C# - ArrayList

In C#, the ArrayList is a non-generic collection of objects whose size is increased dynamically as required. It is the same as Array except that its size is increased dynamically.

An ArrayList can be used to add unknown data where you don't know the types and the size of the data.

Create an ArrayList

The ArrayList class included in the System.Collections namespace. Create an object of the ArrayList using the new keyword.

Example: Create an ArrayList
using System.Collections;

ArrayList arlist = new ArrayList(); 
// or 
var arlist = new ArrayList(); // recommended 

Adding Elements in ArrayList

Use the Add() method or object initializer syntax to add elements in an ArrayList.

An ArrayList can contain multiple null and duplicate values.

Example: Adding Elements in ArrayList
// adding elements using ArrayList.Add() method
var arlist1 = new ArrayList();
arlist1.Add(1);
arlist1.Add("Bill");
arlist1.Add(" ");
arlist1.Add(true);
arlist1.Add(4.5);
arlist1.Add(null);

// adding elements using object initializer syntax
var arlist2 = new ArrayList()
                {
                    2, "Steve", " ", true, 4.5, null
                };

Use the AddRange(ICollection c) method to add an entire Array, HashTable, SortedList, ArrayList, BitArray, Queue, and Stack in the ArrayList.

Example: Adding Entire Array/ArrayList into ArrayList
var arlist1 = new ArrayList();

var arlist2 = new ArrayList()
                    {
                        1, "Bill", " ", true, 4.5, null
                    };

int[] arr = { 100, 200, 300, 400 };

Queue myQ = new Queue();
myQ.Enqueue("Hello");
myQ.Enqueue("World!");

arlist1.AddRange(arlist2); //adding arraylist in arraylist 
arlist1.AddRange(arr); //adding array in arraylist 
arlist1.AddRange(myQ); //adding Queue in arraylist 

Accessing an ArrayList

The ArrayList class implements the IList interface. So, elements can be accessed using indexer, in the same way as an array. Index starts from zero and increases by one for each subsequent element.

An explicit casting to the appropriate types is required, or use the var variable.

Example: Accessing Elements of ArrayList
var arlist = new ArrayList()
                {
                    1,
                    "Bill",
                    300,
                    4.5f
                };

//Access individual item using indexer
int firstElement = (int) arlist[0]; //returns 1
string secondElement = (string) arlist[1]; //returns "Bill"
//int secondElement = (int) arlist[1]; //Error: cannot cover string to int

//using var keyword without explicit casting
var firstElement = arlist[0]; //returns 1
var secondElement = arlist[1]; //returns "Bill"
//var fifthElement = arlist[5]; //Error: Index out of range

//update elements
arlist[0] = "Steve"; 
arlist[1] = 100;
//arlist[5] = 500; //Error: Index out of range

Iterate an ArrayList

The ArrayList implements the ICollection interface that supports iteration of the collection types. So, use the foreach and the for loop to iterate an ArrayList. The Count property of an ArrayList returns the total number of elements in an ArrayList.

Example: Iterate ArrayList
ArrayList arlist = new ArrayList()
                        {
                            1,
                            "Bill",
                            300,
                            4.5F
                        };

foreach (var item in arlist)
    Console.Write(item + ", "); //output: 1, Bill, 300, 4.5, 
            
for(int i = 0 ; i < arlist.Count; i++)
    Console.Write(arlist[i] + ", "); //output: 1, Bill, 300, 4.5, 

Insert Elements in ArrayList

Use the Insert() method to insert an element at the specified index into an ArrayList.

Signature: void Insert(int index, Object value)

Example: Insert Element in ArrayList
ArrayList arlist = new ArrayList()
                {
                    1,
                    "Bill",
                    300,
                    4.5f
                };

arlist.Insert(1, "Second Item");

foreach (var val in arlist)
    Console.WriteLine(val); 

Use the InsertRange() method to insert a collection in an ArrayList at the specfied index.

Signature: Void InsertRange(int index, ICollection c)

Example: Insert Collection in ArrayList
ArrayList arlist1 = new ArrayList()
                {
                    100, 200, 600
                };

ArrayList arlist2 = new ArrayList()
                {
                    300, 400, 500
                };
arlist1.InsertRange(2, arlist2);

foreach(var item in arlist1)
    Console.Write(item + ", "); //output: 100, 200, 300, 400, 500, 600, 

Remove Elements from ArrayList

Use the Remove(), RemoveAt(), or RemoveRange methods to remove elements from an ArrayList.

Example: Remove Elements from ArrayList
ArrayList arList = new ArrayList()
                {
                    1,
                    null,
                    "Bill",
                    300,
                    " ",
                    4.5f,
                    300,
                };

arList.Remove(null); //Removes first occurance of null
arList.RemoveAt(4); //Removes element at index 4
arList.RemoveRange(0, 2);//Removes two elements starting from 1st item (0 index)

Check Element in ArrayList

Use the Contains() method to determine whether the specified element exists in the ArrayList or not. It returns true if exists otherwise returns false.

Example: Check for Elements
ArrayList arList = new ArrayList()
                {
                    1,
                    "Bill",
                    300,
                    4.5f,
                    300
                };

Console.WriteLine(arList.Contains(300)); // true
Console.WriteLine(arList.Contains("Bill")); // true
Console.WriteLine(arList.Contains(10)); // false
Console.WriteLine(arList.Contains("Steve")); // false
Note:
It is not recommended to use the ArrayList class due to performance issue. Instead, use List<object> to store heterogeneous objects. To store data of same data type, use Generic List<T>.

ArrayList Class

The following diagram illustrates the ArrayList class.

C# ArrayList

ArrayList Properties

Properties Description
Capacity Gets or sets the number of elements that the ArrayList can contain.
Count Gets the number of elements actually contained in the ArrayList.
IsFixedSize Gets a value indicating whether the ArrayList has a fixed size.
IsReadOnly Gets a value indicating whether the ArrayList is read-only.
Item Gets or sets the element at the specified index.

ArrayList Methods

Methods Description
Add()/AddRange() Add() method adds single elements at the end of ArrayList.
AddRange() method adds all the elements from the specified collection into ArrayList.
Insert()/InsertRange() Insert() method insert a single elements at the specified index in ArrayList.
InsertRange() method insert all the elements of the specified collection starting from specified index in ArrayList.
Remove()/RemoveRange() Remove() method removes the specified element from the ArrayList.
RemoveRange() method removes a range of elements from the ArrayList.
RemoveAt() Removes the element at the specified index from the ArrayList.
Sort() Sorts entire elements of the ArrayList.
Reverse() Reverses the order of the elements in the entire ArrayList.
Contains Checks whether specified element exists in the ArrayList or not. Returns true if exists otherwise false.
Clear Removes all the elements in ArrayList.
CopyTo Copies all the elements or range of elements to compitible Array.
GetRange Returns specified number of elements from specified index from ArrayList.
IndexOf Search specified element and returns zero based index if found. Returns -1 if element not found.
ToArray Returns compitible array from an ArrayList.