Linq Efficiency question - foreach vs aggregates

Question

Which is more efficient?

//Option 1
foreach (var q in baseQuery)
{
  m_TotalCashDeposit += q.deposit.Cash
  m_TotalCheckDeposit += q.deposit.Check
  m_TotalCashWithdrawal += q.withdraw.Cash
  m_TotalCheckWithdrawal += q.withdraw.Check
}

//Option 2
m_TotalCashDeposit = baseQuery.Sum(q => q.deposit.Cash);
m_TotalCheckDeposit = baseQuery.Sum(q => q.deposit.Check);
m_TotalCashWithdrawal = baseQuery.Sum(q => q.withdraw.Cash);
m_TotalCheckWithdrawal = baseQuery.Sum(q => q.withdraw.Check);

I guess what I'm asking is, calling Sum will basically enumerate over the list right? So if I call Sum four times, isn't that enumerating over the list four times? Wouldn't it be more efficient to just do a foreach instead so I only have to enumerate the list once?

Answer 1

It might, and it might not, it depends.

The only sure way to know is to actually measure it.

Simple code to measure it:

Stopwatch sw = new Stopwatch();
sw.Start();
// your code here
sw.Stop();
Debug.WriteLine("Time taken: " + sw.ElapsedMilliseconds + " ms");
sw.Reset(); // in case you have more code below that reuses sw

You should run the code multiple times to avoid having JITting having too large an effect on your timings.

Answer 2

I went ahead and profiled this and found that you are correct.

Each Sum() effectively creates its own loop. In my simulation, I had it sum SQL dataset with 20319 records, each with 3 summable fields and found that creating your own loop had a 2X advantage.

I had hoped that LINQ would optimize this away and push the whole burden on the SQL server, but unless I move the sum request into the initial LINQ statement, it executes each request one at a time.