Collection

collection types in C#.
• The .NET framework provides specialized classes for data storage
and retrieval. In one of the previous chapters, we have described
arrays. Collections are enhancement to the arrays.
• There are two distinct collection types in C#.
 The standard collections, which are found under the
System.Collections namespace and
 the generic collections, under System.Collections.Generic.
• The generic collections are more flexible and are the preferred way
to work with data. The generic collections or generics were
introduced in .NET framework 2.0. Generics enhance code reuse,
type safety, and performance.
• Generic programming is a style of computer programming in which
algorithms are written in terms of to-be-specified-later types that
are then instantiated when needed for specific types provided as
parameters.

ArrayList
• ArrayList is a collection from a standard System.Collections
namespace.
• It is a dynamic array. It provides random access to its elements.
• An ArrayList automatically expands as data is added. Unlike arrays,
an ArrayList can hold data of multiple data types.
• Elements in the ArrayList are accessed via an integer index. Indexes
are zero based. Indexing of elements and insertion and deletion at
the end of the ArrayList takes constant time. Inserting or deleting
an element in the middle of the dynamic array is more costly. It
takes linear time.
using System;
using System.Collections;
public class CSharpApp
{
class Empty {}

ArrayList
static void Main()
{
ArrayList da = new ArrayList();
da.Add("Visual Basic");
da.Add(344);
da.Add(55);
da.Add(new Empty());
da.Remove(55);
foreach(object el in da)
{
Console.WriteLine(el);
}}}

ArrayList
In the above example, we have created an ArrayList collection. We
have added some elements to it. They are of various data
type, string, int and a class object.
using System.Collections;
In order to work with ArrayList collection, we need to import
System.Collections namespace.
ArrayList da = new ArrayList();
An ArrayList collection is created.
da.Add("Visual Basic");
da.Add(344);
da.Add(55);
da.Add(new Empty()); da.Remove(55); We add five elements to the
array with the Add() method.
da.Remove(55); We remove one element.

List
• A List is a strongly typed list of objects that can be accessed by
index. It can be found under System.Collections.Generic
namespace.
using System;
using System.Collections.Generic;
{
static void Main()
{
List<string> langs = new List<string>();
langs.Add("Java");
langs.Add("C#");
langs.Add("C");
langs.Add("C++");

List
langs.Add("Ruby");
langs.Add("Javascript");
Console.WriteLine(langs.Contains("C#"));
Console.WriteLine(langs[1]);
Console.WriteLine(langs[2]);
langs.Remove("C#");
langs.Remove("C");
Console.WriteLine(langs.Contains("C#"));
langs.Insert(4, "Haskell");
langs.Sort();
foreach(string lang in langs)
{
Console.WriteLine(lang);
}}}

List
In the preceding example, we work with the List collection.
In order to work with the List collection, we need to import
the System.Collections.Generic namespace.
List<string> langs = new List<string>();
A generic dynamic array is created. We specify that we will
work with strings with the type specified inside <>
characters.
• langs.Add("Java");
• langs.Add("C#");
• langs.Add("C"); ... We add elements to the List using the
Add() method.

List
• Console.WriteLine(langs.Contains("C#")); We check if
the List contains a specific string using the Contains()
method.
• Console.WriteLine(langs[1]);
Console.WriteLine(langs[2]); We access the second and
the third element of the List using the index notation.
• langs.Remove("C#"); langs.Remove("C"); We remove
two strings from the List.
• langs.Insert(4, "Haskell"); We insert a string at a
specific location.
• langs.Sort(); We sort the elements using the Sort()
method.

LinkedList
• LinkedList is a generic doubly linked list in C#.
• LinkedList only allows sequential access.
• LinkedList allows for constant-time insertions or removals, but only sequential
access of elements. Because linked lists need extra storage for references, they are
impractical for lists of small data items such as characters.
• Unlike dynamic arrays, arbitrary number of items can be added to the linked list
(limited by the memory of course) without the need to realocate, which is an
expensive operation.
using System;
{
static void Main()
{
LinkedList<int> nums = new LinkedList<int>(); nums.AddLast(23);

LinkedList

nums.AddLast(34);
nums.AddLast(33);
nums.AddLast(11);
nums.AddLast(6);
nums.AddFirst(9);
nums.AddFirst(7);
LinkedListNode<int> node = nums.Find(6);
nums.AddBefore(node, 5);
foreach(int num in nums)
{
Console.WriteLine(num); } } }
This is a LinkedList example with some of its methods.

LinkedList
• LinkedList<int> nums = new LinkedList<int>(); This is an
integer LinkedList.
• nums.AddLast(23); ... nums.AddFirst(7); We populate
the linked list using the AddLast() and AddFirst()
methods.
• LinkedListNode<int> node = nums.Find(6);
nums.AddBefore(node, 5); A LinkedList consists of
nodes. We find a specific node and add an element
before it.
• foreach(int num in nums) { Console.WriteLine(num); }
Printing all elements to the console.

Dictionary

• A dictionary, also called an associative array, is a collection
of unique keys and a collection of values, where each key is
associated with one value.
• Retrieving and adding values is very fast. Dictionaries take
more memory, because for each value there is also a key.
using System;
{
static void Main()
{

Dictionary
Dictionary<string, string> domains = new Dictionary<string,
string>(); domains.Add("de", "Germany");
domains.Add("sk", "Slovakia");
domains.Add("us", "United States");
domains.Add("ru", "Russia");
domains.Add("hu", "Hungary");
domains.Add("pl", "Poland");
Console.WriteLine(domains["sk"]);
Console.WriteLine(domains["de"]);
Console.WriteLine("Dictionary has {0} items", domains.Count);
Console.WriteLine("Keys of the dictionary:");
List<string> keys = new List<string>(domains.Keys);

Dictionary
foreach(string key in keys)
{
Console.WriteLine("{0}", key);
}
Console.WriteLine("Values of the dictionary:");
List<string> vals = new List<string>(domains.Values);
foreach(string val in vals)
{
Console.WriteLine("{0}", val);
}
Console.WriteLine("Keys and values of the dictionary:");
foreach(KeyValuePair<string, string> kvp in domains)
{
Console.WriteLine("Key = {0}, Value = {1}", kvp.Key, kvp.Value);
}}}

Dictionary
We have a dictionary, where we map domain names to their country names.
Dictionary<string, string> domains = new Dictionary<string, string>();
We create a dictionary with string keys and values.
• domains.Add("de", "Germany");
• domains.Add("sk", "Slovakia");
• domains.Add("us", "United States"); ... We add some data to the
dictionary. The first string is the key. The second is the value.
• Console.WriteLine("Dictionary has {0} items", domains.Count); We print
the number of items by referring to the Count property.
• List<string> keys = new List<string>(domains.Keys);
• List<string> vals = new List<string>(domains.Values); foreach(string val in
vals) { Console.WriteLine("{0}", val); } These lines retrieve all values from
the dictionary.
• foreach(KeyValuePair<string, string> kvp in domains) {
Console.WriteLine("Key = {0}, Value = {1}", kvp.Key, kvp.Value); } Finally, we
print both keys and values of the dictionary.

Queues

• A queue is a First-In-First-Out (FIFO) data structure. The first
element added to the queue will be the first one to be removed.
Queues may be used to process messages as they appear or serve
customers as they come. The first customer which comes should be
served first.
• using System;
• using System.Collections.Generic;
• public class CSharpApp
• {
• static void Main()
• {
• Queue<string> msgs = new Queue<string>();

Queues
• msgs.Enqueue("Message 1");
• Console.WriteLine(msgs.Dequeue());
• Console.WriteLine(msgs.Peek());
• Console.WriteLine(msgs.Peek());
• Console.WriteLine();
• foreach(string msg in msgs)
• {
• Console.WriteLine(msg);
• }}}

Queues
• In our example, we have a queue with messages.
• Queue<string> msgs = new Queue<string>(); A queue
of strings is created.
• msgs.Enqueue("Message 1"); msgs.Enqueue("Message
2"); ... The Enqueue() adds a message to the end of the
queue.
• Console.WriteLine(msgs.Dequeue()); The Dequeue()
method removes and returns the item at the beginning
of the queue.
• Console.WriteLine(msgs.Peek()); The Peek() method
returns the next item from the queue, but does not
remove it from the collection.

Stacks
• A stack is a Last-In-First-Out (LIFO) data structure.
• The last element added to the queue will be the first one to
be removed.
• The C language uses a stack to store local data in a function.
The stack is also used when implementing calculators.
• using System;
• using System.Collections.Generic;
• public class CSharpApp
• {
• static void Main()
• {
• Stack<int> stc = new Stack<int>();

Stacks
• stc.Push(1);
• stc.Push(4);
• stc.Push(3);
• stc.Push(6);
• stc.Push(4);
• Console.WriteLine(stc.Pop());
• Console.WriteLine(stc.Peek());
• Console.WriteLine(stc.Peek());
• Console.WriteLine();
• foreach(int item in stc)
• {
• Console.WriteLine(item);
• } } } We have a simple stack

Stacks
• example above.
• Stack<int> stc = new Stack<int>(); A Stack data
structure is created.
• stc.Push(1); stc.Push(4); ... The Push() method adds an
item at the top of the stack.
• Console.WriteLine(stc.Pop()); The Pop() method
removes and returns the item from the top of the
stack.
• Console.WriteLine(stc.Peek()); The Peek() method
returns the item from the top of the stack. It does not
remove it.
• 4 6 6 6 3 4 1 Output.

Collection

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