In desperation, Eric goes to see his friend Marvin who is a C# specialist. Marvin listens as Eric outlines the problem, and nods when Eric expresses a reluctance to create all those variables. "You're absolutely right to look for another approach," Marvin says. "All those variables and all that duplicate code -- your program would be a time bomb waiting to explode."

Marvin then explains that what Eric needs is a collection object -- an object that can hold a bunch of other objects inside it. "Take the List class for example," Marvin says. "It has methods named Add and RemoveAt that you can use to put objects in a list and remove them."

To create a List object to store other objects, you write a line of code like this:

List<string> list = new List<string>();

This needs a bit of explanation. List is a class in the System.Collections.Generic namespace. List objects can hold any kind of data, but you have to specify what type of data they contain, by specifying a data type in <angle brackets>. So, the data type of the variable list is List<String>. If you wanted to create a list that holds integers, you would write:

List<int> list = new List<int>();

The list is initially empty.

Call the Add() method to add an object to the list:

list.Add("Milk");
list.Add("Peanut Butter");
list.Add("Jelly");

The list would look like this:

Notice that each item is at a certain position (slot #) in the list. The slots are numbered beginning at slot 0, just like the characters in a String object are located in positions numbered from 0.

"What if I want to insert an item in the middle of the list?" Eric wondered. "I don't need to do that in this particular application, but there are some other programs where I might want to do that."

"That's easy," answered Marvin. "There's a method called Insert() that allows you to specify where you want to insert the item in the list."

Consider the list above. To insert "Bread" before "Peanut Butter", call Insert() like this:

list.Insert(1, "Bread");

The list would then look like this:

As you can see, Insert() "pushes down" the rest of the items to make room. Notice how the list renumbers itself after you insert an item in the list.

You can use the Count property to find out how many items are in a list:

int numItems = list.Count;

After you have placed information into a List, you will want to eventually get it back out. To access an item in a List, you use the same notation you use to extract a single character from a string: square brackets -- like this:

string item = list[0]; 

Inside the square brackets, you specify the position number of the item you want. In this case, item would be assigned the value "Milk" -- the value in slot 0 of our list.

Marvin notes, "you have to be careful that you never attempt to access a slot that doesn't exist. For example, if a list has 3 items in it and you try to get the item at slot #3, the system will crash with an ArgumentOutOfRange exception, because the only slots that exist are numbered 0, 1, and 2."

"Now, take a look at this bit of code and see if you can tell me what it does."

for (int i = 0; i < list.Count; i++) {
  Console.WriteLine(list[i]);
}

"Well, the first time through the loop, i is 0," Eric thinks aloud, "so we get the first item in the list and display it."

"Go on," said Marvin.

"The second time, i is 1, so the second item is displayed. It looks like this code is displaying all the items in the list. Right?"

"That's right!" Marvin applauded. "You can tailor the appearance of the list this way. But tell me this. Why is the loop condition

i < list.Count

instead of

i <= list.Count

?"

Eric again gave the right answer. Can you?

"Notice that a cast is not needed when you're simply displaying an item in a list," Marvin notes.

Use the RemoveAt() method to remove an item from the list. You must know the position of the item in the list:

list.RemoveAt(1); // Removes second item from list

"This method doesn't just blank out the item in slot 1," Marvin observes. "It shifts all the other items in the list up to fill the gap. So if the list looked like this beforehand:

"... it will look like this afterwards:"

Use square bracket notation to replace an item in the list. For example, if in the example above we wanted to change "Peanut Butter" to "Bread", this code would do the job:

list[1] = "Bread";

The list would then look like this:

"Now you can write your grocery list manager program," says Marvin. "The List class makes it a piece of cake!"

using System;
using System.Collections.Generic;

class ListManager {
  static void Main() {

    List<string> list = new List<string>();
    
    string command = "";
    while (command != "Q") {
      Console.Write("A)dd item, R)emove item, D)isplay list, Q)uit:");
      command = Console.ReadLine();
      if (command == "A") {
        Console.Write("Enter the item to add to the list: ");
        string newitem = Console.ReadLine();
        list.Add(newitem);

      } else if (command == "R") {
        Console.Write("Enter the item # to remove from the list: ");
        int delitem = Convert.ToInt32(Console.ReadLine());

        list.RemoveAt(delitem); // delete item 

      } else if (command == "D") {
        for (int i = 0; i < list.Count; i++) {
          Console.WriteLine(i + " - " + list[i]);
        }
      }
    }
  }
}

"Wow!" exclaims Eric. "This is a LOT shorter than my first attempt, and it can handle the largest lists Julie will need."

Marvin agrees. "Also, notice that Julie no longer has to enter the position to add a new item. The List puts it at its proper place."