c# - Does foreach execute the query only once?

Question

I have a list of items and a LINQ query over them. Now, with LINQ's deferred execution, would a subsequent foreach loop execute the query only once or for each turn in the loop?

Given this example (Taken from Introduction to LINQ Queries (C#), on MSDN)

    // 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);
    }

Or, in other words, would there be any difference if I had:

    foreach (int num in numQuery.ToList())

And, would it matter, if the underlying data is not in an array, but in a Database?

Answer 1

Now, with LINQ's deferred execution, would a subsequent foreach loop execute the query only once or for each turn in the loop?

Yes, once for the loop. Actually, it may execute the query less than once - you could abort the looping part way through and the (num % 2) == 0 test wouldn't be performed on any remaining items.

Or, in other words, would there be any difference if I had:

foreach (int num in numQuery.ToList())

Two differences:

1. In the case above, ToList() wastes time and memory, because it first does the same thing as the initial foreach, builds a list from it, and then foreachs that list. The differences will be somewhere between trivial and preventing the code from ever working, depending on the size of the results.

2. However, in the case where you are going to repeatedly do foreach on the same results, or otherwise use it repeatedly, the then while the foreach only runs the query once, the next foreach runs it again. If the query is expensive, then the ToList() approach (and storing that list) can be a massive saving.