CODEWITHSUNDEEP
c#linq
Leron
Leron

Reputation: 9876

When to check List<T> for null, when for 0 and when both

I use ADO.NET entity framework and very often there are code snippets like this :

List<Sole> entity = soleService.All()
    .Where(s => (s.ShoeLastID == shoeLastID) && (s.Status == 20))
    .ToList();

Since now I haven't think much about it and just made this check:

if (entity.Count > 0)

believing that it is enough. Now I see that many people check with Any() and for null. How to be sure at a certain situation what kind of checks I need and in this certain scenario which as I said - I use very often is if (entity.Count > 0) enough?

Upvotes: 3

Views: 695

Answers (8)

Maris
Maris

Reputation: 4776

entity.Any() - will return true if there is any entities in your collection. entities.Count() == 0 will do the same. But I would recommend to use any because it will work faster. Because Count will return the amount of data in the collection, but any will trigger on the first item found in your collection. But if you are not sure that your collection is initialized I would recommend you to use next construction:

if(entity!=null && entity.Any())
{
   //Do something. You will get her always without error, And you will be 100% sure that your collection is not empty and it is initialized
}

Hope it helps.

Upvotes: 2

MoonKnight
MoonKnight

Reputation: 23831

Of course a list can be null or empty. If you attempt to create a List<Sole> using LINQ as above and the soleService could be null, in which case you will get a NullReferenceException. So I would check that the soleService is not null or empty first. So

List<Sole> entity = null;
// ...
if (soleService != null && soleService.Count > 0)
    entity = soleService.All()
        .Where(s => (s.ShoeLastID == shoeLastID) && (s.Status == 20))
        .ToList();

I hope this helps.

Upvotes: 2

Andy
Andy

Reputation: 4107

When you call if (entity.Count > 0) but entity == null, you will get an exception because .Count does not exist while entity is not initialized.

Upvotes: 2

ken2k
ken2k

Reputation: 49013

if (entity.Count > 0) or if (entity.Any()) are identical in you case. As you already fetched all the data from the DB, the list has been built and you knows its size. So .Count property doesn't iterate over anything.

In the other hand, do not call the .Count() IEnumerable extension if you didn't fetched all data, because it'll enumerate items for nothing.

Use instead:

bool test = soleService.All()
    .Any(s => (s.ShoeLastID == shoeLastID) && (s.Status == 20));

if (test)
{
...
}

Also, LINQ extensions won't return null but an empty IEnumerable, so don't check for null.

Upvotes: 4

Royi Namir
Royi Namir

Reputation: 148664

Any() will provide better solution cause it stopse after first matching.

in addition

I would suggest also to do ToList() only if Any() is true.

Youll save (micro) performance.

var t = soleService.All()  .Where(s => (s.ShoeLastID == shoeLastID) && (s.Status == 20));
if  (t.Any()) entity =t.ToList();

Upvotes: 2

Zdeslav Vojkovic
Zdeslav Vojkovic

Reputation: 14591

Depends on what you need.

If you just want to know if any of entities match your predicate then use Any(), as it will return immediately upon finding the first matching entity. Count() / Count will need to process all the entities which will typically be much slower.

Also prefer Linq's Count() to List Count as it doesn't have to create the full list in the memory, which can be very expensive with large result sets.

Upvotes: 2

nvoigt
nvoigt

Reputation: 77354

If you have a .ToList() call, then the list is always a list. Maybe empty, but never null.

The check for .Any() instead of .Count() > 0 is a performance improvement for most containers or enumerables because .Any() will only touch the first element if there is one. .Count() would need to count through your container to the end although you are not interested in the result, only in the fact that it's not zero.

Upvotes: 2

Nikolay Kostov
Nikolay Kostov

Reputation: 16983

entity.Any() is better choice. And you don't need to invoke .ToList() since this wll take all the data from the database and then just check its count.

Upvotes: 1

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