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Linq Types

Posted on 2009-07-09
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Last Modified: 2012-05-07
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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Question by:daveamour
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LVL 96

Expert Comment

by:Bob Learned
ID: 24818157
That is the signature for generics.  The class is defined like this:

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

Enumerable = System.Linq.Enumerable
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Author Comment

by:daveamour
ID: 24820855
Ok thanks but what is the + and the '1
Also it says is is System.Linq.Enumerable, not WhereListIterator.
Sorry still clueless.
 
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LVL 96

Expert Comment

by:Bob Learned
ID: 24840428
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.
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LVL 19

Author Comment

by:daveamour
ID: 24846779
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?
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Expert Comment

by:Bob Learned
ID: 24849260
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).
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Author Comment

by:daveamour
ID: 24856813
Hmm if it's private then how come I can get an instance of it?  Confused.
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Expert Comment

by:Bob Learned
ID: 24858452
Please explain, "I can get an instance of it".  Where are you creating an instance of a WhereListIterator?
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LVL 19

Author Comment

by:daveamour
ID: 24858492
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?
 
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LVL 96

Expert Comment

by:Bob Learned
ID: 24858538
"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.
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Author Comment

by:daveamour
ID: 24858542
Ok so what type is it then?
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Expert Comment

by:Bob Learned
ID: 24858562
WhereListIterator, which inherits from System.Linq.Enumerable, and lets you enumerate the objects.
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Author Comment

by:daveamour
ID: 24858596
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()
 
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Accepted Solution

by:
Bob Learned earned 1500 total points
ID: 24859234
Yes, you are getting that through an interface (IEnumerable<TSource>, IEnumerable, IEnumerator<TSource> or IEnumerator):

private abstract class Iterator<TSource> : IEnumerable<TSource>, IEnumerable, IEnumerator<TSource>, IDisposable, IEnumerator
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