Getting the business day of the month

Question

I'm working on figuring out what the current 'business' day of the month it is. For instance, if it's the 20th, but there are 3 weekends, it would be the 14th business day of the month.

Is there a better way of doing this?

var first = DateTime.Today.FirstDayOfMonth();
var businessDayOfMonth = 0;
for (var i = first; i <= DateTime.Today; i = i.AddDays(1))
{
    if (i.DayOfWeek != DayOfWeek.Saturday && i.DayOfWeek != DayOfWeek.Sunday) 
        businessDayOfMonth++;
}

Answer 1

As explained in comments (in which I pointed out a few possible edge cases), even though it's not optimized in terms of performance, this is about as clear as it gets.

A more clever algorithm - calculating the number of weekdays without iterating through all of them - is probably possible, but I bet it would be much less readable. If performance isn't a concern at this point, I'd leave it.

You could maybe replace the loop with a LINQ-based solution.

int WhichWorkingDay(DateTime day)
{
    // edge cases
    switch (day.DayOfWeek)
    {
        case DayOfWeek.Saturday:
            return day.Day > 1
                // Friday the day before
                ? WhichWorkingDay(day.AddDays(-1))
                // or next Monday if previous Friday was on last month
                : WhichWorkingDay(day.AddDays(2));
        // analogically here
        case DayOfWeek.Sunday:
            return day.Day > 2
                ? WhichWorkingDay(day.AddDays(-2))
                : WhichWorkingDay(day.AddDays(1));
    }

    // or you could inline it, obviously
    Func<DateTime, bool> isWorkingDay = d => d.DayOfWeek != DayOfWeek.Saturday 
        && d.DayOfWeek != DayOfWeek.Sunday;

    return Enumerable
        .Range(1, day.Day)
        .Select(d => new DateTime(day.Year, day.Month, d))
        .Count(isWorkingDay);
}

Answer 2

As Konrad Morawski already pointed out, working days are culture related. I've worked out a working day calculator that is extendible so every culture can be supported.

The extension method WorkingDay uses a static class OffDayProvider to determine whether a given day is an day off. By default this OffDayProvider does not consider any day to be an off day. By injecting one or more IOffDayProviders you can determine what days are off days.

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

public class Program
{
    public static void Main()
    {
        var startDate = new DateTime(2015,12,1);
        var date = new DateTime(2015,12,31);

        Console.WriteLine("{0} -- {1}",date.ToShortDateString(), date.WorkingDay(startDate));

        OffDayProvider.Providers.Add(new WeekendProvider());
        OffDayProvider.Providers.Add(new BankHolidayProvider());
        OffDayProvider.Providers.Add(new WinterSolsticeProvider());

        Console.WriteLine("{0} -- {1}",date.ToShortDateString(), date.WorkingDay(startDate));
    }
}

public static class DateTimeExtensions
{
    public static int WorkingDay(this DateTime date, DateTime startDate)
    {
        var result = 0;
        for(var day = startDate; day <= date; day = day.AddDays(1))
        {
            if(OffDayProvider.IsOffDay(day))
            {
                continue;
            }
            result++;
        }
        return result;
    }

    public static int WorkingDay(this DateTime date)
    {
        return date.WorkingDay(new DateTime(date.Year, 1, 1));
    }
}

public static class OffDayProvider
{
    public static readonly ICollection<IOffDayProvider> Providers = new List<IOffDayProvider>();

    public static bool IsOffDay(DateTime date)
    {
        return Providers.Any(x => x.IsOffDay(date));
    }   
}

public interface IOffDayProvider 
{
    bool IsOffDay(DateTime date);
}

public class WeekendProvider : IOffDayProvider
{
    public bool IsWeekend(DateTime date)
    {
        return date.DayOfWeek == DayOfWeek.Sunday 
            || date.DayOfWeek == DayOfWeek.Saturday;
    }

    public bool IsOffDay(DateTime date)
    {
        return IsWeekend(date);
    }
}

public class WinterSolsticeProvider : IOffDayProvider
{
    public bool IsWinterSolstice(DateTime date)
    {
        return date.Month == 12 && date.Day == 21;
    }

    public bool IsOffDay(DateTime date)
    {
        return IsWinterSolstice(date);
    }
}

public class BankHolidayProvider : IOffDayProvider
{
    public bool IsChristmasDay(DateTime date) 
    {
        return date.Month == 12 && date.Day == 25;
    }

    public bool IsBoxingDay(DateTime date) 
    {
        return date.Month == 12 && date.Day == 26;
    }

    public bool IsOffDay(DateTime date)
    {
        return IsChristmasDay(date)
            || IsBoxingDay(date);
    }
}

I did not include checks for corner cases. It's up to you to determine how to handle those.

I did not include any comments either. Well written code doesn't need comments, does it? B-)

Answer 3

From the comments it is clear that FirstDayOfMonth() is an extension method, you are using to get the first Day Of Month (I don't have that method so I'm doing the same in a different way). Anyway first will not be a good name for this variable so I would like to change it more specifically as firstDayOfMonth:

DateTime dateToday=DateTime.Today;
var firstDayOfMonth = new DateTime(dateToday.Year, dateToday.Month, 1); // will give you the First day of this month

I prefer you to use List<DayOfWeek> to specify which days are considered as week-off days (I think this may change as per the country). So that modifications will became more easier. And also it makes the conditions more simple.

You can try the following example:

List<DayOfWeek> weekOffDays = new List<DayOfWeek>() { DayOfWeek.Sunday, DayOfWeek.Saturday };
DateTime dateToday=DateTime.Today;
var firstDayOfMonth = new DateTime(dateToday.Year, dateToday.Month, 1); // will give you the First day of this month
var businessDaysOfMonth = 0; 
for (var dateObject = firstDayOfMonth; dateObject <= dateToday; dateObject = dateObject.AddDays(1))
{
    if (!weekOffDays.Contains(dateObject.DayOfWeek))
       businessDaysOfMonth++;
}
Console.WriteLine("Today is {0}th working day of this month", businessDaysOfMonth);