C#日期操纵

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();

由于某些可能对大多数人来说显而易见的原因，日期不会从test1更改为test3。

---

救命啊

DateTime

是不可变的-当您调用

Subtract

时，它不会更改调用的目标，而是返回一个新值：

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();

previousDay = previousDay.AddDays(-1);

虽然我会使用

-

操作符和

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();

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();

或者更好地使用

AddDays

：

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();

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：）

或：

---

DateTime

是不可变的-当调用

Subtract

时，它不会更改调用的目标，而是返回一个新值：

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();

previousDay = previousDay.AddDays(-1);

虽然我会使用

-

操作符和

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();

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();

或者更好地使用

AddDays

：

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();

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：）

或：

---

DateTime

是不可变的，这意味着对

Subtract

和

AddDays

的每次调用实际上都会返回一个带有值的新实例

previousDay

永远不会更改。

DateTime

是不可变的，这意味着每次调用

Subtract

和

AddDays

实际上都会返回一个带值的新实例<代码>前一天将永远不会更改。

：）这将返回一个新的日期时间，但不会更改调用者对象值

previousDay.AddDays(-1);

：）这将返回新的日期时间，但不会更改调用方对象值

previousDay.AddDays(-1);

Try：（因为类似DateTime的字符串类是不可变的）

等等。

试试：（因为类似日期时间的字符串类是不可变的）

---

等等。

用新值减去返回时间跨度。您继续使用旧值

更多信息：

---

用新值减去返回时间间隔。您继续使用旧值

更多信息： 修改此行：

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

作为

或

如果不想更改上一个变量

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

修改此行：

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

作为

或

如果不想更改上一个变量

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

---

问题是您实际上没有更改前一天的值

您需要实际分配新值：

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();

previousDay = previousDay.AddDays(-1);

---

问题是您实际上没有更改前一天的值

您需要实际分配新值：

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();

previousDay = previousDay.AddDays(-1);

---

您只需从datetime变量中减去新的timespan，而不是将结果赋给datetime变量。因此，这些更改不会影响datetime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();

---

您只需从datetime变量中减去新的timespan，而不是将结果赋给datetime变量。因此，这些更改不会影响datetime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();

---

嗯，我说这对大多数人来说可能是显而易见的！谢谢你这么完整的回答@bukko请将上面的帖子设置为“答案”，嗯，我说这对大多数人来说可能是显而易见的！谢谢你这么完整的回答@bukko请把上面的帖子设为“回答”，我们又被抓了。团结这群弱者，让我们投票击败他吧！我们又被抓了。团结这群弱者，让我们投票击败他吧！