Question

Let say I have

IEnumerable<int> list = new int[] { 1, 2, 3 };
List<int> filtered = list.Select(item => item * 10).Where(item => item < 20).ToList();

The question is are there two iterations or just one.

In other words, is that equivalent in performance to:

IEnumerable<int> list = new int[] { 1, 2, 3 };
List<int> filtered = new List<int>();
foreach(int item in list) {
    int newItem = item * 10;
    if(newItem < 20)
        filtered.Add(newItem);
}

Answer 1

There is a single iteration over the collection performed when you call the .ToArray method so both should be equivalent. .Select is a projection and .Where is a filter, both expressed as expression trees on the original dataset.

Could be easily proven:

public class Foo: IEnumerable<int>
{
    public IEnumerator<int> GetEnumerator()
    {
        yield return 1;
        Console.WriteLine("we are at element 1");
        yield return 2;
        Console.WriteLine("we are at element 2");
        yield return 3;
        Console.WriteLine("we are at element 3");
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        throw new NotImplementedException();
    }
}

class Program
{
    static void Main()
    {
        var filtered = new Foo()
            .Select(item => item * 10)
            .Where(item => item < 20)
            .ToList();
    }
}

when run prints the following:

we are at element 1
we are at element 2
we are at element 3