Array.Resize

This .NET method allocates a new array. It then copies existing element values to the new array. This logic is needed when an array's size is inadequate.

Notes, resizing

We show how Resize reallocates and copies elements. The arguments to Array.Resize are easy to understand (unless you are unfamiliar with the method).

Shrink example

We use Array.Resize to replace a large array with a smaller one. This is useful if we have a large array of data, and want to only keep the first part.

using System;

// Step 1: initialize array for example.
int[] array = new int[4];
array[0] = 10;
array[1] = 20;
array[2] = 30;
array[3] = 40;

for (int i = 0; i < array.Length; i++)
{
    Console.WriteLine("BEFORE: {0}", array[i]);
}

// Step 2: resize the array from 4 to 2 elements.
Array.Resize(ref array, 2);

for (int i = 0; i < array.Length; i++)
{
    Console.WriteLine("AFTER: {0}", array[i]);
}
BEFORE: 10
BEFORE: 20
BEFORE: 30
BEFORE: 40
AFTER: 10
AFTER: 20

Grow example

Next we expand an array's size. This can be useful for certain data structures, such as those that must accommodate more data but have minimal memory footprint.

using System;

// Initialize an array with 5 elements.
char[] array = new char[5];
array[0] = 'p';
array[1] = 'y';
array[2] = 't';
array[3] = 'h';
array[4] = 'o';

// We need an array with 6 elements.
// ... Use Array.Resize to make a new array.
Array.Resize<char>(ref array, 6);

// Assign the last element.
array[5] = 'n';

// Display the array.
Console.WriteLine(new string(array));
python

Notes, internals

A call to Array.Resize runs through an algorithm that determines that the array needs to be larger or smaller. It copies the array, and then changes the reference.

Copying an array completely when resizing it is wasteful in many situations. For these cases, use List—call Add() to add to empty space on the end.

A summary

Array.Resize does not change existing arrays. It allocates and copies elements into a new array. It is useful only when arrays are required.