LINQ: Group by month and year within a datetime field

Question

I have a table with a datetime field. I want to retrieve a result set grouped by the month/year combination and the number of records that appear within that month/year. How can this be done in LINQ?

The closest I've been able to figure out is in TSQL:

select substring(mo,charindex(mo,'/'),50) from (
select mo=convert(varchar(2),month(created)) + '/' + convert(varchar(4), year(created)) 
 ,qty=count(convert(varchar(2),month(created)) + '/' + convert(varchar(4), year(created)))
from posts 
group by convert(varchar(2),month(created)) + '/' + convert(varchar(4), year(created))
) a
order by substring(mo,charindex(mo,'/')+1,50)

But I wouldn't say that works...

Answer 1

var grouped = from p in posts
     group p by new { month = p.Create.Month,year= p.Create.Year } into d
     select new { dt = string.Format("{0}/{1}",d.Key.month,d.Key.year), count = d.Count() };

Here's the list of DateTime functions available in LINQ. For this to work you'll also need to understand multi-column grouping

ordered descending

var grouped = (from p in posts 
  group p by new { month = p.Create.Month,year= p.Create.Year } into d 
  select new { dt = string.Format("{0}/{1}",d.Key.month,d.Key.year), count = d.Count()}).OrderByDescending (g => g.dt);

Answer 2

This is for those who are trying to accomplish the same but using lambda expressions.

Assuming that you already have a collection of entities and each entity has OrderDate as one of its properties.

yourCollection
// This will return the list with the most recent date first.
.OrderByDescending(x => x.OrderDate)
.GroupBy(x => new {x.OrderDate.Year, x.OrderDate.Month})
// Bonus: You can use this on a drop down
.Select(x => new SelectListItem
        {
           Value = string.Format("{0}|{1}", x.Key.Year, x.Key.Month),
           Text = string.Format("{0}/{1} (Count: {2})", x.Key.Year, x.Key.Month, x.Count())
        })
.ToList();

If you do not need the collection of SelectListItem then just replace the select with this one:

.Select(x => string.Format("{0}/{1} (Count: {2})", x.Key.Year, x.Key.Month, x.Count()))