does putting Linq query inside a method affect deferred excecution?

Question

Linq query is not executed until the sequence returned by the query is actually iterated.

I have a query that is used repeatedly, so I am going to encapuslate it inside a method. I'd like to know if it interferes with the deferred execution.
If I encapsulate a Linq query into a method like below, the query gets executed at line 2, not line 1 where the method is called. Is this correct?

public IEnumerable<Person> GetOldPeopleQuery()
{
   return personList.Where(p => p.Age > 60);
}

public void SomeOtherMethod()
{
   var getWomenQuery = GetOldPeopleQuery().Where(p => p.Gender == "F"); //line 1
   int numberOfOldWomen = getWomanQuery.Count();  //line 2
}

P.S. I am using Linq-To-EF, if it makes any difference.

score 2 · Accepted Answer · answered Apr 23 '11 at 06:23

The query is lazy evaluated when you enumerate the result the first time, that is not affected by putting it inside a method.

There is however another thing in your code that will be very inefficient. Once you've returned an IEnumerable the next linq statement applied to the collection will be a linq-to-objects query. That means that in your case you will load all old people from the database and then filter out the women in memory. The same with the count, it will be done in memory.

If you instead return an IQueryable<Person> those two questions will be evaluated using linq-to-entities and the filtering and summing can be done in the database.

There's no indication in this question that a database is actually involved. But if it is, I agree with you about the inefficiency. — Jon Skeet, Apr 23 '11 at 06:39

does putting Linq query inside a method affect deferred excecution?

1 Answers1