C＃日期操作

我做错了吗？C＃日期操作

   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();

由于某种原因，对大多数人来说可能很明显，日期不会从test1更改为test3。
请帮忙！

来源

2012-02-07 CompanyDroneFromSector7G

DateTime是不可改变的 - 当你调用Subtract它不会改变调用的目标，它回报一个新值：

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();

虽然我使用-运营商，并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();

甚至更好，但使用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();

来源

2012-02-07 11:22:11

嗯，我说，这是可能明显对大多数人来说！感谢这样一个完整的答案！ – CompanyDroneFromSector7G 2012-02-07 11:30:47

@bukko请将上面的帖子设置为“答案” – fizzbuzz 2012-02-07 11:36:42

@hotadvice我试过 - 有一段时间延迟。有耐心！ – CompanyDroneFromSector7G 2012-02-07 11:48:06

DateTime是不变的，这意味着每次调用Subtract和AddDays实际上与返回值的新实例。 previousDay永远不会改变。

来源

2012-02-07 11:22:39 Lukazoid

:)这将返回一个新的datetime，但并没有改变主叫对象值

previousDay.AddDays(-1);

来源

2012-02-07 11:23:04

我们再次遭到怀疑。团结！ Stack的弱者，让我们冷静下来！ – 2012-02-07 11:56:48

尝试：（如日期时间像串类是不可变的）

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

等

来源

2012-02-07 11:23:54 Jeb

减用新值返回时间跨度。你继续使用旧的价值。

来源

2012-02-07 11:24:09 David

修改这一行：

previousDay.Subtract(new TimeSpan(1, 0, 0, 0));

为

previousDay = previousDay.Subtract(new TimeSpan(1, 0, 0, 0));

，如果你不想改变一个变量

DateTime updatedPreviousDay = previousDay.Subtract(new TimeSpan(1, 0, 0, 0));

来源

2012-02-07 11:24:21 Azodious

问题是你实际上并没有改变previousDay的价值。

您需要实际分配新值：

previousDay = previousDay.AddDays(-1);

来源

2012-02-07 11:26:42 breathingdust

你只是减去日期时间变量的新的时间跨度，但不分配结果的日期时间变量。所以，这些更改不会影响日期时间变量。所以，最好这样做。

previousDay = previousDay.Subtract(new TimeSpan(1, 0, 0, 0)); 
string test2 = previousDay.ToString(); 

or 

string test2 = (previousDay.Subtract(new TimeSpan(1, 0, 0, 0))).ToString();

来源

2012-02-07 11:26:45

C＃日期操作

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