How to get every monday of given month?

Question

How to get every "monday" day of given month ?

An example;
Input: 11.July.2017 (11.07.2017)
Output: ( 3,10,17,24,31 )
3.7.2017 Monday
10.7.2017 Monday
17.7.2017 Monday
24.7.2017 Monday
31.7.2017

I could get number of days of given month(For july 2017, it's 31 days). Then write an iteration(for loop a.e.) if dayOfWeek equals Monday, add to a list. But this is not good code because for loop will work 31 times. There should be a better algorithm to archive the goal.

I'm using C# .net framework 4.6

UPDATE
Thanks for all for your helps, after some answers I've got so far; I tested all codes with a simple & dirty benchmark codes to find faster algorithm.

Here is my benchmark code;

using System;
using System.Collections.Generic;
using System.Linq;

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Attributes.Columns;
using BenchmarkDotNet.Attributes.Jobs;
using BenchmarkDotNet.Engines;

using X.Core.Helpers;

namespace X.ConsoleBenchmark
{
    [SimpleJob(RunStrategy.ColdStart, targetCount: 5)]
    [MinColumn, MaxColumn, MeanColumn, MedianColumn]
    public class LoopTest
    {
        [Benchmark]
        public void CalculateNextSalaryDateWithLoopAllDays()
        {
            DateTime date = new DateTime(2017, 7, 3);
            const int oneMillion = 1000000;
            for (int i = 0; i < oneMillion; i++)
            {
                List<DateTime> allXDaysInMonth = date.GetAllXDaysInMonthWithLoopAllDays(DayOfWeek.Tuesday);
                if (allXDaysInMonth != null && allXDaysInMonth.FirstOrDefault().Day != 4)
                {
                    throw new ApplicationException("Calculate method has errors.");
                }
            }
        }

        [Benchmark]
        public void CalculateNextSalaryDate()
        {
            DateTime date = new DateTime(2017, 7, 3);
            const int oneMillion = 1000000;
            for (int i = 0; i < oneMillion; i++)
            {
                List<DateTime> allXDaysInMonth = date.GetAllXDaysInMonth(DayOfWeek.Tuesday);
                if (allXDaysInMonth != null && allXDaysInMonth.FirstOrDefault().Day != 4)
                {
                    throw new ApplicationException("Calculate method has errors.");
                }
            }
        }

        [Benchmark]
        public void Maccettura_GetAllDayOfWeekPerMonth()
        {
            DateTime exampleDate = new DateTime(2017, 7, 3);
            const int oneMillion = 1000000;
            for (int i = 0; i < oneMillion; i++)
            {
                var date = new DateTime(exampleDate.Year, exampleDate.Month, 1);

                if (date.DayOfWeek != DayOfWeek.Thursday)
                {
                    int daysUntilDayOfWeek = ((int)DayOfWeek.Thursday - (int)date.DayOfWeek + 7) % 7;
                    date = date.AddDays(daysUntilDayOfWeek);
                }

                List<DateTime> days = new List<DateTime>();

                while (date.Month == exampleDate.Month)
                {
                    days.Add(date);
                    date = date.AddDays(7);
                }

                if (days.FirstOrDefault().Day != 6)
                {
                    throw new ApplicationException("Calculate method has errors.");
                }
            }
        }

        [Benchmark]
        public void ScottHannen_GetWeekdaysForMonth()
        {
            DateTime exampleDate = new DateTime(2017, 7, 3);
            const int oneMillion = 1000000;
            for (int i = 0; i < oneMillion; i++)
            {
                IEnumerable<DateTime> days = ScottHannen_GetDaysInMonth(exampleDate).Where(day => day.DayOfWeek == DayOfWeek.Thursday);

                if (days.FirstOrDefault().Day != 6)
                {
                    throw new ApplicationException("Calculate method has errors.");
                }
            }
        }

        private IEnumerable<DateTime> ScottHannen_GetDaysInMonth(DateTime date)
        {
            var dateLoop = new DateTime(date.Year, date.Month, 1);

            while (dateLoop.Month == date.Month)
            {
                yield return dateLoop;
                dateLoop = dateLoop.AddDays(1);
            }
        }

        [Benchmark]
        public void Trioj_GetWeekdaysForMonth()
        {
            DateTime exampleDate = new DateTime(2017, 7, 3);
            const int oneMillion = 1000000;
            for (int i = 0; i < oneMillion; i++)
            {
                IEnumerable<DateTime> days = Trioj_GetDatesInMonthByWeekday(exampleDate, DayOfWeek.Thursday);

                if (days.FirstOrDefault().Day != 6)
                {
                    throw new ApplicationException("Calculate method has errors.");
                }
            }
        }

        private List<DateTime> Trioj_GetDatesInMonthByWeekday(DateTime date, DayOfWeek dayOfWeek)
        {
            // We know the first of the month falls on, well, the first.
            var first = new DateTime(date.Year, date.Month, 1);
            int daysInMonth = DateTime.DaysInMonth(date.Year, date.Month);

            // Find the first day of the week that matches the requested day of week.
            if (first.DayOfWeek != dayOfWeek)
            {
                first = first.AddDays(((((int)dayOfWeek - (int)first.DayOfWeek) + 7) % 7));
            }

            // A weekday in a 31 day month will only occur five times if it is one of the first three weekdays.
            // A weekday in a 30 day month will only occur five times if it is one of the first two weekdays.
            // A weekday in February will only occur five times if it is the first weekday and it is a leap year.
            // Incidentally, this means that if we subtract the day of the first occurrence of our weekday from the 
            // days in month, then if that results in an integer greater than 27, there will be 5 occurrences.
            int maxOccurrences = (daysInMonth - first.Day) > 27 ? 5 : 4;
            var list = new List<DateTime>(maxOccurrences);

            for (int i = 0; i < maxOccurrences; i++)
            {
                list.Add(new DateTime(first.Year, first.Month, (first.Day + (7 * i))));
            }

            return list;
        }

        [Benchmark]
        public void Jonathan_GetWeekdaysForMonth()
        {
            DateTime exampleDate = new DateTime(2017, 7, 3);
            const int oneMillion = 1000000;
            for (int i = 0; i < oneMillion; i++)
            {
                IEnumerable<DateTime> days = Jonathan_AllDatesInMonth(exampleDate.Year, exampleDate.Month).Where(x => x.DayOfWeek == DayOfWeek.Thursday);

                if (days.FirstOrDefault().Day != 6)
                {
                    throw new ApplicationException("Calculate method has errors.");
                }
            }
        }

        private static IEnumerable<DateTime> Jonathan_AllDatesInMonth(int year, int month)
        {
            int days = DateTime.DaysInMonth(year, month);
            for (int day = 1; day <= days; day++)
            {
                yield return new DateTime(year, month, day);
            }
        }

        [Benchmark]
        public void Swatsonpicken_GetWeekdaysForMonth()
        {
            DateTime exampleDate = new DateTime(2017, 7, 3);
            const int oneMillion = 1000000;
            for (int i = 0; i < oneMillion; i++)
            {
                IEnumerable<DateTime> days = Swatsonpicken_GetDaysOfWeek(exampleDate, DayOfWeek.Thursday);

                if (days.FirstOrDefault().Day != 6)
                {
                    throw new ApplicationException("Calculate method has errors.");
                }
            }
        }

        private static IEnumerable<DateTime> Swatsonpicken_GetDaysOfWeek(DateTime startDate, DayOfWeek desiredDayOfWeek)
        {
            var daysOfWeek = new List<DateTime>();
            var workingDate = new DateTime(startDate.Year, startDate.Month, 1);
            var offset = ((int)desiredDayOfWeek - (int)workingDate.DayOfWeek + 7) % 7;

            // Jump to the first desired day of week.
            workingDate = workingDate.AddDays(offset);

            do
            {
                daysOfWeek.Add(workingDate);

                // Jump forward seven days to get the next desired day of week.
                workingDate = workingDate.AddDays(7);
            } while (workingDate.Month == startDate.Month);

            return daysOfWeek;
        }

        [Benchmark]
        public void AliaksandrHmyrak_GetWeekdaysForMonth()
        {
            DateTime exampleDate = new DateTime(2017, 7, 3);
            const int oneMillion = 1000000;
            for (int i = 0; i < oneMillion; i++)
            {
                IEnumerable<DateTime> days = AliaksandrHmyrak_GetDaysOfWeek(exampleDate, DayOfWeek.Thursday);

                if (days.FirstOrDefault().Day != 6)
                {
                    throw new ApplicationException("Calculate method has errors.");
                }
            }
        }

        private static List<DateTime> AliaksandrHmyrak_GetDaysOfWeek(DateTime date, DayOfWeek dayOfWeek)
        {
            var daysInMonth = DateTime.DaysInMonth(date.Year, date.Month);
            var i = 1;

            List<DateTime> result = new List<DateTime>(5);

            do
            {
                var testDate = new DateTime(date.Year, date.Month, i);

                if (testDate.DayOfWeek == dayOfWeek)
                {
                    result.Add(testDate);
                    i += 7;
                }
                else
                {
                    i++;
                }

            } while (i <= daysInMonth);

            return result;
        }

    }
}

And this is the results table;

I can remove any code and picture-name if you want
I marked Jonathan's answer. Simple, clean and faster (interestingly).

Answer 1

Other answers work, but I would prefer to make use of Jon Skeet's AllDaysInMonth function from foreach day in month

public static IEnumerable<DateTime> AllDatesInMonth(int year, int month)
    {
        int days = DateTime.DaysInMonth(year, month);
        for (int day = 1; day <= days; day++)
        {
            yield return new DateTime(year, month, day);
        }
    }

And then you can call with LINQ like so:

var mondays = AllDatesInMonth(2017, 7).Where(i => i.DayOfWeek == DayOfWeek.Monday);

But I guess it depends on how many times you're going to use it as to wherther or not it's worth breaking out into a separate function.

Answer 2

Try something like this:

public static IEnumerable<DateTime> GetAllDayOfWeekPerMonth(int month, int year, DayOfWeek dayOfWeek)
{
    var date = new DateTime(year, month, 1);

    if(date.DayOfWeek != dayOfWeek)
    {
        int daysUntilDayOfWeek = ((int) dayOfWeek - (int) date.DayOfWeek + 7) % 7;
        date = date.AddDays(daysUntilDayOfWeek);
    }

    List<DateTime> days = new List<DateTime>();

    while(date.Month == month)
    {
        days.Add(date);
        date = date.AddDays(7);         
    }

    return days;
}

Demo fiddle here