Adding values to a C# array


Probably a really simple one this - I'm starting out with C# and need to add values to an array, for example:

int[] terms;

for(int runs = 0; runs < 400; runs++)
    terms[] = runs;

For those who have used PHP, here's what I'm trying to do in C#:

$arr = array();
for ($i = 0; $i < 10; $i++) {
    $arr[] = $i;
8/24/2018 6:15:17 PM

Accepted Answer

You can do this way -

int[] terms = new int[400];
for (int runs = 0; runs < 400; runs++)
    terms[runs] = value;

Alternatively, you can use Lists - the advantage with lists being, you don't need to know the array size when instantiating the list.

List<int> termsList = new List<int>();
for (int runs = 0; runs < 400; runs++)

// You can convert it back to an array if you would like to
int[] terms = termsList.ToArray();

Edit: a) for loops on List<T> are a bit more than 2 times cheaper than foreach loops on List<T>, b) Looping on array is around 2 times cheaper than looping on List<T>, c) looping on array using for is 5 times cheaper than looping on List<T> using foreach (which most of us do).

5/23/2017 12:26:38 PM

If you're writing in C# 3, you can do it with a one-liner:

int[] terms = Enumerable.Range(0, 400).ToArray();

This code snippet assumes that you have a using directive for System.Linq at the top of your file.

On the other hand, if you're looking for something that can be dynamically resized, as it appears is the case for PHP (I've never actually learned it), then you may want to use a List instead of an int[]. Here's what that code would look like:

List<int> terms = Enumerable.Range(0, 400).ToList();

Note, however, that you cannot simply add a 401st element by setting terms[400] to a value. You'd instead need to call Add(), like this:


Using Linq's method Concat makes this simple

int[] array = new int[] { 3, 4 };

array = array.Concat(new int[] { 2 }).ToArray();

result 3,4,2


Answers on how to do it using an array are provided here.

However, C# has a very handy thing called System.Collections :)

Collections are fancy alternatives to using an array, though many of them use an array internally.

For example, C# has a collection called List that functions very similar to the PHP array.

using System.Collections.Generic;

// Create a List, and it can only contain integers.
List<int> list = new List<int>();

for (int i = 0; i < 400; i++)

Using a List as an intermediary is the easiest way, as others have described, but since your input is an array and you don't just want to keep the data in a List, I presume you might be concerned about performance.

The most efficient method is likely allocating a new array and then using Array.Copy or Array.CopyTo. This is not hard if you just want to add an item to the end of the list:

public static T[] Add<T>(this T[] target, T item)
    if (target == null)
        //TODO: Return null or throw ArgumentNullException;
    T[] result = new T[target.Length + 1];
    target.CopyTo(result, 0);
    result[target.Length] = item;
    return result;

I can also post code for an Insert extension method that takes a destination index as input, if desired. It's a little more complicated and uses the static method Array.Copy 1-2 times.


Based on the answer of Thracx (I don't have enough points to answer):

public static T[] Add<T>(this T[] target, params T[] items)
        // Validate the parameters
        if (target == null) {
            target = new T[] { };
        if (items== null) {
            items = new T[] { };

        // Join the arrays
        T[] result = new T[target.Length + items.Length];
        target.CopyTo(result, 0);
        items.CopyTo(result, target.Length);
        return result;

This allows to add more than just one item to the array, or just pass an array as a parameter to join two arrays.