C# 0 The Complete Reference

PART I

C h a p t e r   1 9 :  

L I N Q  

567

PART IPART I

The following program creates a class called 

Item

, which encapsulates an item’s name 

with its number. It creates another class called 

InStockStatus

, which links an item number 

with a Boolean property that indicates whether or not the item is in stock. It also creates a 

class called 

Temp

, which has two fields: one 

string

 and one 

bool

. Objects of this class will 

hold the result of the query. The query uses 

join

 to produce a list in which an item’s name 

is associated with its in-stock status.

// Demonstrate join.

using System;

using System.Linq;

// A class that links an item name with its number.

class Item {

  public string Name { get; set; }

  public int ItemNumber { get; set; }

  public Item(string n, int inum) {

    Name = n;

    ItemNumber = inum;

  }

}

// A class that links an item number with its in-stock status.

class InStockStatus {

  public int ItemNumber { get; set; }

  public bool InStock { get; set; }

  public InStockStatus(int n, bool b) {

    ItemNumber = n;

    InStock = b;

  }

}

// A class that encapsulates a name with its status.

class Temp {

  public string Name { get; set; }

  public bool InStock { get; set; }

  public Temp(string n, bool b) {

    Name = n;

    InStock  = b;

  }

}

class JoinDemo {

  static void Main() {

    Item[] items = {

         new Item("Pliers", 1424),

         new Item("Hammer", 7892),

         new Item("Wrench", 8534),

         new Item("Saw", 6411)

    };

www.freepdf-books.com

---

568

 

P a r t   I :  

T h e   C #   L a n g u a g e

    InStockStatus[] statusList = {

         new InStockStatus(1424, true),

         new InStockStatus(7892, false),

         new InStockStatus(8534, true),

         new InStockStatus(6411, true)

    };

    // Create a query that joins Item with InStockStatus to

    // produce a list of item names and availability. Notice

    // that a sequence of Temp objects is produced.

    var inStockList = from item in items

                      join entry in statusList

                        on item.ItemNumber equals entry.ItemNumber

                      select new Temp(item.Name, entry.InStock);

    Console.WriteLine("Item\tAvailable\n");

    // Execute the query and display the results.

    foreach(Temp t in inStockList)

      Console.WriteLine("{0}\t{1}", t.Name, t.InStock);

  }

}

The output is shown here:

Item    Available

Pliers  True

Hammer  False

Wrench  True

Saw     True

To understand how 

join

 works, let’s walk through each line in the query. The query 

begins in the normal fashion with this 

from

 clause:

var inStockList = from item in items

This clause specifies that 

item

 is the range variable for the data source specified by