I am using the following code in order to substract a day of the DateTime until I am getting Monday:
DateTime currentWeek = new DateTime(beginDate.Year, beginDate.Month, beginDate.Day);
while (currentWeek.DayOfWeek.ToString() != "Monday")
{
currentWeek.AddDays(-1);
MessageBox.Show(currentWeek.Day.ToString());
MessageBox.Show(currentWeek.DayOfWeek.ToString());
}
beginDate is in the first run set to the current Date of DateTime.Now.
For me this loops forever, and the day of currentWeek always stays the same (29) even though I am substracting 1 everytime I am looping through.
I am already using another function that takes a DateTime and a bool Parameter, which does pretty much the same and works:
private void ErstenTagDerWocheAuswaehlen(DateTime date, bool anfangDerWoche = true)
{
string wochentagName;
int incrementor;
if(anfangDerWoche == true)
{
wochentagName = "Monday";
incrementor = -1;
}
else
{
wochentagName = "Friday";
incrementor = 1;
}
while(date.DayOfWeek.ToString() != wochentagName)
{
date = date.AddDays(incrementor);
}
}
Can someone explain to me why the upper code doesn't work whilst the lower one does?
You have to assign the resulting value, DateTime is immutable.
currentWeek = currentWeek.AddDays(-1);
About your 2nd question:
Use the enum for day of the week, do not try to convert a day of the week to a string for a comparison. The type is DayOfWeek.
Again, a DateTime is not mutable so you have to return a DateTime instance as you can't mutate the one that was passed in (without passing it as ref)
Code change
private DateTime ErstenTagDerWocheAuswaehlen(DateTime date, bool anfangDerWoche = true)
{
System.DayOfWeek wochentagName;
int incrementor;
if(anfangDerWoche == true)
{
wochentagName = System.DayOfWeek.Monday;
incrementor = -1;
}
else
{
wochentagName = System.DayOfWeek.Friday;
incrementor = 1;
}
while(date.DayOfWeek != wochentagName)
{
date = date.AddDays(incrementor);
}
return date;
}
DateTime is an immutable struct, so you need to store the value returned from AddDays():
var t2 = currentWeek.AddDays(-1);
Then use t2. The call to AddDays() doesn't actually change currentWeek.
As DateTime is immutable, when using the AddDays it returns a new DateTime structure with the new information and does not change the given one.
Method documentation states:
Returns a new System.DateTime that adds the specified number of days to the value of this instance.
You must assign it to a variable:
currentWeek = currentWeek.AddDays(-1);
Related
I need to compare two dates including the seconds. I searched the web and got two methods. One is import Microsoft.VisualBasic dll. The result is not same when it runs the same data. I think C# should do the same thing without import VisualBasic. Can someone point the way to me to made it work in C#?
Thanks in advance.
There is one using Microsoft.VisualBasic
if (Math.Abs(DateAndTime.DateDiff(DateInterval.Second,
Conversions.ToDate(colFilesFound[RuntimeHelpers.GetObjectValue(rw["file_path"])]),
Conversions.ToDate(rw["last_modified_timestamp"]),
FirstDayOfWeek.Sunday, FirstWeekOfYear.Jan1)) == 0L)
{
unchangedFileNum++;
Console.WriteLine("unchange");
}
else
{
modifiedFileNum++;
Console.WriteLine("change");
}
There is another method not using Visual.dll:
DateTime fileLastModifiedDate = Conversions.ToDate(colFilesFound[rw["file_path"]]);
DateTime dataLastModifiedDate = Conversions.ToDate(rw["last_modified_timestamp"]);
if (Math.Abs((fileLastModifiedDate - dataLastModifiedDate).TotalSeconds) == 0L)
{
Console.WriteLine("File Date: " + colFilesFound[rw["file_path"]] +
" <> Database Date: " + Conversions.ToString(rw["last_modified_timestamp"]));
unchangedFileNum++;
Console.WriteLine("unchange");
}
else
{
modifiedFileNum++;
Console.WriteLine("change");
}
The correct way to compare DateTimes in .NET is using the == operator or calling the DateTime.Compare() method:
DateTime fileLastModifiedDate = ...;
DateTime dataLastModifiedDate = ...;
if (fileLastModifiedDate == dataLastModifiedDate)
{
...
}
You need to take into account the precission. Sometimes, you cannot have enough precission to consider milliseconds. In this case you need to compare the dates without having into account the milliseconds:
public static bool IsSameDateWithoutMilliseconds(DateTime d1, DateTime d2)
{
return d1.Subtract(d2).TotalSeconds == 0;
}
Your problem is probably the milliseconds since those are probably not equal the way you check the DateTimes.
DateTime fileLastModifiedDate = Conversions.ToDate(colFilesFound[rw["file_path"]]);
DateTime dataLastModifiedDate = Conversions.ToDate(rw["last_modified_timestamp"]);
fileLastModifiedDate = fileLastModifiedDate.AddMilliseconds(-fileLastModifiedDate.Millisecond);
dataLastModifiedDate = dataLastModifiedDate.AddMilliseconds(-dataLastModifiedDate.Millisecond);
if (DateTime.Compare(fileLastModifiedDate, dataLastModifiedDate) == 0)
{
// dates are equal
}
else
{
// dates are not equal
}
I was wondering if there is any neat way to check is data is in allowed range. I mean in c# we can represent data from 0001-01-01 to (I think) 9999-01-01. However if we try to do something like that
DateTime result = DateTime.Parse("0001-01-01").Subtract(TimeSpan.FromDays(1))
I get an exception. Is there any neat way to check is it is possible to do DateTime operations (addition subtraction etc)
Just use the comparison operators (>, <, >=, <=, == and !=), as they are implemented in DateTime.
Example:
DateTime lowerAllowedDate = new DateTime(1,1,1); // 01/01/0001
DateTime upperAllowedDate = new DateTime(3000, 12, 31) // 31/12/3000
DateTime now = DateTime.Now
if (lowerAllowedDate <= now && now < upperAllowedDate)
{
//Do something with the date at is in within range
}
Consider these extension methods.
public static class ValidatedDateTimeOperations
{
public static bool TrySubtract (this DateTime dateTime, TimeSpan span, out DateTime result)
{
if (span < TimeSpan.Zero)
return TryAdd (dateTime, -span, out result);
if (dateTime.Ticks >= span.Ticks)
{
result = dateTime - span;
return true;
}
result = DateTime.MinValue;
return false;
}
public static bool TryAdd (this DateTime dateTime, TimeSpan span, out DateTime result)
{
if (span < TimeSpan.Zero)
return TrySubtract (dateTime, -span, out result);
if (DateTime.MaxValue.Ticks - span.Ticks >= dateTime.Ticks)
{
result = dateTime + span;
return true;
}
result = DateTime.MaxValue;
return false;
}
}
The can be called like this:
DateTime result;
if (DateTime.MinValue.TrySubtract (TimeSpan.FromDays(1), out result)
{
// Subtraction succeeded.
}
Checking for an overflow in a given operation beforehand is cumbersome and I'm not really sure it's really worth it against simply handling the exception.
You could for example do the following when subtracting:
DateTime date;
TimeSpan subtractSpan;
if ((date - DateTime.MinValue) < subtractSpan)
{
//out of range exception: date - subtractSpan
}
Worth it? Your call.
Take a look at the DateTime structure documentation in MSDN.
In particular, you can take a look at:
TryParse and TryParseExact
The comparison operators
MinValue and MaxValue
You can also put try..catch (ArgumentOutOfRangeException) around the DateTime values you are trying to use.
However, if you are consistently (or ever?) running into this kind of exception, I'd take a closer look at your design. Unless you are doing some serious date-crunching, I don't know of any instance where I would be bumping into the min and max values.
Is is possible to change the value returned by DateTime? Or at least assign it to a variable then change that variable?
internal int hour;
internal int minute;
DateTime time = DateTime.Now;
public int incrementHour(int step)
{
if (step > 0 && hour < 24)
{
//step = step % hour;
hour = (hour + step) % 24;
time.AddHours(hour);
return hour;
}//end of if
else
{
MessageBox.Show("Please enter a positive number.");
return 0;
}//end of else
}//end of incrementHour
Addhours doesn't really do anything from the looks of it.
Instances of DateTime are immutable - AddHours() returns a new instance of DateTime that reflects the changed value - so you have to re-assign the changed value to your time variable:
time = time.AddHours(hour);
MSDN for AddHours method (applies to all other methods of DateTime as well):
This method does not change the value
of this DateTime. Instead, it returns
a new DateTime whose value is the
result of this operation. The Kind
property of the returned DateTime
object is the same as that of value.
all you need to do is
time = time.AddHours(hour)
and that will be it
See documentation of DateTime.AddHours, it does not change the parameter value but returns a new DateTime instance. Try:
time = time.AddHours(hour);
I'm reading Fowler Clean Code book and I think that my code is a little messy, I want some suggestions:
I have a simple business requirement that is return the date of new execution of my Thread.
I've two class fields: _hour and _day.
If actual day is higher than my _day field I must return true, so I'll add a month to "executionDate"
If the day is the same, but the actual hour is higher than _hour I should return true too.
So I did this simple method:
private bool ScheduledDateGreaterThanCurrentDate (DateTime dataAtual) {
if (dateActual.Day > _day) {
return true;
}
if (dateActual.Day == _day && dateActual.Hour > _hour) {
return true;
}
if (dateActual.Day == _day && dateActual.Hour == _hour)
if (dateActual.Minute>0 || dateActual.Second>0)
return true;
return false;
}
I'm programming with TDD, so I know that the return is correct, but this is bad maintain code right?
var compareDate = new DateTime(
dateActual.Year,
dateActual.Month,
_day,
_hour,
0,
0);
return dateActual> compareDate;
DateTime objects can be compared against one another.
For example, say my class has this field:
// Using DateTime.Today as an example
DateTime _date = DateTime.Today;
I can then do this instead of a method call
if (dataAtual > _date) {
// logic here
}
If you use the datetime object in .NET it will do all this logic for you.
In the below thisIsAlwaysTrue should always be true.
DateTime d = DateTime.Now;
bool thisIsAlwaysTrue = d == d;
But does DateTime.Now work in such a way that isThisAlwaysTrue is guaranteed to be true? Or can the clock change between references to the Now property?
bool isThisAlwaysTrue = DateTime.Now == DateTime.Now;
The clock can definitely change between two back-to-back calls to DateTime.Now;
The DateTime.Now property is volatile, meaning it definitely can change between uses. But the variable you assign it to is not volatile.
So this should always set result to true:
DateTime d = DateTime.Now;
bool result = d == d;
It assigns the value returned by DateTime.Now to the d variable, not the property itself. Thus d will always equal d in that code.
But this will not always set result to true:
bool result = DateTime.Now == DateTime.Now;
I would have to recommend you try this for yourself. This code takes a fraction of second in the Release build:
using System;
class Program {
static void Main(string[] args) {
while (DateTime.UtcNow == DateTime.UtcNow) ;
Console.WriteLine("oops");
Console.ReadLine();
}
}
I trust it will repro well.
DateTime is immutable, so it will never ever change once assigned. Your call to DateTime.Now doesn't "link" them - it just assigns whatever value DateTime.Now is at the time of calling to the variable d - it will not assign some sort of reference.
So if you have a delay like this:
DateTime d = DateTime.Now; // Let's assume it's 9:05:10
Thread.Sleep(100);
Console.WriteLine(d); // will still be 9:05:10, even though it's much later now