如何使用datetime获取前一天 [英] How to get previous day using datetime
问题描述
我想在00:00:00将DateTime属性设置为前一天。我不知道为什么DateTime.AddDays(-1)不工作。或者为什么DateTime.AddTicks(-1)不工作。
首先应该这样工作吗?
I want to set a DateTime property to previous day at 00:00:00. I don't know why DateTime.AddDays(-1) isn't working. Or why DateTime.AddTicks(-1) isn't working. First should this work?
我有2个对象。每个对象都有DateTime字段ValidFrom,ValidTo。
I have 2 objects. Each object have DateTime fields ValidFrom, ValidTo.
编辑:从工作回家后,我尝试获得与业务对象相同的行为。以下是我试图复制它的工作原理的代码。当然,这是在家工作,但不在工作。好的事情是,我得到了很好的答案,并+1。 =)
After coming home from work I tried to get the same behavior as my business objects behave. Below are the code I tried to replicate how it looks at work. Of course this is working at home but not at work. The good thing is I got good answers and +1 on all! =)
public class RuleValue
{
public DateTime ValidFrom, ValidTo;
public RuleValue(DateTime _validFrom, DateTime _validTo)
{
ValidFrom = _validFrom;
ValidTo = _validTo;
}
// oldObject.ValidFrom = 1900-01-01
// oldObject.ValidTo = 9999-12-31
// newObject.ValidFrom = 2010-03-22
// newObject.ValidTo = 9999-12-31
public void ChangeOldDate(RuleValue oldObject, RuleValue newObject)
{
/*
* 1: When first object (oldObject) have ValidTo set to SQL-server maxdate (9999-12-12 23:59:59 etc)
* I want to allow for a new object to be created
* 2: oldObject timespan ValidFrom-ValidTo should not be overlapping with newObjects timespan(i have checks for that)
* 3: oldObject.ValidTo should be newObject.ValidFrom - one day/or one tick
*/
if (oldObject.ValidTo == DateTime.MaxValue)
{
oldObject.ValidTo = newObject.ValidFrom.AddTicks(-1); //now works
}
}
}
class Program
{
static void Main(string[] args)
{
RuleValue rv1 = new RuleValue(DateTime.Parse("1900-01-01"), DateTime.MaxValue);
RuleValue rv2 = new RuleValue(DateTime.Parse("2010-03-22"), DateTime.MaxValue);
Console.WriteLine("First: ");
Console.WriteLine("Old = " + rv1.ValidFrom + " - " + rv1.ValidTo);
Console.WriteLine("New = " + rv2.ValidFrom + " - " + rv2.ValidTo);
rv1.ChangeOldDate(rv1, rv2);
Console.WriteLine("After: ");
Console.WriteLine("Old = " + rv1.ValidFrom + " - " + rv1.ValidTo);
Console.WriteLine("New = " + rv2.ValidFrom + " - " + rv2.ValidTo);
Console.ReadKey();
}
}
//Output:
//First:
//Old = 1900-01-01 00:00:00 - 9999-12-31 23:59:59
//New = 2010-03-22 00:00:00 - 9999-12-31 23:59:59
//After:
//Old = 1900-01-01 00:00:00 - 2010-03-21 23:59:59
//New = 2010-03-22 00:00:00 - 9999-12-31 23:59:59
// ALL CORRECT! :D
推荐答案
DateTime是一个不可变的结构。 当您致电 AddDays( )
或 AddTicks()
它返回一个新的DateTime实例,它不会修改你打电话给你的例子。确保将结果分配给变量,或者您的代码中没有可见的更改:
DateTime is an immutable struct. When you call AddDays()
or AddTicks()
it returns a new instance of a DateTime, it does NOT modify the instance you called it on. Make sure you assign the result to a variable or there is no visible change in your code:
DateTime d1 = DateTime.Now;
d1 = d1.AddDays(-1); // assign back to see the new instance
如果您需要将日期的时间部分重置为午夜,你需要使用一个明确的构造函数调用:
If you need to reset the time portion of the date to midnight, you will need to use an explicit constructor call:
DateTime d1 = DateTime.Now;
DateTime d2 = new DateTime( d1.Year, d1.Month, d1.Day, 0, 0, 0 );
DateTime d3 = d1.Date; // a simpler alternative to the above...
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