c# - 为什么将 DateTime 格式化为字符串会截断而不是舍入毫秒？

Question

当 Double 被格式化为字符串舍入时。例如

Console.WriteLine(12345.6.ToString("F0"));

输出

12346

However, when a DateTime is formatted as a string truncation is used. E.g.

var ci = CultureInfo.InvariantCulture;
var dateTime = DateTime.Parse("2011-09-14T15:18:42.999", ci);
Console.WriteLine(dateTime.ToString("o", ci));
Console.WriteLine(dateTime.ToString("s", ci));
Console.WriteLine(dateTime.ToString("yyyy-MM-hhThh:mm:ss.f", ci));

输出

2011-09-14T15:18:42.9990000
2011-09-14T15:18:42
2011-09-14T15:18:42.9

What is the reasoning (if any) behind this behavior?

---

Rounding to nearest second can be achieved by adding half a second before formatting as a string:

var ci = CultureInfo.InvariantCulture;
var dateTime = DateTime.Parse("2010-12-31T23:59:59.999", ci);
Console.WriteLine(dateTime.ToString("s", ci));
var roundedDateTime = dateTime.AddMilliseconds(500);
Console.WriteLine(roundedDateTime.ToString("s", ci));

输出

2010-12-31T23:59:59
2011-01-01T00:00:00

Answer 1

这有点主观，但我会说舍入日期和时间值而不是截断它们会导致“更多”意外行为。

例如，四舍五入 new DateTime(2011, 1, 1, 23, 59, 59, 999) 将完全是新的一天。这听起来比仅仅截断值要奇怪得多。