Array, initialize

In .NET, an array can be initialized in several ways. With array initializer syntax, we can specify individual elements directly.

Initialize methods

We can assign each element in an array to one value. This is often needed in programs, and the syntax is important to review.

Array initializers

An array initializer uses curly brackets with elements in comma-separated lists. The length (and number of dimensions) is inferred from the expression.

using System;

// Part 1: declare arrays with curly brackets.
// ... It is allowed to omit the type.
int[] array1 = { 1, 2, 3 };
int[] array2 = new int[] { 1, 2, 3 };
int[] array3 = [1, 2, 3];

// Part 2: use array initializations with strings.
// ... We can specify two-dimensional arrays.
// ... We can use empty arrays.
string[] array4 = { "dot", "net", "perls" };
string[] array5 = new string[] { "DOT", "NET", "PERLS", null };
string[] array6 = { };
string[,] array7 = { { "dot", "perls" }, { "framework", "4.0" } };


// Part 3: print the length and ranks.
Console.WriteLine($"int {array1.Length} {array2.Length} {array3.Length}");
Console.WriteLine($"string {array4.Length} {array5.Length} {array6.Length}");
Console.WriteLine($"       {array7.Length} / {array7.Rank}");
int 3 3 3
string 3 4 0
       4 / 2

Example, for-loop

We can initialize arrays with for-loops, which (overall) may be best for a team—it uses the more standard style. We create a helper method for this purpose.

using System;

class Program
{
    static void Main()
    {
        // Initialize an array of -1 integers.
        int[] arr1 = new int[10];
        InitIntArray(arr1);
        foreach (int i in arr1)
        {
            Console.Write(i);
        }
        Console.WriteLine();

        // Initialize an array of space chars.
        char[] arr2 = new char[5];
        InitCharArray(arr2);
        foreach (char c in arr2)
        {
            Console.Write(c);
        }
        Console.WriteLine();
    }

    /// <summary>
    /// Initialize array to -1
    /// </summary>
    static void InitIntArray(int[] arr)
    {
        for (int i = 0; i < arr.Length; i++)
        {
            arr[i] = -1;
        }
    }

    /// <summary>
    /// Initialize array to ' '
    /// </summary>
    static void InitCharArray(char[] arr)
    {
        for (int i = 0; i < arr.Length; i++)
        {
            arr[i] = ' ';
        }
    }
}
-1-1-1-1-1-1-1-1-1-1

Enumerable.Repeat

Here we use Enumerable.Repeat to assign a new array to a single value series. We ensure the System.Linq namespace is included.

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        // Initialize an array of -1 integers.
        int[] arr1 = Enumerable.Repeat(-1, 10).ToArray();
        foreach (int i in arr1)
        {
            Console.Write(i);
        }
        Console.WriteLine();

        // Initialize an array of space chars.
        char[] arr2 = Enumerable.Repeat(' ', 5).ToArray();
        foreach (char c in arr2)
        {
            Console.Write(c);
        }
        Console.WriteLine();
    }
}
-1-1-1-1-1-1-1-1-1-1

Enumerable.Range

Here we use Enumerable.Range to initialize an array to an entire range of numbers or other values. This can be replaced with a loop. We include the System.Linq namespace.

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        // Initialize array of 5 sequential integers.
        int[] arr1 = Enumerable.Range(5, 5).ToArray();
        foreach (int i in arr1)
        {
            Console.WriteLine(i);
        }
    }
}
5
6
7
8
9

Initialize, benchmark

Here we benchmark Enumerable.Repeat. This method is elegant, but if it causes a performance loss, it may be necessary to avoid it in some programs.

using System;
using System.Diagnostics;
using System.Linq;

const int _max = 1000000;
// Version 1: initialize with for-loop.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    int[] array = new int[100];
    for (int z = 0; z < array.Length; z++)
    {
        array[z] = -1;
    }
}
s1.Stop();
// Version 2: use Enumerable.Repeat.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    int[] array = Enumerable.Repeat(-1, 100).ToArray();
}
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"));
}
52.89 ns    for-loop
36.40 ns    Enumerable.Repeat, ToArray

Discussion

The C# language specification describes array initializers. We see that an array initializer is converted to a sequence of assignments into the newly-allocated arrays.

Array.CreateInstance

With this method, we can create an array based on runtime parameters. So a method can create a string or int array (for example) based on its arguments.

Summary

The C# compiler can understand and infer array creations. The dimensions and the length of the arrays is inferred from the initializers.