C＃日期操作

我做錯了嗎？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();

由於某種原因，對大多數人來說可能很明顯，日期不會從test1更改爲test3。
請幫忙！

來源

2012-02-07 CompanyDroneFromSector7G

DateTime是不可改變的 - 當你調用Subtract它不會改變調用的目標，它回報一個新值：

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();

雖然我使用-運營商，並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();

來源

2012-02-07 11:22:11

嗯，我說，這是可能明顯對大多數人來說！感謝這樣一個完整的答案！ – CompanyDroneFromSector7G 2012-02-07 11:30:47

@bukko請將上面的帖子設置爲「答案」 – fizzbuzz 2012-02-07 11:36:42

@hotadvice我試過 - 有一段時間延遲。有耐心！ – CompanyDroneFromSector7G 2012-02-07 11:48:06

DateTime是不變的，這意味着每次調用Subtract和AddDays實際上與返回值的新實例。 previousDay永遠不會改變。

來源

2012-02-07 11:22:39 Lukazoid

:)這將返回一個新的datetime，但並沒有改變主叫對象值

previousDay.AddDays(-1);

來源

2012-02-07 11:23:04

我們再次遭到懷疑。團結！ Stack的弱者，讓我們冷靜下來！ – 2012-02-07 11:56:48

嘗試：（如日期時間像串類是不可變的）

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

等

來源

2012-02-07 11:23:54 Jeb

減用新值返回時間跨度。你繼續使用舊的價值。

來源

2012-02-07 11:24:09 David

修改這一行：

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

爲

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

，如果你不想改變一個變量

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

來源

2012-02-07 11:24:21 Azodious

問題是你實際上並沒有改變previousDay的價值。

您需要實際分配新值：

previousDay = previousDay.AddDays(-1);

來源

2012-02-07 11:26:42 breathingdust

你只是減去日期時間變量的新的時間跨度，但不分配結果的日期時間變量。所以，這些更改不會影響日期時間變量。所以，最好這樣做。

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();

來源

2012-02-07 11:26:45

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