What is the difference between List and ArrayList in .NET?

ArrayListis a class that was created since version 1 of C# and the .NET Framework before Generics was added , while it List<T>is the class that has replaced it since C# 2, the .NET Framework 2.0, and VB.NET 2008 (8.0) onwards .

The main difference is that an instance of ArrayListallows multiple types to be added to the collection.

Example:

C#

var arrayList = new ArrayList();
arrayList.Add(1); // Puede contener enteros
arrayList.Add("Dos"); // Tambien strings
arrayList.Add(false); // y booleans, etc

VB

Dim arrayList As New ArrayList()
arrayList.Add(1) ' Puede contener enteros
arrayList.Add("Dos") ' Tambien strings
arrayList.Add(False) ' y booleans, etc

While it List<T>needs the type of the elements of the list to be defined when it is instantiated

C#

var list = new List<int>();
list.Add(1);  // permite agregar enteros
list.Add("Dos"); // error de sintaxis, solo se pueden agregar enteros

VB

Dim list As New List(Of Integer)
list.Add(1) ' permite agregar enteros
list.Add("Dos") ' error de sintaxis, solo se pueden agregar enteros

And about the performance the difference is in the terms boxing and unboxing

Boxing refers to the fact of wrapping a value of type Reference type (such as int, char, float, double, ... among others) inside a reference if it is to be assigned to a variable or parameter of type object. Unboxing refers to the reverse process, that is, to obtain the value of the object in a variable of type reference type .

The problem is that this boxing and unboxing process occurs in the case of ArrayListwhen a value is added or obtained.

Examples:

C#

var arrayList = new ArrayList();
arrayList.Add(1); // Boxing. Ya que el método Add espera un object
var numero = (int)arrayList[0]; // Unboxing. El indexer devuelve un object que debe ser casteado a `int`

VB

Dim arrayList As New ArrayList()
arrayList.Add(1) ' Boxing. Ya que el método Add espera un object
Dim numero As Integer = arrayList(0) ' Unboxing. El indexer devuelve un object que debe ser casteado a `int`

This process takes execution time so it is slower than the generic case.

In the generic case with List<T>this process it is non-existent since the compiler generates the code for the exact data types, (in this case int) and does not require any type of conversion.

C#

var list = new List<int>;
list.Add(1); // No hay boxing puesto que el método espera un int
var numero = list[0]; // No hay cast ni unboxing puesto que el indexer devuelve un int

VB

Dim list As New List(Of Integer)
list.Add(1) ' No hay boxing puesto que el método espera un int
Dim numero As Integer = list(0) ' No hay cast ni unboxing puesto que el indexer devuelve un int

I recommend that you always use List<T>the truth, I do not know of a case where it is convenient to use ArrayListit, since although it has not been marked as obsolete, in practice it is no longer used.

Basically, ArrayList needs to box and unbox each time an array element is saved/accessed. This is so because ArrayList stores references to objects of type object, so it needs to perform the cast task continuously.

On the other hand, List doesn't need to perform that task, since the compiler implements a concrete (specialized?) list for the type T to work with. For this reason, it is also more secure (although it does not ensure eye ordering either).

The difference could be understood like this:

• By declaring List you assign it a type (String, integer, Boolean) and as you add elements it is a process of assigning values ​​of a specific type to a memory sector.

• ArrayList instead converts the information it receives into a completely new object before allocating space in memory (boxing) and so on for each item in the arrayList. When reading it you must not only get it from RAM space, but also "create" it and all of its properties. which consumes a lot of resources

The List(Of T) class -T refers to Type (a fixed type)- is very powerful. One of its greatest qualities is to implement the IEnumerable(Of T) interface, which allows access to many methods. An object of this class can be easily transformed into an array using the ToArray method. We can read the MSDN documentation about this class at: List(Of T) on MSDN

An important advantage of List(Of T) is that objects of this class can be part of LINQ language queries, allowing us to transform lists of objects of one type into lists of objects of another type or into a statistical object as the first element of a query. list, last, max, min... LINQ