C#实现大数字的运算 
1、添加引用:System.Numerics.dll
2、添加命名空间:using System.Numerics;
3、实例:
3.1判断一个数字是不是质数
3.2实现两个大数的加减乘除
            Console.Write("请输入第二个大数字:");
            string bigNum2 = Console.ReadLine();
            BigInteger bigInt2 = BigInteger.Parse(bigNum2);
            Console.Write(Environment.NewLine);
            BigInteger addNum = bigInt1 + bigInt2;
            BigInteger subNum = bigInt1 - bigInt2;
            BigInteger purNum = bigInt1 * bigInt2;
            BigInteger divNum = bigInt1 / bigInt2;
            Console.WriteLine("两大数相加结果为:{0}",addNum);
            Console.WriteLine("两大数相减结果为:{0}",subNum);
            Console.WriteLine("两大数相乘结果为:{0}",purNum);
            Console.WriteLine("两大数相除结果为:{0}",divNum);
            Console.ReadLine();
        }
