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C# ArrayList ExamplesUse the ArrayList type from System.Collections. ArrayList is a resizable array.
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.
List
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.Collections; class Program { static void Main() { // 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.
Array.Copy
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; class Program { static void Main() { // // 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.
Int The Count property returns an int. This will always be a positive value. No calculation takes place in the property itself.
Int, uint
Clear You can call the instance method Clear on your ArrayList. Internally, this calls the Array.Clear method.
Array.Clear
using System; using System.Collections; class Program { static void Main() { // // 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.
Elements The ArrayList Sort method works on different element types. The example here shows strings.
using System; using System.Collections; class Program { static void Main() { // // 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; class Program { static void Main() { // // 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.
Indexer
Cast The "as" cast in C# is probably the best way to cast reference types such as string.
As
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; class Program { static void Main() { // // 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.
SetRange 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; class Program { static void Main() { // // 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.
IndexOf
using System; using System.Collections; class Program { static void Main() { 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; class Program { static void Main() { 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
Performance, sort. The ArrayList Sort method ends up in an internal TrySZSort or QuickSort method when it doesn't throw an exception.
Note TrySZSort is optimized for one-dimensional arrays, also known as "Zero" arrays or vectors.
Info The TrySZSort method used in the base class libraries is implemented in native code. It has been heavily optimized.
Thus Using Sort on ArrayList or on Array is faster than most custom implementations.
Convert. Arrays offer more performance and compatibility, so you will want to convert ArrayLists to arrays. We use casting to convert an ArrayList to an array.
Convert ArrayList, Array
List versus ArrayList. It is usually better to use List. But older applications may be using ArrayList. And it is often best not to have to rewrite them.
Note Lists not only avoid boxing or unboxing, but they also lead to clearer and less bug-prone code.
And With List, the compiler can check your code for type integrity before runtime. This improves reliability.
BinarySearch. This method implements the binary searching algorithm. This uses a "divide and conquer" approach to finding the correct element. It only works on presorted data.
Thus Never use BinarySearch if your ArrayList might not be already sorted. The results could be invalid.
BinarySearch List
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.
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