C# get difference in months?

Question

I am developing a project at C#. I have a model named ExamResult which has a field named Date, which is defined as String.

I then define the following

        var executionQuery = (from x in db.ExamResult
                     where x.Student.Equals(iStudent)
                     orderby x.Date
                     select *);

The Date gets values in the format <YEAR>-<MONTH> like this

2014-01
2013-04
2013-09

What I want to do, is to create a table which gets the minimum value of all the dates returned, and creates a table which have the difference in months from that minimum date.

Example:

When we have results like above, I want to get the following table (if we get that the minimum value is 2013-04)

9
0
5

I tried to do the following but I get a System.NotSupported exception

var dates = executionQuery.Select(x => int.Parse(x.Date.Substring(0,
4)) * 12 + int.Parse(x.Date.Substring(5, 2)) -
int.Parse(minDate.Substring(0, 4)) * 12 -
int.Parse(minDate.Substring(5, 2)));

Do you know how I can do this?

Answer 1

I'd use a small Func<TIn, TOut> delegate to convert your string dates into DateTimes, then they can be sorted properly.

First, a simple method to convert the date string to a DateTime object:

// Split the string and instantiate new DateTime object to sort by later
Func<string, DateTime> getDate = s => {
    int[] dateParts = s
        .Split(new char[] {'-'})
        .Select(dp => int.Parse(dp))
        .ToArray();

    // Let's use the new DateTime(int year, int month, int day) constructor overload
    // dateParts[0] is the year and dateParts[1] is the month;
    // the magic number 1 below  is just a day to give to the DateTime constructor
    return new DateTime(dateParts[0], dateParts[1], 1); 
};

Your code might look something like this; I can't test your code so this will be up to you to make it work:

Note I separated your Linq query and am doing the ordering in C#; so you can get the stuff from the DB anyway you want, then order the items. I hope this works; otherwise, you have to call my getDate Func twice - once on orderby, and once on select; I didn't like that option.

// The select now builds an anonymous object; You can also create a new class, ExamResultWithDate,
// for example, that has all fields of ExamResult plus a DateTime field; OR you can just add that 
// property to the partial class generated by EF or Linq-to-Sql or whatever right on the ExamResult 
// entity.

var executionQuery = (from x in db.ExamResult
                     where x.Student.Equals(iStudent)
                     select new { Entity = x, ActualDate = getDate(x.Date) }); // note select * as in your OP doesn't compile :)

var orderedQuery = executionQuery
    .OrderBy(eq => eq.ActualDate)
    .Select(er => er.Entity); // gets you just the entities in this case and discards the dates

To get dates with difference, just do some simple calculations on your minimum date: Again this is pseudo code for your program;

// Let's get the minimum date and difference in months;
DateTime minDate = executionQuery
    .ToList()
    .Select(o => o.ActualDate)
    .Min();

// I am just using the dates here but you can easily use your entire entity or whatever you need
Dictionary<DateTime, int> datesWithMonthDifference = executionQuery
    .ToDictionary(
        eq => eq.ActualDate
        eq => ((eq.Year - minDate.Year) * 12) + eq.Month - minDate.Month // this formula calculates month difference as an integer 
    );

Here's a working program that does what you need: Note this is just an example that needs to be fit into your project.

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

namespace DateTimeFromString
{
    class Program
    {

        static void Main(string[] args)
        {

            List<string> dates = new List<string>()
            {
                "2014-01",
                "2013-04",
                "2013-09"
            };

            // Split the string and instantiate new DateTime object to sort by later
            Func<string, DateTime> getDate = s => {
                int[] dateParts = s
                    .Split(new char[] {'-'})
                    .Select(dp => int.Parse(dp))
                    .ToArray();

                // Let's use the new DateTime(int year, int month, int day) constructor overload
                // dateParts[0] is the year and dateParts[1] is the month;
                // the magic number 1 below  is just a day to give to the DateTime constructor
                return new DateTime(dateParts[0], dateParts[1], 1); 
            };

            List<DateTime> sortedDates = dates
                .Select(d => getDate(d))
                .OrderBy(d => d)
                .ToList();

            Console.WriteLine(" Sorted Dates: ");
            sortedDates.ForEach(d => Console.WriteLine(d.Year.ToString() + " - " + d.Month.ToString()));

            // Let's get the minimum date and difference in months;
            DateTime minDate = sortedDates.Min();

            Dictionary<DateTime, int> datesWithMonthDifference = sortedDates
                .ToDictionary(
                    sd => sd,
                    sd => ((sd.Year - minDate.Year) * 12) + sd.Month - minDate.Month
                );

            Console.WriteLine();
            Console.WriteLine("Sorted dates with month difference:");

            foreach (var key in datesWithMonthDifference.Keys)
            {
                Console.WriteLine("{0} has difference of {1}", key, datesWithMonthDifference[key]);
            }
            Console.ReadKey();
        }
    }
}

The result of my test program looks like this:

Answer 2

Converting your strings to actual 'DateTime' objects will make things simpler.

// Getting DateTime objects instead of strings
var dates = executionQuery.ToArray().Select(
                 x => DateTime.ParseExact(x.Date,"yyyy-MM", CultureInfo.InvariantCulture));

// calculating smallest date
var minDate = dates.Min(x => x);

// this will help you get a collection of integers  
var diffDates = dates.Select(
               x => ((x.Year - minDate.Year) * 12) + x.Month - minDate.Month);