string value = File.ReadAllText(@"f:\test.txt"); //假设test.txt里只有一行1,2,3
string[] numberStrs = value.Split(',');
int thirdNumber = int.Parse(numberStrs[2]) + 7; //第三个数加上7
numberStrs[2] = thirdNumber.ToString();
File.WriteAllText(@"f:\test.txt", string.Join(",", numberStrs));
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