Question

I’ve got a class like the following:

public class Invoice
{
    public int InvoiceId {get;set;}
    public int VersionId {get;set;}
}

Each time an Invoice is modified, the VersionId gets incremented, but the InvoiceId remains the same. So given an IEnumerable<Invoice> which has the following results:

InvoiceId VersionId
1         1
1         2
1         3
2         1
2         2

How can I get just the results:

InvoiceId VersionId
1         3
2         2

I.e. I want just the Invoices from the results which have the latest VersionId. I can easily do this in T-SQL, but cannot for the life of me work out the correct LINQ syntax. I’m using Entity Framework 4 Code First.

Answer 1

Order by the VersionId, group them by InvoiceId, then take the first result of each group. Try this:

var query = list.OrderByDescending(i => i.VersionId)
                .GroupBy(i => i.InvoiceId)
                .Select(g => g.First());

EDIT: how about this approach using Max?

var query = list.GroupBy(i => i.InvoiceId)
                .Select(g => g.Single(i => i.VersionId == g.Max(o => o.VersionId)));

Try using FirstOrDefault or SingleOrDefault in place of Single as well… it would give the same result although Single shows the intention better.