What is List<T>

The List<T> is a collection of strongly typed objects that can be accessed by index and having methods for sorting, searching, and modifying list. It is the generic version of the ArrayList that comes under System.Collections.Generic namespace.

List<T> Characteristics

• List<T> equivalent of the ArrayList, which implements IList<T>.

• It comes under System.Collections.Generic namespace.

• List<T> can contain elements of the specified type. It provides compile-time type checking and doesn't perform boxing-unboxing because it is generic.

• Elements can be added using the Add(), AddRange() methods or collection-initializer syntax.

• Elements can be accessed by passing an index e.g. myList[0]. Indexes start from zero.

• List<T> performs faster and less error-prone than the ArrayList.

Create List and adding items

List<int> primeNumbers = new List<int>();
primeNumbers.Add(1); // adding elements using add() method
primeNumbers.Add(3);
primeNumbers.Add(5);
primeNumbers.Add(7);

var cities = new List<string>();
cities.Add("New York");
cities.Add("London");
cities.Add("Mumbai");
cities.Add("Chicago");
cities.Add(null);// nulls are allowed for reference type list

//adding elements using collection-initializer syntax
var bigCities = new List<string>()
                    {
                        "New York",
                        "London",
                        "Mumbai",
                        "Chicago"                    
                    };

Add Custom Class Objects in List

var students = new List<Student>() { 
                new Student(){ Id = 1, Name="Bill"},
                new Student(){ Id = 2, Name="Steve"},
                new Student(){ Id = 3, Name="Ram"},
                new Student(){ Id = 4, Name="Abdul"}
            };

Adding an Array in a List

string[] cities = new string[3]{ "Mumbai", "London", "New York" };
var popularCities = new List<string>();
// adding an array in a List
popularCities.AddRange(cities);
var favouriteCities = new List<string>();
// adding a List 
favouriteCities.AddRange(popularCities);

Accessing a List

List<int> numbers = new List<int>() { 1, 2, 5, 7, 8, 10 };
Console.WriteLine(numbers[0]); // prints 1
Console.WriteLine(numbers[1]); // prints 2
Console.WriteLine(numbers[2]); // prints 5
Console.WriteLine(numbers[3]); // prints 7

// using foreach LINQ method
numbers.ForEach(num => Console.WriteLine(num + ", "));//prints 1, 2, 5, 7, 8, 10,

// using for loop
for(int i = 0; i < numbers.Count; i++){
    Console.WriteLine(numbers[i]);
}

Accessing a List using LINQ

var students = new List<Student>() { 
                new Student(){ Id = 1, Name="Bill"},
                new Student(){ Id = 2, Name="Steve"},
                new Student(){ Id = 3, Name="Ram"},
                new Student(){ Id = 4, Name="Abdul"}
            };

//get all students whose name is Bill
var result = from s in students
         where s.Name == "Bill"
         select s;

foreach(var student in result){
    Console.WriteLine(student.Id + ", " + student.Name);
}

Insert Elements in List

var numbers = new List<int>(){ 10, 20, 30, 40 };
numbers.Insert(1, 11);// inserts 11 at 1st index: after 10.
foreach (var num in numbers){
    Console.Write(num);
}

Remove Elements from List

var numbers = new List<int>(){ 10, 20, 30, 40, 10 };
numbers.Remove(10); // removes the first 10 from a list
numbers.RemoveAt(2); //removes the 3rd element (index starts from 0)
//numbers.RemoveAt(10); //throws ArgumentOutOfRangeException

foreach (var el in intList){
    Console.Write(el); //prints 20 30;
}

Check Elements in List:

Use the Contains() method to determine whether an element is in the List<T> or not.

var numbers = new List<int>(){ 10, 20, 30, 40 };
numbers.Contains(10); // returns true
numbers.Contains(11); // returns false
numbers.Contains(20); // returns true

List<T> Class Properties and Methods:

The following table lists the important properties and methods of List<T> class:

Dictionary<TKey, TValue>

The Dictionary<TKey, TValue> is a generic collection that stores key-value pairs in no particular order.

Dictionary Characteristics

• Dictionary<TKey, TValue> stores key-value pairs.

• Comes under System.Collections.Generic namespace.

• Implements IDictionary<TKey, TValue> interface.

• Keys must be unique and cannot be null.

• Values can be null or duplicate.

• Values can be accessed by passing associated key in the indexer e.g. myDictionary[key]

• Elements are stored as KeyValuePair<TKey, TValue> objects.

Creating a Dictionary

IDictionary<int, string> numberNames = new Dictionary<int, string>();
numberNames.Add(1,"One"); //adding a key/value using the Add() method
numberNames.Add(2,"Two");
numberNames.Add(3,"Three");

//The following throws run-time exception: key already added.
//numberNames.Add(3, "Three"); 

foreach(KeyValuePair<int, string> kvp in numberNames){
    Console.WriteLine("Key: {0}, Value: {1}", kvp.Key, kvp.Value);
}

//creating a dictionary using collection-initializer syntax
var cities = new Dictionary<string, string>(){
    {"UK", "London, Manchester, Birmingham"},
    {"USA", "Chicago, New York, Washington"},
    {"India", "Mumbai, New Delhi, Pune"}
};

foreach(var kvp in cities){
    Console.WriteLine("Key: {0}, Value: {1}", kvp.Key, kvp.Value);
}

Access Dictionary Elements:

var cities = new Dictionary<string, string>(){
    {"UK", "London, Manchester, Birmingham"},
    {"USA", "Chicago, New York, Washington"},
    {"India", "Mumbai, New Delhi, Pune"}
};

Console.WriteLine(cities["UK"]); //prints value of UK key
Console.WriteLine(cities["USA"]);//prints value of USA key
//Console.WriteLine(cities["France"]); // run-time exception: Key does not exist

//use ContainsKey() to check for an unknown key
if(cities.ContainsKey("France")){  
    Console.WriteLine(cities["France"]);
}

//use TryGetValue() to get a value of unknown key
string result;

if(cities.TryGetValue("France", out result))
{
    Console.WriteLine(result);
}

//use ElementAt() to retrieve key-value pair using index
for (int i = 0; i < cities.Count; i++)
{
    Console.WriteLine("Key: {0}, Value: {1}", 
                                            cities.ElementAt(i).Key, 
                                            cities.ElementAt(i).Value);
}

Update Dictionary

var cities = new Dictionary<string, string>(){
    {"UK", "London, Manchester, Birmingham"},
    {"USA", "Chicago, New York, Washington"},
    {"India", "Mumbai, New Delhi, Pune"}
};

cities["UK"] = "Liverpool, Bristol"; // update value of UK key
cities["USA"] = "Los Angeles, Boston"; // update value of USA key
//cities["France"] = "Paris"; //throws run-time exception: KeyNotFoundException

if(cities.ContainsKey("France")){
    cities["France"] = "Paris";
}

Remove Elements in Dictionary

var cities = new Dictionary<string, string>(){
    {"UK", "London, Manchester, Birmingham"},
    {"USA", "Chicago, New York, Washington"},
    {"India", "Mumbai, New Delhi, Pune"}
};

cities.Remove("UK"); // removes UK 
//cities.Remove("France"); //throws run-time exception: KeyNotFoundException

if(cities.ContainsKey("France")){ // check key before removing it
    cities.Remove("France");
}

cities.Clear(); //removes all elements

Hashtable

The Hashtable is a non-generic collection that stores key-value pairs, similar to generic Dictionary<TKey, TValue> collection. It optimizes lookups by computing the hash code of each key and stores it in a different bucket internally and then matches the hash code of the specified key at the time of accessing values.

Hashtable Characteristics

• Hashtable stores key-value pairs.

• Comes under System.Collections namespace.

• Implements IDictionary interface.

• Keys must be unique and cannot be null.

• Values can be null or duplicate.

• Values can be accessed by passing associated key in the indexer e.g. myHashtable[key]

• Elements are stored as DictionaryEntry objects.

Creating a Hashtable

Hashtable numberNames = new Hashtable();
numberNames.Add(1,"One"); //adding a key/value using the Add() method
numberNames.Add(2,"Two");
numberNames.Add(3,"Three");

//The following throws run-time exception: key already added.
//numberNames.Add(3, "Three"); 

foreach(DictionaryEntry de in numberNames){
    Console.WriteLine("Key: {0}, Value: {1}", de.Key, de.Value);
}

//creating a Hashtable using collection-initializer syntax
var cities = new Hashtable(){
    {"UK", "London, Manchester, Birmingham"},
    {"USA", "Chicago, New York, Washington"},
    {"India", "Mumbai, New Delhi, Pune"}
};

foreach(DictionaryEntry de in cities){
    Console.WriteLine("Key: {0}, Value: {1}", de.Key, de.Value);
}

Add Dictionary in Hashtable

Dictionary<int, string> dict = new Dictionary<int, string>();
dict.Add(1, "one");
dict.Add(2, "two");
dict.Add(3, "three");

Hashtable ht = new Hashtable(dict);

Update Hashtable

//creating a Hashtable using collection-initializer syntax
var cities = new Hashtable(){
    {"UK", "London, Manchester, Birmingham"},
    {"USA", "Chicago, New York, Washington"},
    {"India", "Mumbai, New Delhi, Pune"}
};

string citiesOfUK = (string) cities["UK"]; //cast to string
string citiesOfUSA = (string) cities["USA"]; //cast to string

Console.WriteLine(citiesOfUK);
Console.WriteLine(citiesOfUSA);

cities["UK"] = "Liverpool, Bristol"; // update value of UK key
cities["USA"] = "Los Angeles, Boston"; // update value of USA key

if(!cities.ContainsKey("France")){
    cities["France"] = "Paris";
}

Remove Elements in Hashtable

var cities = new Hashtable(){
    {"UK", "London, Manchester, Birmingham"},
    {"USA", "Chicago, New York, Washington"},
    {"India", "Mumbai, New Delhi, Pune"}
};

cities.Remove("UK"); // removes UK 
//cities.Remove("France"); //throws run-time exception: KeyNotFoundException

if(cities.ContainsKey("France")){ // check key before removing it
    cities.Remove("France");
}

cities.Clear(); //removes all elements

Thanks for your time. I hope this post was helpful and gave you a mental model for List, Dictionay & HashTable in C#!