Slow foreach() on a LINQ query - ToList() boosts performance immensely - why is this?

Question

I kind of grasp the whole delayed execution concept, but the following has me puzzled...

On a DataTable containing about 1000 rows, I call AsEnumerable(). I

Answer 1

Indeed you seem to have no clear idea what is execution of code and what is definition of the intent to (possibly) execute later when the results are actually used. I suggest reading up one this part of LINQ.

And possibly try executing both of your variants with a debugger attached so you can actually see what code is executing in which order and what is actually happening to your data. You might be in for a (big?) surprise here...

Answer 2

You don't understand which methods are deferred and which are not, so you don't understand when your code defines operations vs performs operations.

These are all deferred. They define, but do not execute, an operation.

source.AsEnumerable
source.Select
source.Where

These enumerate the source and so are not deferred.

source.ToList
source.First
source.Single
foreach(var x in source)

Answer 3

It will not get all items from database until you type

 ToList or First or Single

In foreach, you send a query into database for each item. So it works slower. Open your sql profiler to understand better.