Develop Reference

c# direct equivalent of VB IsDate()

I have a 'simple' requirement - a quick check to see if an object is a valid DateTime and, if not, return a known default date.
There are a number of alternative solution:
Use the VB function directly by adding a reference to the VB library - simple but adding an additional library to the solution feels like bloat
Using DateTime.TryParse() - need to convert to object to string first and don't need the result
try catch block around Convert.ToDateTime() - feels clumsy but simple
They each have different pros and cons and they all have overheads - have a missed a simpler solution

Use the is operator:
Object foo = new DateTime();
if( foo is DateTime dt )
{
Console.WriteLine( "The date is: {0:yyyy-MM-dd}", dt );
return dt;
}
else
{
return someDefaultDate;
}
If you want the same behaviour as VB's IsDate function, add a call to DateTime.TryParse if the is DateTime check evaluates to false:
static DateTime GetDateOrDefault( Object o, DateTime defaultValue )
{
if( o is DateTime dt )
{
return dt;
}
else if( DateTime.TryParse( o?.ToString(), out dt ) )
{
return dt;
}
else
{
return defaultValue;
}
}

DateTime substracting days doesn't work

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);

Compare datetime get different result

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
}

System.OutOfMemoryException thrown when computing date ranges

It might be a simple fix, but I can't for the life of me think of how to do this. I compute a bunch of StartDates and End Dates into a bunch of arrays of dates using this query:
this.Reserved = unit.Reservations.Where(r => r.Active.HasValue && r.Active.Value).SelectMany(r => Utilities.DateRangeToArray(r.StartDate, r.EndDate)).ToArray();
Utilities.DateRangeToArray() is defined as follows:
public static IEnumerable<DateTime> DateRangeToArray(DateTime start, DateTime end) {
DateTime curDate = start;
while (curDate <= end) {
yield return curDate;
curDate.AddDays(1);
}
}
Is there a way to make this less memory intensive?
Thanks!

Your code is broken - AddDays doesn't change the existing value, it returns a new value. You're ignoring that new value, thus creating an infinite loop.
Change your code to:
public static IEnumerable<DateTime> DateRangeToArray(DateTime start,
DateTime end) {
DateTime curDate = start;
while (curDate <= end) {
yield return curDate;
curDate = curDate.AddDays(1);
}
}
Another hint: unit testing can help you find this sort of problem long before you try to use the method in a LINQ query. I'd also change the name, given that it's not returning an array.

You're sure you don't have any reservations where r.StartDate > r.EndDate, right? If you do, you'll get an infinite loop, I think.

I assume the out of memory is when converting the result to the array. Two points:
The output will contain duplicate dates for overlapping reservations.
Perhaps Reserved should be a collection of date ranges (start,end) rather than containing every date?

Better way to clean this messy bool method

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.