是否推迟执行

Question

Are below comments correct about DEFERRED EXECUTION?

1. var x = dc.myTables.Select(r=>r);//yes
2. var x = dc.myTables.Where(..).Select(r=>new {..});//yes
3. var x = dc.myTables.Where(..).Select(r=>new MyCustomClass {..});//no

In other words, I always thought projecting custom class objects will always cause eager execution. But I couldn't find references supporting/denying it (though I am seeing results contradicting it, hence the post)

Answer 1

Every statement in your question is an example of deferred execution. The contents of the Select and Where statement have no effect on whether or not the resulting value is deferred executed or not. The Select + Where statements themselves dictate that.

As a counter example consider the Sum method. This is always eagerly executed irrespective of what the input is.

var sum = dc.myTables.Sum(...);  // Always eager 

Answer 2

To prove your point, your test should look like this:

var tracer = string.Empty;
Func<inType, outType> map = r => {
       tracer = "set";
       return new outType(...);
    } 

var x = dc.myTables.Where(..).Select(map);

// this confirms x was never enumerated as tracer would be "set".
Assert.AreEqual(string.Empty, tracer);
// confirm that it would have enumerated if it could
CollectionAssert.IsNotEmpty(x);

Answer 3

It has been my observation that the only way to force execution right away is to force iteration of the collection. I do this by calling .ToArray() on my LINQ.

Answer 4

Generally methods that return a sequence use deferred execution:

IEnumerable<X> ---> Select ---> IEnumerable<Y>

and methods that return a single object doesn't:

IEnumerable<X> ---> First ---> Y

So, methods like Where , Select , Take , Skip , GroupBy and OrderBy use deferred execution because they can, while methods like First , Single , ToList and ToArray doesn't because they can't.

from here

Answer 5

.Select(...) is always deferred.

When you're working with IQueryable<T> , this and other deferred execution methods build up an expression tree and this isn't ever compiled into an actual executable expression until it's iterated. That is, you need to:

• Do a for-each on the projected enumerable.
• Call a method that internally enumerates the enumerable (ie .Any(...) , .Count(...) , .ToList(...) , ...).