Question

I’m trying to convert some SQL queries into Linq to avoid multiple trips to the database.

The old SQL I’m trying to convert does:

SELECT
    AVG(CAST(DATEDIFF(ms, A.CreatedDate, B.CompletedDate) AS decimal(15,4))),
    AVG(CAST(DATEDIFF(ms, B.CreatedDate, B.CompletedDate) AS decimal(15,4)))
FROM
    dbo.A
INNER JOIN
    dbo.B ON B.ParentId = A.Id

So I’ve created two C# classes:

class B
{
    public Guid Id { get; set; }
    public DateTime CreatedDate { get; set; }
    public DateTime CompletedDate { get; set; }
}

class A
{
    public Guid Id { get; set; }
    public DateTime CreatedDate { get; set; }
    public List<B> BList { get; set; }
}

And I’ve got a List<A> object that I want to query. It’s populated from the database, so each A in the list has a sub-list with a load of Bs. I want to use Linq-to-objects to query this list.

So I need to use Linq to get the average time between an A’s start and the completion of its child Bs, and the average time between each B’s start and completion. I didn’t write the original SQL so I’m not entirely sure it does what it’s supposed to!

I’ve got several of these averages to calculate, so I’d like to do them all inside one magical Linq query. Is this possible?

Answer 1

The more interesting question would be how to do such a thing on the server, for example to make the following query translate to LINQ to SQL.

        var q = from single in Enumerable.Range(1, 1)
                let xs = sourceSequence
                select new
                {
                    Aggregate1 = xs.Sum(),
                    Aggregate2 = xs.Average(),
                    // etc
                };

But there’s no point trying to cram it into one query if you’re using LINQ to Objects. Just write the aggregates separately:

var aggregate1 = xs.Sum();
var aggregate2 = xs.Average();
// etc

In other words:

var xs = from a in listOfAs
         from b in a.Bs
         select new { A=a, B=b };

var average1 = xs.Average(x => (x.B.Completed - x.A.Created).TotalSeconds);
var average2 = xs.Average(x => (x.B.Completed - x.B.Created).TotalSeconds);

Did I understand right?

Edit: David B answered similarly. I forgot to convert the TimeSpan to a simple numeric type supported by the Average method. Use TotalMilliseconds/Seconds/Minutes or whatever scale is appropriate.