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Linq Types

I am wondering about what type is returned when we do a Linq select.

Consider the attached code for discussion.

I know that the fortiesPeople in var fortiesPeople = from p in people where p.Age > 40 select p; returns an object which implements IEnumerable. When I try and see what type it is though I get:

fortiesPeople type = System.Linq.Enumerable+WhereListIterator`1[LinqExamples.Person]

So what is all that - what is the + and the '1 etc

Thanks

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;
 
namespace LinqExamples
{
    public class Program
    {
        static void Main(string[] args)
        {
            Person dave = new Person { FirstName = "Dave", LastName = "Amour", Age = 42 };
            Person fred = new Person { FirstName = "Fred", LastName = "Smith", Age = 22 };
            Person tiff = new Person { FirstName = "Tiff", LastName = "Bloggs", Age = 24 };
            Person kate = new Person { FirstName = "Kate", LastName = "Brown", Age = 33 };
            Person bill = new Person { FirstName = "Bill", LastName = "Green", Age = 38 };
            Person ian = new Person { FirstName = "Ian", LastName = "Smith", Age = 19 };
            Person fiona = new Person { FirstName = "Fiona", LastName = "Williams", Age = 28 };
            Person sonia = new Person { FirstName = "Sonia", LastName = "Johnson", Age = 43 };
            
            List<Person> people = new List<Person>();
 
            people.Add(dave);
            people.Add(fred);
            people.Add(tiff);
            people.Add(kate);
            people.Add(bill);
            people.Add(ian);
            people.Add(fiona);
            people.Add(sonia);
 
            var fortiesPeople = from p in people where p.Age > 40 select p;
 
            Console.WriteLine("fortiesPeople type = " + fortiesPeople.GetType().ToString());
 
            foreach (Person person in fortiesPeople)
            {
                Console.WriteLine(person);
            }
 
            Console.Read();
        }
    }
 
    public class Person
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Age { get; set; }
 
        public override string ToString()
        {
            return FirstName + " " + LastName + " (Age " + Age.ToString() + ")";
        }
    }
}

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Bob Learned

That is the signature for generics. The class is defined like this:

private class WhereListIterator<TSource> : Enumerable.Iterator<TSource>

Enumerable = System.Linq.Enumerable

daveamour

ASKER

Ok thanks but what is the + and the '1
Also it says is is System.Linq.Enumerable, not WhereListIterator.
Sorry still clueless.

Bob Learned

Any time you see WhereListIterator`1, that means that it uses generics (WhereListIterator<TSource>). The "+" symbol means that the first class is a base class (System.Linq.Enumerable), and the second class is the deriving class (WhereListIterator).

If you look at the class descriptor with Reflector, as shown above, you can see that WhereListIterator implements Enumerable.Iterator<TSource>, which is from the System.Linq.Enumerable class.

daveamour

ASKER

So the type is WhereListIterator which has a base class of System.Linq.Enumerable.
Where is the WhereListIterator class - I can't find it anywhere?

Bob Learned

private class WhereListIterator

WhereListIterator is a private class in the framework, so you can't see it, but reflection can (I used Reflector to find it).

daveamour

ASKER

Hmm if it's private then how come I can get an instance of it? Confused.

Bob Learned

Please explain, "I can get an instance of it". Where are you creating an instance of a WhereListIterator?

daveamour

ASKER

I am trying to find out what fortiesPeople is below
var fortiesPeople = from p in people where p.Age > 40 select p;
Did you not say it was a WhereListIterator or have I misunderstood?

Bob Learned

"var fortiesPeople" uses type inference, which builds instances inside of the framework, that you can use to enumerate. You are not creating an instance, the CLR is. When you are checking the type, the CLR tells you what type it created.

daveamour

ASKER

Ok so what type is it then?

Bob Learned

WhereListIterator, which inherits from System.Linq.Enumerable, and lets you enumerate the objects.

daveamour

ASKER

I don't understand how I can have an instance of a class (created by the CLR) which is of a private type. Is it in fact expressed as an interface - ie IEnumerbable - ie is there a method somewhere like this
public IEnumerable MyMethod()

ASKER CERTIFIED SOLUTION

Bob Learned

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