Find the last thursday of a month between two dates C#

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I am trying to get the last Thursday of every month between two days. Some of my results are correct, others are occurring a day late on Fridays. Below is my code and the results.
Any help will be appreciated.

What I have tried:

C#

public class BusinessWeekDays
        {
            public DateTime StartDateMonth;
            public DateTime EndMonth;
        }      
     
        static void Main(string[] args)
        {
            var StartDate = DateTime.Parse("04/25/2019");
            var SeriesEndDate = DateTime.Parse("09/30/2019");

            var months = new List<businessweekdays>();
            var Monthly = 1;
            for (var i = StartDate; i < SeriesEndDate; i = i.AddMonths(Monthly))
            {
                const int thursday = 4;
                const int dayDiff = 7 - thursday;
                
                var Day = DateTime.DaysInMonth(i.Year, i.Month);
                var DayOfWeek = (int)new DateTime(i.Year, i.Month, Day).DayOfWeek;
                
                var _dtNew = new DateTime(i.Year, i.Month, Day).AddDays(-(DayOfWeek < thursday ? DayOfWeek + dayDiff : DayOfWeek - thursday));
                var e = _dtNew.AddMonths(+1).AddDays(-2);
                months.Add(new BusinessWeekDays{ StartDateMonth = _dtNew, EndMonth = e});
                Console.WriteLine(months);
            }
           }
        }

Results:
------------

[0]	 		
		StartDateMonth	{4/25/2019 12:00:00 AM}	System.DateTime  ----->Correct Date and Day
		EndMonth	{5/23/2019 12:00:00 AM}	System.DateTime
		[1]	 
		StartDateMonth	{5/30/2019 12:00:00 AM}	System.DateTime -----Incorrect Date and Day Friday 
		EndMonth	{6/28/2019 12:00:00 AM}	System.DateTime
		
		[2]	
        StartDateMonth	{6/27/2019 12:00:00 AM}	System.DateTime		
		EndMonth	{7/25/2019 12:00:00 AM}	System.DateTime -----> Correct Date and day 
		
		[3]	 
		StartDateMonth	{7/25/2019 12:00:00 AM}	System.DateTime
		EndMonth	{8/23/2019 12:00:00 AM}	System.DateTime ---Incorrect Date and day
		
		[4]	 
		StartDateMonth	{8/29/2019 12:00:00 AM}	System.DateTime
		EndMonth	{9/27/2019 12:00:00 AM}	System.DateTime ---- Incorrect Date and Day

Posted 6-Apr-19 19:24pm

Member 1283347

Updated 8-Apr-19 10:07am

Maciej Los

v2

Add a Solution

3 solutions

Solution 1

Have a look here: DateTime Extensions to Make Some Simple Tasks a Little More Readable[^] - it may not solve your problem exactly, but it'll give you a base to work from - it includes a "last ...day of the month" method which you could modify quite easily.

Posted 6-Apr-19 20:00pm

OriginalGriff

Solution 2

Quote:
Some of my results are correct, others are occurring a day late on Fridays.

May be your evaluation of correctness of result is wrong too.

Quote:

StartDateMonth {4/25/2019 12:00:00 AM} System.DateTime ----->Correct Date and Day
EndMonth {5/23/2019 12:00:00 AM} System.DateTime

Last thursday of May is 30, not 23 !
Check this part

C#

var e = _dtNew.AddMonths(+1).AddDays(-2);

Months have different number of days!

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

Debugging C# Code in Visual Studio - YouTube[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

Posted 6-Apr-19 20:22pm

Patrice T

Comments

Member 1283347 7-Apr-19 14:03pm

I know what my issue is not that I am not debugging my code. Maybe I am not explaining my issue well. The code can give me the last Thursday of each month between two dates correctly.
What I want to do next is just display the start date and the end date for each month between the two days. That is where I am having an issue.
This portion of my code: var e = _dtNew.AddMonths(+1).AddDays(-2);
months.Add(new BusinessWeekDays{ StartDateMonth = _dtNew, EndMonth = e});
is just to show the beginning and end date for each month. I know I am not getting it correct here hence the reason for asking for help. Without these two lines the code gives me correct Last Thursday of every month.

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Maciej Los · Accepted Answer · 2019-04-08T10:08:00

A linq (lambda) query which "finds" last thursdays of months between two dates:

C#

int datediff = (SeriesEndDate - StartDate).Days+1;
var lastThursdaysBetweenDates = Enumerable.Range(0, datediff)
    .Select(x=>StartDate.AddDays(x))
    .Where(d=> d.DayOfWeek==DayOfWeek.Thursday)
    .GroupBy(d=>d.Month)
    .Select(grp=>grp.Max(d=>d));

Good luck!