In this article, we are going to talk more about Generic List and Dictionary in C#. A List<T> and Dictionary are very useful collections in C#, and we are going to discover its features in the rest of the article.

If you want to see complete navigation of this tutorial, you can do that here C# Intermediate Tutorial.

To download the source code, you can visit the Generic List and Dictionary in C# Source Code. 

We are going to split this article into the following sections:

List<T>

A List<T> represents a strongly typed collection of objects that can be accessed by index.

To instantiate a List<T> we need to provide a type between the angle brackets:

List<int> numberList = new List<int>();
List<Student> students = new List<Student>();

It has two more constructors that we can use to initialize a List object. With the first one, we can set initial capacity:
List<int> numbers = new List<int>(2);

With the second one, we can populate our list with the IEnumerable collection:
int[] nums = new int[5] { 1, 2, 3, 4, 5 };
List<int> numbers = new List<int>(nums);

To access any element we can specify its index position:
int number = numbers[1];

Methods and Properties

The Add method adds the element inside a list:

List<int> numbers = new List<int>();
numbers.Add(34);
numbers.Add(58);
numbers.Add(69);

foreach (int number in numbers)
{
    Console.WriteLine(number);
}

AddRange adds the elements of the specified collection to the end of a list:
List<int> numbers = new List<int>();
numbers.Add(34);
numbers.Add(58);
numbers.Add(69);

int[] nums = new int[] { 1, 22, 44 };

numbers.AddRange(nums);

foreach (int number in numbers)
{
     Console.WriteLine(number);
}

Contains determines whether an element exists in the list:
if(numbers.Contains(34))
{
     Console.WriteLine("The number 34 exists in a list");
}

The IndexOf method returns the position of an element as an integer number. If an element couldn’t be found, this method returns -1:
int index;
if((index = numbers.IndexOf(58)) != -1)
{
    Console.WriteLine($"The number 58 is on the index: {index}");
}

LastIndexOf is similar to a previous method except it returns a last occurrence of the element.

CopyTo method copies the entire collection to a compatible array, starting from the beginning of that array:

int[] copyArray = new int[6];

numbers.CopyTo(copyArray);

foreach (int copyNumber in copyArray)
{
     Console.WriteLine(copyNumber);
}

The Remove method removes the first occurrence of a specific element from the list:
numbers.Remove(69);

The Clear method clears all the elements from a list:
numbers.Clear();

We can check how many elements a list has by using the Count property:
Console.WriteLine(numbers.Count);

Dictionary

Dictionary represents a collection of keys and values. To instantiate an object we can use the following syntax:

Dictionary<KeyType, ValueType> Name = new Dictionary<KeyType, ValueType>();

The KeyType represents a type for our key in a collection. The ValueType represents the value assigned to the key. So, we can extract our value from a collection by using the key inside the square brackets:
DictionaryName[key];

Dictionary has several constructors we can use to instantiate objects:
Dictionary<string, int> dictExample = new Dictionary<string, int>();

Dictionary<string, int> dictExample1 = new Dictionary<string, int>(5); //to set initial size

Dictionary<string, int> dictExample2 = new Dictionary<string, int>(dictExample1); //accepts all the elements from created Key-Value collection

Methods and Properties

The Add method adds the key-value pair inside a collection:

Dictionary<string, int> dictExample = new Dictionary<string, int>();

dictExample.Add("First", 100);
dictExample.Add("Second", 200);
dictExample.Add("Third", 300);

foreach (var item in dictExample)
{
     Console.WriteLine(dictExample[item.Key]);
}

Remove removes the key-value pair from a collection based on the specified key:
dictExample.Remove("Second");
foreach (var item in dictExample)
{
     Console.WriteLine(dictExample[item.Key]);
}

ContainsKey determines if a collection contains a specific key.

ContainsValue determines if a collection contains a specific value:

if(dictExample.ContainsKey("First"))
{
     Console.WriteLine("It contains key");
}

if(dictExample.ContainsValue(300))
{
      Console.WriteLine("It contains value");
}

The Clear method removes all key-value pairs from a collection:
dictExample.Clear();

If we want to count all of our elements inside a collection, we can use the Count property. If we want to get a collection of containing Keys or containing Values from a dictionary, we can use the Keys and Values properties:
Console.WriteLine(dictExample.Count);

foreach (var key in dictExample.Keys)
{
     Console.WriteLine(key);
}

foreach (var value in dictExample.Values)
{
     Console.WriteLine(value);
}

Conclusion

In this article, we have learned:

  • To use the List<T> collection with its methods
  • To use a Dictionary with its methods and properties

In the next article, we are going to talk about Delegates in C#.