Calculate working days and excluding specific dates (Holidays)

Question

I read some questions/answers related to functions in C# to calculate the working days; some use an extended code to achieve that.

I have a data table with more than 50,000 rows and I required an effective method to calculate this information.

I created the following extension in C# that return a number of days (Integer), between two dates; excluding dates (Based on a list of DateTime)

/// <summary> Get working days between two dates (Excluding a list of dates - Holidays) </summary>
/// <param name="dtmCurrent">Current date time</param>
/// <param name="dtmFinishDate">Finish date time</param>
/// <param name="lstExcludedDates">List of dates to exclude (Holidays)</param>
public static int fwGetWorkingDays(this DateTime dtmCurrent, DateTime dtmFinishDate, List<DateTime> lstExcludedDates)
{
    Func<DateTime, bool> workDay = currentDate =>
    (
        currentDate.DayOfWeek == DayOfWeek.Saturday ||
        currentDate.DayOfWeek == DayOfWeek.Sunday ||
        lstExcludedDates.Exists(evalDate => evalDate.Date.Equals(currentDate.Date))
    );

    return Enumerable.Range(0, 1 + (dtmFinishDate - dtmCurrent).Days).Count(intDay => workDay(dtmCurrent.AddDays(intDay)));
}

Does there exists some way to improve this method.

Answer 1

Bug

There is a bug in this method. It counts non-working days because the workDay condition needs to be negated.

return 
    Enumerable
        .Range(0, (finishDate - current).Days)
        .Count(day => !isExcludedDate(current.AddDays(day)));

---

Naming

Then comes the naming. What's with the prefixes fw, dtm, lst? In C# we don't use the hungarian notation. You should use either the camelCase or PascalCase naming where appropriate.

---

Closures

You could take advantage of closures and simplify the Count

public static int GetWorkingDays(this DateTime current, DateTime finishDateExclusive, List<DateTime> excludedDates)
{
    Func<int, bool> isWorkingDay = days =>
    {
        var currentDate = current.AddDays(days);
        var isNonWorkingDay =
            currentDate.DayOfWeek == DayOfWeek.Saturday ||
            currentDate.DayOfWeek == DayOfWeek.Sunday ||
            excludedDates.Exists(excludedDate => excludedDate.Date.Equals(currentDate.Date));
        return !isNonWorkingDay;
    };
   
    return Enumerable.Range(0, (finishDateExclusive - current).Days).Count(isWorkingDay);
}  

Depending on how long the List<DateTime> excludedDates is, you may consider using a HashSet<DataTime>.

Upper range

1 + (dtmFinishDate - dtmCurrent).Days

Most .NET APIs consider the upper range exclusive thus it might be a good idea to follow this style by removig the +1 and renaming the finishDate to finishDateExclusive.