directory loop - Page 666

How to join two collections with LINQ

- by JustinGreenwood

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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