C# 0 The Complete Reference

Storing User-Defined Classes in Collections

For the sake of simplicity, the foregoing examples have stored built-in types, such as 

int

,

string

, or 

char

, in a collection. Of course, collections are not limited to the storage of built-in 

objects. Quite the contrary. The power of collections is that they can store any type of object, 

including objects of classes that you create.

Let’s begin with an example that uses the non-generic class 

ArrayList

 to store inventory 

information that is encapsulated by the 

Inventory

 class:

// A simple inventory example.

using System;

using System.Collections;

class Inventory {

  string name;

  double cost;

  int onhand;

  public Inventory(string n, double c, int h) {

    name = n;

    cost = c;

    onhand = h;

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  }

  public override string ToString() {

    return

      String.Format("{0,-10}Cost: {1,6:C}  On hand: {2}",

                    name, cost, onhand);

  }

}

class InventoryList {

  static void Main() {

    ArrayList inv = new ArrayList();

    // Add elements to the list

    inv.Add(new Inventory("Pliers", 5.95, 3));

    inv.Add(new Inventory("Wrenches", 8.29, 2));

    inv.Add(new Inventory("Hammers", 3.50, 4));

    inv.Add(new Inventory("Drills", 19.88, 8));

    Console.WriteLine("Inventory list:");

    foreach(Inventory i in inv) {

      Console.WriteLine("   " + i);

    }

  }

}

The output from the program is shown here:

Inventory list:

   Pliers    Cost:  $5.95  On hand: 3

   Wrenches  Cost:  $8.29  On hand: 2

   Hammers   Cost:  $3.50  On hand: 4

   Drills    Cost: $19.88  On hand: 8

In the program, notice that no special actions were required to store objects of type 

Inventory

 in a collection. Because all types inherit 

object

, any type of object can be stored in 

any non-generic collection. Thus, using a non-generic collection, it is trivially easy to store 

objects of classes that you create. Of course, it also means the collection is not type-safe.

To store objects of classes that you create in a type-safe collection, you must use one of 

the generic collection classes. For example, here is a version of the preceding program 

rewritten to use 

List

. The output is the same as before.

// Store Inventory Objects in a List collection.

using System;

using System.Collections.Generic;

class Inventory {

  string name;

  double cost;

  int onhand;

  public Inventory(string n, double c, int h) {

    name = n;

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