Deferred execution value unchanged

Question

Say I have IQueryable<Car> cars.

I iterate it such as:

foreach (Car c in cars)
{
    c.Doors = 2;
}

Why does c.Doors contain the original value after the foreach, instead of the changed value?

Thank you in advance

Answer 1

You indicated that IQueryable is the result of a linq to entities query which is not a correct statement. MyDatabaseContext.Cars.Where(x => x.Name == "Test") returns a IQueryable which on iteration perfoms a query on the database. (Iteration is when you perform a foreach over it). So it doesnt contain the result set yet, just the query.

looping twice over cars generates 2 identical queries to the database and returns 2 identical result sets. If you want to preserve the data. you need to call ToArray or ToList or after manipulation, perform a save of the changes before you iterate again.

Answer 2

Iterating over an IQueryable retrieves the result set from the database as it goes, as I'm sure you know. What I've observed in this and other situations is that this result is not cached, so you'll often find that iterating over the IQueryable again will actually run the query again, thus your modifications aren't preserved because you're getting a new result set which was never affected by that code.

The reason it works when you call ToList() first and iterate over the result of that is because ToList() retrieves and materialises the entire result set, which is then no longer linked to the database content and is really just a copy of what the database returned.

The only way to be sure your changes will stick is to operate on the local copy of the data, i.e. lift it out of IQueryable land. This can be as simple as saving an IEnumerable of the result set (i.e. the result of ToEnumerable()), which will then return the same result set each time you enumerate it, but unlike ToList() it won't cause evaluation immediately.