如何减去日期？ [英] How to subtract dates?

问题描述

嗨我有下面的代码，但是当我运行程序时，计数总是会停留在零......

例如。如果我输入2014年3月3日和daysInWeek为4，它应该返回2计数，但它给我0代替。任何帮助将不胜感激。

  string  datetime = txtEffectiveDate.Text ; 
 DateTime startDate = Convert.ToDateTime（datetime）;  //  用户输入的日期 
  int  daysInWeek = Convert.ToInt32（txtDaysInWeek.Text）;  //   no。用户输入的天数 
 DateTime previousDate = startDate.AddDays（-daysInWeek）;  //  从上面的ori日期中扣除后的日期 
 
 TimeSpan preDiff = previousDate  - 开始日期; 
  int  preDays = preDiff.Days; 
 
  for （ var  j =  0 ; j <  = preDays; j ++）
 {
  var  pretestDate = startDate.AddDays（-j）; 
  switch （pretestDate.DayOfWeek）
 {
  case  DayOfWeek。星期六：
 count ++; 
  break ; 
  case  DayOfWeek.Sunday：
 count ++; 
  break ; 
 
} 
} 
 
 MessageBox.Show（ 您有 + count +  休假日）; 

解决方案

嗯...除非是周六或周日，否则你不会做任何计数，所以你可能需要看一下，但是主要问题非常明显，如果你用调试器快速运行。

在这一行放一个断点：

  code-keyword> var  j =  0 ; j <  = preDays; j ++）

并查看preDays的值。

它是什么？

答案，-4。

并且是0< = -4？

编号所以你的循环永远不会被执行 - 就像你曾经尝试过的那样使用调试器逐步执行代码。也许你想扭转这种局面：

 TimeSpan preDiff = previousDate  -  startDate; 

对此：

< pre lang =c＃> TimeSpan preDiff = startDate - previousDate;

但是......因为这只是你想到的数字 - daysInWeek - 为什么要这样做？

使用调试器逐步执行该代码会向您显示您的 TimeSpan 值为负数。将其更改为：

 TimeSpan preDiff = startDate  -  previousDate; 

Hi i have the codes below, but when i run the program, the count would always stuck at zero...
eg. if i entered 3/3/2014 and daysInWeek as 4, it should return 2 for the count but it give me 0 instead. Any help would be appreciated.

string datetime = txtEffectiveDate.Text;
DateTime startDate = Convert.ToDateTime(datetime); // date entered by users
int daysInWeek = Convert.ToInt32(txtDaysInWeek.Text); //no. of days entered by users
DateTime previousDate = startDate.AddDays(-daysInWeek); // date after subtracted from ori dates above

TimeSpan preDiff = previousDate - startDate;
int preDays = preDiff.Days;
 
for (var j = 0; j <= preDays; j++)
{
   var pretestDate = startDate.AddDays(-j);
   switch (pretestDate.DayOfWeek)
   {
      case DayOfWeek.Saturday:
      count++;
      break;
      case DayOfWeek.Sunday:
      count++;
      break;
 
   }
}
 
MessageBox.Show("You have " + count + " days off");

解决方案

Well...you don't do anything with count, unless it's a Saturday or Sunday, so you probably need to look at that, but the main problem is pretty obvious, if you quickly run trough with the debugger.
Put a breakpoint on this line:

for (var j = 0; j <= preDays; j++)

And look at the value of preDays.
What is it?
Answer, -4.
And is 0 <= -4 ?
No. So your loop is never executed - as you would have seen if you had tried using the debugger to step through the code. Probably, you wanted to reverse this:

TimeSpan preDiff = previousDate - startDate;

To this:

TimeSpan preDiff = startDate - previousDate;

But...since this just works out the number you first thought of - daysInWeek - why do it at all?

Stepping through that code with your debugger would have shown you that your TimeSpan value was negative. Change it to:

TimeSpan preDiff = startDate - previousDate;

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