Linq deferred operations

Question

I mostly understand deferred execution, but I have a question about a particular case:

Given a code fragment such as

                        var resultsOfInterest = from r in ...
                                                select r;
                        foreach (var x in resultsOfInterest)
                        {
                            //do something with x
                        }

how many times is the query resultsOfInterest executed? Once when setting up the foreach loop, or once for every element 'x'? Would it be more efficient with

                        foreach (var x in resultsOfInterest.ToArray())
                        {
                            //do something with x
                        }

?

TIA

Answer 1

It will be executed once, jut before loop, when GetEnumerator() method will be executed over query variable. Here is how foreach loop looks like:

var enumerator = resultsOfInterest.GetEnumerator(); // query executed here

while(enumerator.MoveNext()) // iterating over results of query execution
{
   var x = enumerator.Current;
   // do something with x
}

Second sample will not be more efficient, it simply stores results of query execution in array, and then calls array iterator:

var enumerator = resultsOfInterest.ToArray().GetEnumerator();
// loop stays same