LINQ: Group by date including the year

Question

I have dataset looks like this:

FileName     Date 
ABC      -    01/10/16
DBC      -    01/11/16
ZYX      -    03/10/16
ABX2      -   01/10/17
IOS       -   01/09/17

How can I group them into a list of groups of months while ensuring that the year is taken into account in the clause?

I'm currently using a LINQ Query is creating groups by month but not including the year, so I have a group of ABC, ZYX and ABX2. even though ABX2 was a 2017 report but the same month so should be in a different group.

I've been trying different ways of doing this but none of have been successful as of yet.

        var newList = from x in list
                      group x
        by x.Properties.LastModified.Value.Month into lastMod
                      where lastMod.Count() > 1
                      select lastMod;

Once I have them in separate groups, I will find out which one was written last and save that and remove the rest. I'm quite stuck and been on this issue for half day. would appreciate fresh eyes on it.

You can group by month and year, Take a look at [How to Datetime Groupby Month with Entity Framework](https://stackoverflow.com/questions/46868420/how-to-datetime-groupby-month-with-entity-framework) — Shyju, Nov 01 '17 at 15:42

score 3 · Accepted Answer · answered Nov 01 '17 at 15:43

You can group by a composite year-month key, like this:

var newList = list
    .Where(x => x.Properties.LastModified.HasValue)
    .GroupBy(x => new {
        x.Properties.LastModified.Value.Month
    ,   x.Properties.LastModified.Value.Year
    })
    .Where(g => g.Count() > 1);

You need to ensure that LastModified has non-null value before accessing its Value property.

score 1 · Answer 2 · answered Nov 01 '17 at 15:44

I can't test this at the moment but I think grouping by an anonymous type that contains both the month and year should do it.

var newList = from x in list
                          group x
            by new {x.Properties.LastModified.Value.Month, x.Properties.LastModified.Value.Year} into lastMod
                          where lastMod.Count() > 1
                          select lastMod;

LINQ: Group by date including the year

2 Answers2