Solved

Linq Types

Posted on 2009-07-09
13
328 Views
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() + ")";

        }

    }

}

Open in new window

0
Comment
Question by:daveamour
  • 7
  • 6
13 Comments
 
LVL 96

Expert Comment

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

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

Enumerable = System.Linq.Enumerable
0
 
LVL 19

Author Comment

by:daveamour
Comment Utility
Ok thanks but what is the + and the '1
Also it says is is System.Linq.Enumerable, not WhereListIterator.
Sorry still clueless.
 
0
 
LVL 96

Expert Comment

by:Bob Learned
Comment Utility
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.
0
 
LVL 19

Author Comment

by:daveamour
Comment Utility
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?
0
 
LVL 96

Expert Comment

by:Bob Learned
Comment Utility
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).
0
 
LVL 19

Author Comment

by:daveamour
Comment Utility
Hmm if it's private then how come I can get an instance of it?  Confused.
0
How your wiki can always stay up-to-date

Quip doubles as a “living” wiki and a project management tool that evolves with your organization. As you finish projects in Quip, the work remains, easily accessible to all team members, new and old.
- Increase transparency
- Onboard new hires faster
- Access from mobile/offline

 
LVL 96

Expert Comment

by:Bob Learned
Comment Utility
Please explain, "I can get an instance of it".  Where are you creating an instance of a WhereListIterator?
0
 
LVL 19

Author Comment

by:daveamour
Comment Utility
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?
 
0
 
LVL 96

Expert Comment

by:Bob Learned
Comment Utility
"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.
0
 
LVL 19

Author Comment

by:daveamour
Comment Utility
Ok so what type is it then?
0
 
LVL 96

Expert Comment

by:Bob Learned
Comment Utility
WhereListIterator, which inherits from System.Linq.Enumerable, and lets you enumerate the objects.
0
 
LVL 19

Author Comment

by:daveamour
Comment Utility
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()
 
0
 
LVL 96

Accepted Solution

by:
Bob Learned earned 500 total points
Comment Utility
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
0

Featured Post

How your wiki can always stay up-to-date

Quip doubles as a “living” wiki and a project management tool that evolves with your organization. As you finish projects in Quip, the work remains, easily accessible to all team members, new and old.
- Increase transparency
- Onboard new hires faster
- Access from mobile/offline

Join & Write a Comment

Summary: Persistence is the capability of an application to store the state of objects and recover it when necessary. This article compares the two common types of serialization in aspects of data access, readability, and runtime cost. A ready-to…
Introduction Hi all and welcome to my first article on Experts Exchange. A while ago, someone asked me if i could do some tutorials on object oriented programming. I decided to do them on C#. Now you may ask me, why's that? Well, one of the re…
Internet Business Fax to Email Made Easy - With eFax Corporate (http://www.enterprise.efax.com), you'll receive a dedicated online fax number, which is used the same way as a typical analog fax number. You'll receive secure faxes in your email, fr…
This tutorial demonstrates a quick way of adding group price to multiple Magento products.

763 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

15 Experts available now in Live!

Get 1:1 Help Now