Question

If I wrote :

for (int i = 0; i < Strutture.Count(); i++)
{
}

and Strutture is an IEnumerable with 200 elements, IIS crash. That’s because I see every time I do Strutture.Count() it executes all LINQ queries linked with that IEnumerable.

So, how can I get the “current” number of elements? I need a list?

Answer 1

“That’s because I see every time I do Strutture.Count() it executes all LINQ queries linked with that IEnumerable.”

Without doing such, how is it going to know how many elements there are?

For example:

Enumerable.Range(0,1000).Where(i => i % 2==0).Skip(100).Take(5).Count();

Without executing the LINQ, how could you know how many elements there are?

If you want to know how many elements there are in the source (e.g. Enumerable.Range) then I suggest you use a reference to that source and query it directly. E.g.

 var numbers = Enumerable.Range(0,1000);
 numbers.Count();

Also keep in mind some data sources don’t really have a concept of ‘Count’ or if they do it involves going through every single item and counting them.

Lastly, if you’re using .Count() repetitively [and you don’t expect the value to actually change] it can be a good idea to cache:

 var count = numbers.Count();
 for (int i =0; i<count; i++) //  Do Something

Supplemental:

“At first Count(), LINQ queries are executes. Than, for the next, it just “check” the value 🙂 Not “execute the LINQ query again…” :)” – Markzzz

Then why don’t we do that?

var query = Enumerable.Range(0,1000).Where(i => i % 2==0).Skip(100).Take(5).Count();
var result = query.ToArray() //Gets and stores the result!
result.Length;   

🙂

“But when I do the first “count”, it should store (after the LINQ queries) the new IEnumerable (the state is changed). If I do again .Count(), why LINQ need to execute again ALL queries.” – Markzzz

Because you’re creating a query that gets compiled down into X,Y,Z. You’re running the same query twice however the result may vary.

For example, check this out:

    static void Main(string[] args)
    {
        var dataSource = Enumerable.Range(0, 100).ToList();
        var query = dataSource.Where(i => i % 2 == 0);

        //Run the query once and return the count:
        Console.WriteLine(query.Count());  //50

        //Now lets modify the datasource - remembering this could be a table in a db etc.
        dataSource.AddRange(Enumerable.Range(100, 100));

        //Run the query again and return the count:
        Console.WriteLine(query.Count());  //100
        Console.ReadLine();
    }

This is why I recommended storing the results of the query above!