lastResult.Substring(1);
43                 }
44                 return lastResult;
45             }