How to check if ObjectResult collection was empty?

Question

I have this piece of code which check the last customer_id from the db.

ObjectResult<int?> last_customer_id_collection = MainForm.eggsContext.ExecuteStoreQuery<int?>("select MAX(customer_id) from customers");

Then I get the next id required like this

new_customer_id = last_customer_id_collection.FirstOrDefault<int?>().Value + 1;

It's working, HOWEVER when dealing with a new empty db the statment last_customer_id_collection.FirstOrDefault<int?>() throws an InvalidOperationException.

How can I check if last_customer_id_collection was empty without try catch?

(p.s. I was trying to check if last_customer_id_collection.Any<int?>() or last_customer_id_collection.Count<int?>() or DefaultIfEmpty etc. but everything I try causes this exception)

Answer 1

You get an InvalidOperationException because Nullable<T>.HasValue would return false.

You will need to modify the code to not call .Value if the query return value is null.

var custId = last_customer_id_collection.FirstOrDefault<int?>();
if(custId.HasValue)
    DoStuffWithId();
else
    DoStuffWithNullResult();

Small Example that demonstrates the potentialy confusing behaviors of Nullables This will print:

"Null!", "No Value but no NullReferenceException" and then "Oops, InvalidOperationException"

List<int?> values = new List<int?>();

var test = values.FirstOrDefault();

if (test == null)
    Console.WriteLine("Null!");

if (test.HasValue)
    Console.WriteLine(test.Value);
else
    Console.WriteLine("No Value but no NullReferenceException");

try
{
    int value = test.Value;
}
catch(InvalidOperationException)
{
    Console.WriteLine("Oops, InvalidOperationException");
}