C# 日期操作

Question

我做错了吗？

                DateTime previousDay = new DateTime(year, month, day);
                string test1 = previousDay.ToString();
                previousDay.Subtract(new TimeSpan(1, 0, 0, 0));
                string test2 = previousDay.ToString();
                previousDay.AddDays(-1);
                string test3 = previousDay.ToString();

由于某些对大多数人来说可能显而易见的原因，日期不会从 test1 更改为 test3。
请帮忙!

Answer 1

DateTime 是不可变的 - 当您调用 Subtract 时，它不会更改调用的目标，它返回一个新值:

DateTime previousDay = new DateTime(year, month, day);
string test1 = previousDay.ToString();
previousDay = previousDay.Subtract(new TimeSpan(1, 0, 0, 0));
string test2 = previousDay.ToString();
previousDay = previousDay.Subtract(new TimeSpan(1, 0, 0, 0));
string test3 = previousDay.ToString();

尽管我会使用 - 运算符和 TimeSpan.FromDays:

DateTime previousDay = new DateTime(year, month, day);
string test1 = previousDay.ToString();
previousDay = previousDay - TimeSpan.FromDays(1);
string test2 = previousDay.ToString();
previousDay = previousDay - TimeSpan.FromDays(1);
string test3 = previousDay.ToString();

或者更好的是，使用 AddDays:

DateTime previousDay = new DateTime(year, month, day);
string test1 = previousDay.ToString();
previousDay = previousDay.AddDays(-1);
string test2 = previousDay.ToString();
previousDay = previousDay.AddDays(-1);
string test3 = previousDay.ToString();

或者更好的是，使用 Noda Time :)

LocalDate date = new LocalDate(year, month, day);
string test1 = previousDay.ToString();
date = date.PlusDays(-1);
string test2 = previousDay.ToString();
date = date.PlusDays(-1);
string test3 = previousDay.ToString();

或者:

LocalDate date = new LocalDate(year, month, day);
string test1 = previousDay.ToString();
date = date + Period.FromDays(-1);
string test2 = previousDay.ToString();
date = date + Period.FromDays(-1);
string test3 = previousDay.ToString();