The document discusses collections in programming and introduces some common collection classes. It explains that a collection gathers objects together and allows methods to manage the objects. Arrays have fixed sizes and require manual management, while collection classes are more flexible and remove disadvantages of arrays. Generic collection classes like List, Stack, and Queue provide type safety by only allowing objects of a specified type.

1. Collections
• A Collection is a group of items or objects put together
• A Book collection is a number of books grouped or put together on a shelf
• A Collection in programming has similar meaning
• Gathering number of objects together, for example collecting names of all
student in a list
• More formally a Collection in programming is a class that allows
putting objects together
• Collection classes also allow methods to manage the objects within
the collection, such as, addition, removal, search etc.
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2. Collections
• An Array can be used to group a bunch of objects together
• Disadvantages of using an Array
• Array is fixed size. It cannot grow or shrink
• int[] numbers = new int[100]; // size of the numbers array is fixed to 100
• Programmer has to manage the collection, such as, adding, removing,
searching and moving items within the array etc.
• Collection classes removes the above disadvantages
• To work with Collection classes in C# the following namespaces have
to be included in your project:
• Systems.Collections
• Systems.Collections.Generic
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3. Collections
• Classes within the System.Collection namespace do not check the type of object
in the collection
• Example of some of these classes are:
• ArrayList, Stack, Queue
• Classes within the System.Collection.Generic namespace check the type of object
in the collection
• Following example shows difference between using the two namespaces
System.Collection System.Collection.Generic
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ArrayList scList = ArrayList(); List<string> scgList = new List<string>();
scList.Add(“saif”); scgList.Add(“saif”);
scList.Add(32); scgList.Add(32); // Error
// scList allows strings and ints //csgList only allows strings to be added to the
// to be added to the list. No Type // List. This is called type checking.
// checking.

4. Collections
• Generic classes enforce type checking.
• When using a Generic class the type of the object is specified as a type parameter
List<string>
• Type parameter appears within the <> brackets
• The type of object in the example above is string. So this list can only store
strings.
• Some of the collections that are part of the System.Collection.Generic
namespace are:
List<T>, LinkedList<T>, Stack<T>, Queue<T>
<T> refers to the type parameter. In the example above T = string.
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5. Collections
• The collection classes we will be using are:
• Note that we will only use the Generic Stack and Queue collections. Also note
ArrayList is not generic. Even though it is covered we should use List<T> instead.
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Collection Class Description Namespace
List<T> Stores objects that can be accessed by index, like arrays. List
can dynamically grow.
System.Collection.Generic
ArrayList Similar to list. Consider using List instead. System.Collection
LinkedList<T> Implements List as a collection of nodes linked together. The list
is a doubly-linked list.
System.Collection.generic
Stack<T> Collection of objects inserted and retrieved in Last-In-First-Out
order.
System.Collection.Generic
Queue<T> Collection of objects inserted and retrieved in First-In-First-Out
order.
System.Collection.Generic