调试易

int*int得到的还是int，得到溢出后的结果再赋给long肯定是1
试试：(long)int1 * int2;

     Console.WriteLine(int.MaxValue);     //2147483647     int a = int.MaxValue;
     int b = int.MaxValue;
     int t = a * b;                //1
     long c =(long) a *  (long)b;     Console.WriteLine("c:{0}",(long)a * (long)b);//c:4611686014132420609
     Console.WriteLine("Math.BigMul:" + Math.BigMul(a,b));//Math.BigMul:4611686014132420609