ArrayList. Sometimes we do not know how many elements to place in a collection. Usually the C# List generic type is ideal for this. But older programs can use ArrayList.
ArrayList details. Like the List, the ArrayList dynamically resizes. It grows in capacity as elements are added (if space is needed).
Add method. This appends a new element object to the end. We can keep adding elements to the collection until memory runs out. The objects are stored in the managed heap.
Info When you run this program, 3 elements (all strings) are added to the ArrayList.
using System;
using System.Collections;
// Create an ArrayList and add 3 elements.
ArrayList list = new ArrayList();
list.Add("One");
list.Add("Two");
list.Add("Three");
Console.WriteLine("DONE");DONE
Arguments. It is easy to pass ArrayList as an argument. However, in the receiving function, you have to know (or find out) the type of each element.
Next In this program we pass the ArrayList as an argument to the Example() method.
Info You can also use the ArrayList as a return value. It is usually best to reuse the same Array List.
using System;
using System.Collections;
class Program
{
static void Main()
{
// Create an ArrayList and add two ints.
ArrayList list = new ArrayList();
list.Add(5);
list.Add(7);
// Use ArrayList with method.
Example(list);
}
static void Example(ArrayList list)
{
foreach (int i in list)
{
Console.WriteLine(i);
}
}
}5
7
Combine. We can use AddRange to combine two ArrayLists. Internally, AddRange uses the Array.Copy or CopyTo methods, which have better performance than some loops.
Here The first ArrayList has two elements added to it. Next, the second ArrayList has two elements added.
Then The second ArrayList is appended to the first using the AddRange method. The example finally shows the output.
using System;
using System.Collections;
// Create an ArrayList with two values.
ArrayList list = new ArrayList();
list.Add(5);
list.Add(7);
// Second ArrayList.
ArrayList list2 = new ArrayList();
list2.Add(10);
list2.Add(13);
// Add second ArrayList to first.
list.AddRange(list2);
// Display the values.
foreach (int i in list)
{
Console.WriteLine(i);
}5
7
10
13
Count. This is a virtual property. When you use Count, no counting is done—instead a cached field value is returned. This means that Count is fairly fast.
Here The example shows the Count property. It also shows the Clear method, and how this affects the count.
Note The Count property returns an int. This will always be a positive value. No calculation takes place in the property itself.
using System;
using System.Collections;
// Create an ArrayList with two values.
ArrayList list = new ArrayList();
list.Add(9);
list.Add(10);
// Show number of elements in ArrayList.
Console.WriteLine(list.Count);
// Clear the ArrayList.
list.Clear();
// Show count again.
Console.WriteLine(list.Count);2
0
Sort, reverse. Many dynamic arrays (such as ArrayList) must be frequently sorted. We call the instance Sort method and then Reverse. These methods work in-place.
Tip You can sort subsets (ranges) of elements in your ArrayList using the third overload. This is useful in rare situations.
Also You can Reverse only a range of your ArrayList. This is useful even less often.
Note The Sort method in the base class library is an instance method (with no parameters) on ArrayList.
Detail The ArrayList Sort method works on different element types. The example here shows strings.
using System;
using System.Collections;
// Create an ArrayList with four strings.
ArrayList list = new ArrayList();
list.Add("Cat");
list.Add("Zebra");
list.Add("Dog");
list.Add("Cow");
// Sort the ArrayList.
list.Sort();
// Display the ArrayList elements.
foreach (string value in list)
{
Console.WriteLine(value);
}
// Reverse the ArrayList.
list.Reverse();
// Display the ArrayList elements again.
foreach (string value in list)
{
Console.WriteLine(value);
}Cat
Cow
Dog
Zebra
Zebra
Dog
Cow
Cat
Insert, Remove. Here we insert and remove elements in an ArrayList. We see the RemoveAt method for erasing a single element, and then Insert and RemoveRange.
Tip The first argument to Insert is the position: this is equivalent to the index of the element.
using System;
using System.Collections;
// Create an ArrayList with three strings.
ArrayList list = new ArrayList();
list.Add("Dot");
list.Add("Net");
list.Add("Perls");
// Remove middle element in ArrayList.
list.RemoveAt(1); // It becomes [Dot, Perls]// Insert word at the beginning of ArrayList.
list.Insert(0, "Carrot"); // It becomes [Carrot, Dot, Perls]// Remove first two words from ArrayList.
list.RemoveRange(0, 2);
// Display the result ArrayList.
foreach (string value in list)
{
Console.WriteLine(value); // <-- "Perls"
}Perls
Loop. The for-loop is popular and useful. We need to cast elements after accessing them. The "i" part in the example shows how to use the indexer on the ArrayList.
Tip After you cast, you can check the result for null before using the variable, to see if the cast succeeded.
using System;
using System.Collections;
// Create an ArrayList with three strings.
ArrayList list = new ArrayList();
list.Add("man");
list.Add("woman");
list.Add("plant");
// Loop over ArrayList.
for (int i = 0; i < list.Count; i++)
{
string value = list[i] as string;
Console.WriteLine(value);
}man
woman
plant
GetRange. This will return a subset of the original ArrayList in a new ArrayList. This is ideal when you know a certain part of your ArrayList has a different purpose or behavior.
Tip The SetRange method on ArrayList is also useful when you need to replace a range.
However I have not found SetRange to be useful, as often you will just want to replace elements in a for-loop.
using System;
using System.Collections;
// Create an ArrayList with 4 strings.
ArrayList list = new ArrayList();
list.Add("fish");
list.Add("amphibian");
list.Add("bird");
list.Add("plant");
//Get last two elements in ArrayList.
ArrayList range = list.GetRange(2, 2);
// Display the elements.
foreach (string value in range)
{
Console.WriteLine(value); // bird, plant
}bird
plant
IndexOf. The IndexOf and LastIndexOf methods on ArrayList are similar to those on strings. You pass in the value you are looking for, the start index, the number of elements to search.
Info IndexOf will return -1 if the element could not be located. This value must be specially tested.
using System;
using System.Collections;
ArrayList list = new ArrayList();
list.Add(5);
list.Add(7);
// Call IndexOf.
int result = list.IndexOf(7);
Console.WriteLine("RESULT: {0}", result);
Console.WriteLine("NOT FOUND: {0}", list.IndexOf(900));RESULT: 1
NOT FOUND: -1
Benchmark, List, ArrayList. Here we compare the performance of List (a generic collection) to ArrayList. We add, remove, test and clear each collection.
Version 1 This version of the code uses the List from System.Collections.Generic. Many iterations are run.
Version 2 Here we use the ArrayList from System.Collections. We perform the same actions as in version 1.
Result List performs with more than twice the speed of ArrayList. It performs this simple benchmark faster.
using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
List<int> list = new List<int>();
ArrayList array = new ArrayList();
const int max = 1000000;
var s1 = Stopwatch.StartNew();
// Version 1: use List.
for (int i = 0; i < max; i++)
{
list.Add(10);
list.Add(20);
list.Add(30);
list.Remove(30);
if (list[list.Count - 1] != 20)
{
return;
}
list.Clear();
}
s1.Stop();
var s2 = Stopwatch.StartNew();
// Version 2: use ArrayList.
for (int i = 0; i < max; i++)
{
array.Add(10);
array.Add(20);
array.Add(30);
array.Remove(30);
if ((int)array[array.Count - 1] != 20)
{
return;
}
array.Clear();
}
s2.Stop();
Console.WriteLine(s1.Elapsed.TotalMilliseconds);
Console.WriteLine(s2.Elapsed.TotalMilliseconds); 67.4821 ms, List add, remove, test, clear
151.8429 ms, ArrayList add, remove, test, clear
A review. ArrayList holds objects. It stores elements of type object—casting is needed. We use as-casts, numeric casts to test elements. ArrayList is a collection that is best avoided.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Nov 10, 2023 (simplify).
