How to join two collections with LINQ

Posted by JustinGreenwood on Geeks with Blogs See other posts from Geeks with Blogs or by JustinGreenwood
Published on Thu, 15 Mar 2012 12:31:51 GMT Indexed on 2012/03/18 17:59 UTC
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Here is a simple and complete example of how to perform joins on two collections with LINQ. I wrote it for a friend to show him, in one simple file, the power of LINQ queries and anonymous objects.
In the file below, there are two simple data classes defined: Person and Item. In the beginning of the main method, two collections are created. Note that the Item's OwnerId field reference the PersonId of a Person object. The effect of the LINQ query below is equivalent to a SQL statement looking like this:

select 
    Person.PersonName as OwnerName, 
    Item.ItemName as OwnedItem
from Person 
inner join Item on Item.OwnerId = Person.PersonId
order by Item.ItemName desc;

using System;
using System.Collections.Generic;
using System.Linq;

namespace LinqJoinAnonymousObjects
{
    class Program
    {
        class Person
        {
            public int PersonId { get; set; }
            public string PersonName { get; set; }
        }

        class Item
        {
            public string ItemName { get; set; }
            public int OwnerId { get; set; }
        }

        static void Main(string[] args)
        {
            // Create two collections: one of people, and another with their possessions.
            var people = new List<Person> { 
                new Person { PersonId=1, PersonName="Justin" },
                new Person { PersonId=2, PersonName="Arthur" },
                new Person { PersonId=3, PersonName="Bob" }
            };

            var items = new List<Item> { 
                new Item { OwnerId=1, ItemName="Armor" },
                new Item { OwnerId=1, ItemName="Book" },
                new Item { OwnerId=2, ItemName="Chain Mail" },
                new Item { OwnerId=2, ItemName="Excalibur" },
                new Item { OwnerId=3, ItemName="Bubbles" },
                new Item { OwnerId=3, ItemName="Gold" }
            };

            // Create a new, anonymous composite result for person id=2.
            var compositeResult = from p in people
                                   join i in items on p.PersonId equals i.OwnerId
                                   where p.PersonId == 2
                                   orderby i.ItemName descending
                                   select new
                                   {
                                       OwnerName = p.PersonName,
                                       OwnedItem = i.ItemName
                                   };

            // The query doesn't evaluate until you iterate through the query or convert it to a list
            Console.WriteLine("[" + compositeResult.GetType().Name + "]");

            // Convert to a list and loop through it.
            var compositeList = compositeResult.ToList();
            Console.WriteLine("[" + compositeList.GetType().Name + "]");
            foreach (var o in compositeList)
            {
                Console.WriteLine("\t[" + o.GetType().Name + "] " + o.OwnerName + " - " + o.OwnedItem);
            }

            Console.ReadKey();
        }
    }
}

The output of the program is below:

[WhereSelectEnumerableIterator`2]
[List`1]
        [<>f__AnonymousType1`2] Arthur - Excalibur
        [<>f__AnonymousType1`2] Arthur - Chain Mail

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How to join two collections with LINQ - Developer IT

How to join two collections with LINQ

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