IEnumerable<T>.Count() vs List<T>.Count with Entity Framework

Question

I am retrieving a list of items using Entity Framework and if there are some items retrieved I do something with them.

var items = db.MyTable.Where(t => t.Expiration < DateTime.Now).ToList();
if(items.Count != 0)
{
    // Do something... 
}

The if statement could also be written as

if(items.Count() != 0)
{
    // Do something... 
}

In the first case, the .Count is a List<T>.Count property. In the second case, the .Count() is IEnumerable<T>.Count() extension method.

Although both approaches achieve the same result, however, is one more preferred than the other? (Possibly some difference in performance?)

Answer 1

Enumerable.Count<T> (the extension method for IEnumerable<T>) just calls Count if the underlying type is an ICollection<T>, so for List<T> there is no difference.

Queryable.Count<T> (the extension method for IQueryable<T>) will use the underlying query provider, which in many cases will push the count down to the actual SQL, which will perform faster than counting the objects in memory.

If a filter is applied (e.g. Count(i => i.Name = "John")) or if the underlying type is not an ICollection<T>, the collection is enumerated to compute the count.

is one more preferred than the other?

I generally prefer to use Count() since 1) it's more portable (the underlying type can be anything that implements IEnumerable<T> or IQueryable<T>) and 2) it's easier to add a filter later if necessary.

As Tim states in his comment, I also prefer using Any() to Count() > 0 since it doesn't have to actually count the items - it will just check for the existence of one item. Conversely I use !Any() instead of Count() == 0.