Array

An array in C# is a region of memory that stores a certain number of elements—each element has the same type. Arrays are harder to use than Lists, but can be more efficient.

Arrays are inside many things—in this sense, they are a foundation that we use to build more complex types. Sometimes they are required as arguments, or returned from methods.

String arrays

We begin with string arrays. We use square brackets to set array sizes and access elements, and the Length property returns the element count.

using System;

// Version 1: create empty string array.
// ... Assign into the array.
string[] animals = new string[3];
animals[0] = "deer";
animals[1] = "moose";
animals[2] = "boars";
Console.WriteLine("ARRAY 1: " + animals.Length);

// Version 2: use array initializer.
string[] animals2 = new string[] { "deer", "moose", "boars" };
Console.WriteLine("ARRAY 2: " + animals2.Length);

// Version 3: minimal array initializers.
string[] animals3 = { "deer", "moose", "boars" };
string[] animals4 = ["deer", "moose", "boars"];
string[] animals5 = new[] { "deer", "moose", "boars" };

// Test if lengths are equal.
if (animals3.Length == animals4.Length && animals4.Length == animals5.Length)
{
    Console.WriteLine("All lengths are equal");
}
ARRAY 1: 3
ARRAY 2: 3
All lengths are equal

First and last

The first element is at index 0. And the last element is always at the array's length minus one. These elements can be accessed by using the indexer syntax.

using System;

// Create an array.
int[] array = new int[4];
// Populate the array.
int value = 10;
for (int i = 0; i < array.Length; i++)
{
    array[i] = value++ * 10;
}
// Access first and last elements.
int first = array[0];
int last = array[array.Length - 1];
Console.WriteLine($"FIRST: {first}");
Console.WriteLine($"LAST: {last}");
FIRST: 100
LAST: 130

Foreach-loop

With this loop, no indexes are needed—the loop itself handles the indexes. For many programs, a foreach-loop is the clearest loop. We use string interpolation to display the colors.

string[] array = { "red", "blue", "green" };
// Loop with foreach and write colors with string interpolation.
foreach (string color in array)
{
    System.Console.WriteLine($"Color = {color}");
}
Color = red
Color = blue
Color = green

For-loop

With this loop we use indexes. We have a start, and end, and increment expression. Here we use a for-loop to access and print each element in a string array.

using System;

string[] array = ["Socrates", "Plato"];

// Use for-loop on array.
for (int i = 0; i < array.Length; i++)
{
    // Get element, and print index and element value.
    string element = array[i];
    Console.WriteLine("INDEX: {0}, VALUE: {1}", i, element);
}
INDEX: 0, VALUE: Socrates
INDEX: 1, VALUE: Plato

Index from end

Suppose we want to access the last element in an array. The simplest syntax to do this is to use the "^1" notation—this means "last."

using System;

int[] values = {10, 20, 30};
// Use indexes from the end with caret.
int last = values[^1];
int middle = values[^2];
int first = values[^3];
Console.WriteLine($"{first} {middle} {last}");
10 20 30

Int array, parameter

In structural programming we pass arrays as arguments to methods. The entire contents of the array are not copied—just the small reference.

using System;

class Program
{
    static void Main()
    {
        // Step 1: create 3-element array.
        int[] array = { -5, -6, -7 };

        // Step 2: pass array reference to method.
        int result = MultiplyFirstElement(array);
        Console.WriteLine("FIRST ELEMENT MULTIPLIED: {0}", result);
    }

    static int MultiplyFirstElement(int[] array)
    {
        // Step 3: multiply the first element by 2 and return it.
        return array[0] * 2;
    }
}
-10

Return

We can return arrays from methods—a method might need to generate data for the array beforehand. In this program, we create an array of 3 strings and return it in GetTreeNameArray.

using System;

class Program
{
    static void Main()
    {
        // Get tree array, and merge its results into a single string.
        string result = string.Join("; ", GetTreeNameArray());

        // Write the tree names.
        Console.WriteLine("TREES: {0}", result);
    }

    static string[] GetTreeNameArray()
    {
        // Put 3 tree names in the array and return it.
        string[] array = { "Elm", "Oak", "Palm" };
        return array;
    }
}
TREES: Elm; Oak; Palm

Empty

Here is another syntax example. To create an empty string array, we can use an empty initializer expression. Or we can specify a 0 length.

using System;

// Create string array with no elements.
var empty1 = new string[] { };
Console.WriteLine(empty1.Length == 0);

// This syntax has the same result.
var empty2 = new string[0];
Console.WriteLine(empty2.Length == 0);
True
True

IndexOf

Sometimes we want to search for a value in an array without using a for-loop. Consider the Array.IndexOf method. We can use this to search an array by value.

using System;

string[] array = { "cat", "dog", "bird", "fish" };

// The dog string is at index 1.
int dogIndex = Array.IndexOf(array, "dog");
Console.WriteLine(dogIndex);

// There is no monkey string in the array.
// ... So IndexOf returns -1.
int monkeyIndex = Array.IndexOf(array, "monkey");
Console.WriteLine(monkeyIndex);
1
-1

Class, indexer

We use arrays as fields (or properties) in classes. This is useful for storing values. In the Test class here, we have a string array field.

class Program
{
    static void Main()
    {
        // Create new instance with string array.
        Test test = new Test();

        // Loop over elements with property.
        foreach (string element in test.Elements)
        {
            System.Console.WriteLine(element);
        }
        // Get first string element.
        System.Console.WriteLine(test[0]);
    }
}

public class Test
{
    string[] _elements = { "one", "two", "three" };

    public string[] Elements
    {
        get { return _elements; }
    }

    public string this[int index]
    {
        get { return _elements[index]; }
    }
}
one
two
three
one

Convert string arrays

It is possible to use built-in methods like Join and Split to convert a string array into a string, and back again. We can also use loops and StringBuilder.

using System;

string[] elements = ["cat", "dog", "fish"];
Console.WriteLine(elements[0]);

// Part 1: join strings into a single string.
string joined = string.Join("|", elements);
Console.WriteLine(joined);

// Part 2: separate joined strings with Split.
string[] separated = joined.Split('|');
Console.WriteLine(separated[0]);
cat
cat|dog|fish
cat

String args

When a C# program is started, an optional string array is received from the operating system. This array, args, contains string arguments.

using System;

class Program
{
    static void Main(string[] args)
    {
        // ... Loop over arguments passed to this program.
        foreach (string value in args)
        {
            Console.WriteLine("Argument: {0}", value);
        }
    }
}
Argument: hello
Argument: world

Return ref, array element

With the ref keyword, we can return a reference to an array element. Here we have an int array. FirstElement returns a ref to the element at index 0.

class Program
{
    static ref int FirstElement(int[] array)
    {
        // Return ref to first element in array.
        return ref array[0];
    }

    static void Main()
    {
        int[] codes = { 10, 20, 30 };

        // Change first element to a new value.
        FirstElement(codes) = 60;

        // Display modified array.
        for (int i = 0; i < codes.Length; i++)
        {
            System.Console.WriteLine(codes[i]);
        }
    }
}
60
20
30

Random array

We can use the Random NextBytes method to fill a byte array with random values. This does not require much custom code.

using System;

// Part 1: fill up a random byte array.
byte[] array = new byte[4];
Random random = new Random();
random.NextBytes(array);

// Part 2: display random array elements.
foreach (byte value in array)
{
    Console.WriteLine("RANDOM ELEMENT: {0}", value);
}
RANDOM ELEMENT: 240
RANDOM ELEMENT: 197
RANDOM ELEMENT: 121
RANDOM ELEMENT: 218

IEnumerable

Arrays implement the IEnumerable interface for us automatically. This allows us to use arrays with LINQ extension methods, or pass arrays to methods that receive IEnumerable.

using System;
using System.Collections.Generic;

class Program
{
    static void Display(IEnumerable<int> values)
    {
        // This method can be used with arrays or Lists.
        foreach (int value in values)
        {
            Console.WriteLine("IENUMERABLE: " + value);
        }
    }

    static void Main()
    {
        int[] ids = { 10400, 20800, 40100 };
        // Pass the int array to the Display method, which accepts it as an IEnumerable.
        Display(ids);
    }
}
IENUMERABLE: 10400
IENUMERABLE: 20800
IENUMERABLE: 40100

Count extension

It is usually best to use the Length property for an element count. But the Count() extension method can be also used—here we use a lambda to only count certain elements.

using System;
using System.Linq;

var array = new int[] { 10, 20, 30 };
// Count elements greater than or equal to 20.
int c = array.Count(x => x >= 20);
Console.WriteLine(c);
2

Combine

Suppose two arrays exist—we can combine them. The LINQ namespace provides us the Concat extension method, which we can use with 2 arrays.

using System;
using System.Linq;

var values1 = new int[] { 1, 2 };
var values2 = new int[] { 3, 4 };
// Use Concat and ToArray to merge the values into a single array.
var result = values1.Concat(values2).ToArray();
foreach (var value in result)
{
    Console.WriteLine(value);
}
1
2
3
4

Jagged, 2D arrays

We can place an array inside another array, giving us a jagged array. Two-dimensional arrays can also be used (they use a comma in the index syntax).

using System;

// Use jagged array.
var values = new int[][] { new int[] { 1, 2 }, new int[] { 3, 4 }};
// The top left item.
Console.WriteLine(values[0][0]);
// The bottom right item.
Console.WriteLine(values[1][1]);
1
4

Is operator

Suppose we want to test an array for a known list of constant elements. We can do this with the is-operator in modern versions of the C# language.

using System;

int[] values = [100, 400, 600];

// Test array with pattern matching syntax.
if (values is [100, 200])
{
    Console.WriteLine("???");
}
else if (values is [100, 400, 600])
{
    Console.WriteLine("Matched 100, 400, 600");
}
Matched 100, 400, 600

Exceptions

We must only access elements in an array that are present in the array. The values can be anything, but the accesses must occur within the length of the array.

using System;

class Program
{
    static void Main()
    {
        int[] test = { 10, 20, 30 };
        Console.WriteLine(test[4]);
    }
}
Unhandled Exception: System.IndexOutOfRangeException: Index was outside the bounds of the array.
at Program.Main() in ...Program.cs:line 7

Benchmark, array caching

Creating an array has some cost—memory needs to be allocated, and it will need to be garbage-collected. We can optimize by caching arrays of the required sizes.

using System;
using System.Diagnostics;

const int _max = 1000000;
int[] emptyArrayCache = new int[0];
// Version 1: use an empty array cache to avoid creating more than 1 array.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    var temp = emptyArrayCache;
    if (temp.Length != 0)
    {
        return;
    }
}
s1.Stop();
// Version 2: use a new array on each method call.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    var temp = new int[0];
    if (temp.Length != 0)
    {
        return;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
0.81 ns    Cached array
6.36 ns    New array

Arrays in C# are memory regions that contain elements. They store string references, ints, bytes (or any type). Even if not used directly, they are a core part of all programs.