How does a LINQ IEnumerable work under the hood?

Question

After reading various LINQ tutorials I am wondering how exactly it works under the hood and I am hoping someone could confirm some of the questions I have with the following example code:

// The Three Parts of a LINQ Query:
//  1. Data source.
int[] numbers = new int[7] { 0, 1, 2, 3, 4, 5, 6 };

// 2. Query creation.
// numQuery is an IEnumerable<int>
var numQuery =
    from num in numbers
    where (num % 2) == 0
    select num;

// 3. Query execution.
foreach (int num in numQuery)
{
    Console.Write("{0,1} ", num);
}

My questions refer to part 2:

1. Here we are using LINQ syntax to create an IEnumerable object. Is this no different to any other IEnumerable object used within a foreach statement, such as List?
2. As with all IEnumerables the contents are iterated over using a foreach statement. My query comes from LINQ IEnumerables using deferred execution - when are the contents of the IEnumerable actually gathered? Is it when the GetEnumerator() is called?

With question 2 my assumption would be that the object generated from the LINQ query produces code within the GetEnumerator function which retrieves the data in a fashion represented by our LINQ syntax. Just my guess... Unless I'm thinking about LINQ completely wrong...

Answer 1

Both of your questions are closely related.

So, yes, there is a difference. LINQ query operator return decorator sequences and this decoration enables deferred execution. Decorator sequence is a wrapper over another sequence (int[] in your case) that also knows which query to apply on wrapped sequence. Actual execution occurs at GetEnumerator().MoveNext() call.