Question

I have DateStart, DateEnd Periodicity, TypePeriodicity fields.

We have a query:

var result = Events.Where(e => e.DateStart <=today && e.DateEnd >= today).ToList();

I want that this query to check Periodicity.

For example:

name  - record1
DateStart = 2012-02-02
DateEnd = 2012-03-31
Periodicity = 2
TypePeriodicity = 1 ( it's mean a week, may be also day = 0, month=2): 

I want the following, if current date equals:

2,3,4,5 February - return `record1`   
6,7,8..12 - not return, because TypePeriodicity = 1 and Periodicity = 2, which means every 2 weeks
13..19 - return `record1`
20..26 - not return  
and so on until `DateEnd`

Thanks.

PS. Maybe not LINQ, but simple method that recieve result as parameter.

Answer 1

Here is something to get you started:

You could define a DateEvaluator delegate like so:

    delegate bool DateEvaluator(DateTime startDate, DateTime endDate, DateTime dateToCheck, int periodicity);

The purpose of the delegate would be to evaluate for a given periodicity type if a date should be considered as within range. We would have hence 3 date evaluators.
One for each period type: Lets call them dayPeriodicityChecker, weekPeriodicityChecker and monthPeriodicityChecker

Our dayPeriodicityChecker is straightforward:

        DateEvaluator dayPeriodicityChecker = (startDate, endDate, dateToCheck, periodicity) =>
            {
                if ((dateToCheck < startDate) || (dateToCheck > endDate))
                    return false;

                TimeSpan dateDiff = dateToCheck - startDate;
                return dateDiff.Days % periodicity == 0;
            };

Our weekPeriodicityChecker needs to account for the start day of week, so the start date would need to be adjusted to the date in which the startDate week actually starts:

        DateEvaluator weekPeriodicityChecker = (startDate, endDate, dateToCheck, periodicity) =>
            {
                if ((dateToCheck < startDate) || (dateToCheck > endDate))
                    return false;

                DateTime adjustedStartDate = startDate.AddDays(-(int)startDate.DayOfWeek + 1);
                TimeSpan dateDiff = dateToCheck - adjustedStartDate;
                return (dateDiff.Days / 7) % periodicity == 0;
            };

Our monthPeriodicityChecker needs to cater for months with a variable number of days:

        DateEvaluator monthPeriodicityChecker dateToCheck, periodicity) =>
            {
                if ((dateToCheck < startDate) || (dateToCheck > endDate))
                    return false;

                int monthDiff = 0; 
                while (startDate.AddMonths(1) < dateToCheck)
                {
                    monthDiff++
                    // i'm sure there is a speedier way to calculate the month difference, but this should do for the purpose of this example 
                }

                return (monthDiff - 1) % periodicity == 0;
            };

Once you have all your date evaluators defined you could put them in an array like so:

        DateEvaluator[] dateEvaluators = new DateEvaluator[] 
        {
            dayPeriodicityChecker,
            weekPeriodicityChecker,
            monthPeriodicityChecker
        };

This will allow you to do :

int periodicityType = 0; // or 1=week or 2=months
bool isDateIn = dateEvaluators[periodicityType ](startDate, endDate, dateTocheck, Periodicity)

So lets test this:

        PeriodicityEvent pEvent = new PeriodicityEvent
        {
            Name = "record1",
            DateStart = new DateTime(2012, 02, 02),
            DateEnd = new DateTime(2012, 03, 31),
            PeriodicityType = 1,
            Periodicity = 2
        };

        DateTime baseDate = new DateTime(2012, 02, 01);
        for (int i = 0; i < 30; i++)
    {
            DateTime testDate = baseDate.AddDays(i);
            if (dateEvaluators[pEvent.PeriodicityType](pEvent.DateStart, pEvent.DateEnd, testDate, pEvent.Periodicity))
            {
                Console.WriteLine("{0} is in", testDate.ToString("dd MMM"));
            }
            else
            {
                Console.WriteLine("{0} is out", testDate.ToString("dd MMM"));
            }
   }

This will produce the desired output as below:

To use you would simply do:

Events.Where(e => dateEvaluators[e.PeriodType](e.DateStart, e.DateEnd, today, e.Periodicity).ToList();

Good luck!