LINQ to Entities Group By and then select the min value

LINQ to Entities Group By and then select the min value

I'm using VS2012 and SQL Compact Edition and I have difficult time to
create my database. I'm trying to use the designer from VS2012 but I still
can't do simple things like defining Foreign keys and stuff like that, but
I need to prepare a Demo fast so instead I decided to leave the learning
for later and for now I work with only one table in my database (it's a
small project) and I have less than 500 rows so it's not that big of a
problem. However I need to do this :
I have insurance policies everyone of which has unique Policy number. But
for one Policy there might be several dates when the customer need to make
payments. So based on the information above in my table I have say four
rows with different IDs but with the same Policy number and different date
for each record.
I use MDI Windows Forms and I have a form where I show info for all
clients and here comes the problem. If one policy has several dates or in
other words several records in my table I want to show only the record
with the upcoming date, and only if the user clicks "details" to show all
date for this insurance policy.
So what I need is to query the table group the results by PolicyNumber and
for each policy that has several dates to take the record with the most
recent date.
For now I have this:
using (RalBaseEntities ctx = new RalBaseEntities())
{
var mainGridInfo = from cs in ctx.MainInfo
where cs.Id != null
select cs;
IList<MainInfo> mainGridInfoList =
mainGridInfo.ToList<MainInfo>();
dgvClients.DataSource = mainGridInfoList;
}
So having this I try to use group by like this:
var mainGridInfo = from cs in ctx.MainInfo
where cs.Id != null
group cs by cs.PolicyNumber into test
select test;
But then I get this marked as error : IList mainGridInfoList =
mainGridInfo.ToList<MainInfo>();
And the error is :
Error 1
'System.Linq.IQueryable<System.Linq.IGrouping<string,SHAdmin.MainInfo>>'
does not contain a definition for 'ToList' and the best extension method
overload
'System.Linq.ParallelEnumerable.ToList<TSource>(System.Linq.ParallelQuery<TSource>)'
has some invalid arguments
C:\Users\X-Man\Desktop\SHAdmin\SHAdmin\Forms\Clients.cs 44 52 SHAdmin
And even if this worker I still don't know how to extract only the record
with the most recent date.