Summing linq data by year - asp.net

Question

I've seen dozens of posts similar to this, but I just can't get it to work.
Using asp.net MVC framework, I have a table named Contributions that contains a "ContributionDate" column and an "Amount" column. I'm loading the dates and amounts to display in a chart:
var results = db.Contributions.Where(c => c.Amount > 0);
ArrayList xValue = new ArrayList();
ArrayList yValue = new ArrayList();
results.ToList().ForEach(c => xValue.Add(c.ContributionDate));
results.ToList().ForEach(c => yValue.Add(c.Amount));
The above works. Now I'd liked to sum (i.e., total) the Amounts for each year. I've seen examples that are similar to the following, but I'm clearly clueless (in this example, the compiler doesn't like the "c.ContributionDate" in the new{} statement):
var results = db.Contributions
.Where(c => c.Amount > 0)
.GroupBy( c => c.ContributionDate )
.Select(c => new {Amount = c.Sum(b => b.Amount), Date=c.ContributionDate});
Thanks for your help!

Answer 1

When you perform a GroupBy, the key by which you're grouping elements is represented by the Key property.
Try this:
var results = db.Contributions
.Where(c => c.Amount > 0)
.GroupBy( c => c.ContributionDate )
.Select(c => new { Amount = c.Sum(b => b.Amount), Date = c.Key });
But this will group items by the entire ContributionDate value, not just by the year. To do that, you'd have to do something like this:
var results = db.Contributions
.Where(c => c.Amount > 0)
.GroupBy( c => c.ContributionDate.Year)
.Select(c => new
{
Amount = c.Sum(b => b.Amount),
Date = new DateTime(c.Key, 1, 1)
});
But since this appears to be Entity Framework, you probably need to use the CreateDateTime function:
using System.Data.Entity;
...
var results = db.Contributions
.Where(c => c.Amount > 0)
.GroupBy( c => c.ContributionDate.Year)
.Select(c => new
{
Amount = c.Sum(b => b.Amount),
Date = EntityFunctions.CreateDateTime(c.Key, 1, 1, 0, 0, 0)
});