Collections

In this chapter we will deal with C# collections. 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. 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. This approach, pioneered by Ada in 1983, permits writing common functions or types that differ only in the set of types on which they operate when used, thus reducing duplication. (Wikipedia)

ArrayList

An 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
    {}
    
    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);
        }
    }
}

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 use the 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());

We add four elements to the array with the Add() method.

da.Remove(55);

We remove one element with the Remove() method.

foreach(object el in da)
{
    Console.WriteLine(el);
}

We iterate through the array and print its elements to the console.

$ ./arraylist.exe 
Visual Basic
344
CSharpApp+Empty

This is the output of the example.

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;

public class ListExample 
{
    static void Main()
    {
        List<string> langs = new List<string>();

        langs.Add("Java");
        langs.Add("C#");
        langs.Add("C");
        langs.Add("C++");
        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);
        }     
    }
}

In the preceding example, we work with the List collection.

using System.Collections.Generic;

The List collection is located in 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.

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.

$ ./list.exe 
True
C#
C
False
C++
Haskell
Java
Javascript
Ruby

This is the outcome of the example.

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;
using System.Collections.Generic;

public class LinkedListExample 
{
    static void Main()
    {
        LinkedList<int> nums = new LinkedList<int>();

        nums.AddLast(23);
        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<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);
}

We are 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;
using System.Collections.Generic;

public class DictionaryExample 
{
    static void Main()
    {
        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);

        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);
        }
    }
}

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(domains["sk"]);
Console.WriteLine(domains["de"]);

Here we retrieve two values by their keys.

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);

foreach(string key in keys)
{
    Console.WriteLine("{0}", key);
}  

These lines retrieve all keys from the dictionary.

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.

$ ./dictionary.exe 
Slovakia
Germany
Dictionary has 6 items
Keys of the dictionary:
de
sk
us
ru
hu
pl
Values of the dictionary:
Germany
Slovakia
United States
Russia
Hungary
Poland
Keys and values of the dictionary:
Key = de, Value = Germany
Key = sk, Value = Slovakia
Key = us, Value = United States
Key = ru, Value = Russia
Key = hu, Value = Hungary
Key = pl, Value = Poland

This is the output of the example.

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 QueueExample 
{
    static void Main()
    {
        Queue<string> msgs = new Queue<string>();

        msgs.Enqueue("Message 1");
        msgs.Enqueue("Message 2");
        msgs.Enqueue("Message 3");
        msgs.Enqueue("Message 4");
        msgs.Enqueue("Message 5");

        Console.WriteLine(msgs.Dequeue());
        Console.WriteLine(msgs.Peek());
        Console.WriteLine(msgs.Peek());

        Console.WriteLine();

        foreach(string msg in msgs)
        {
            Console.WriteLine(msg);
        }
    }
}

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.

$ ./queue.exe 
Message 1
Message 2
Message 2

Message 2
Message 3
Message 4
Message 5

The Dequeue() method removes the "Message 1" from the collection. The Peek() method does not. The "Message 2" remains in 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 StackExample 
{
    static void Main()
    {
        Stack<int> stc = new Stack<int>();

        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 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.

$ ./stack.exe 
4
6
6

6
3
4
1

This is the output of the stack.exe program.

This part of the C# tutorial was dedicated to Collections in C#.