怎么取得两字符串的差值："9999999999999999999"-"9999999999999999899"?
单个字符大于长整型，差值在有效的整数范围内。
两字符位数相同。

[解决办法]
decimal de1 = 9999999999999999999m;
decimal de2 = 9999999999999999899m;
long i = (long)(de1-de2);
string str = (de1-de2).ToString();