Question

I’ve found some interesting behaviour in PowerShell Arrays, namely, if I declare an array as:

$array = @()

And then try to add items to it using the $array.Add("item") method, I receive the following error:

Exception calling “Add” with “1” argument(s): “Collection was of a fixed size.”

However, if I append items using $array += "item", the item is accepted without a problem and the “fixed size” restriction doesn’t seem to apply.

Why is this?

Answer 1

When using the $array.Add()-method, you’re trying to add the element into the existing array. An array is a collection of fixed size, so you will receive an error because it can’t be extended.

$array += $element creates a new array with the same elements as old one + the new item, and this new larger array replaces the old one in the $array-variable

You can use the += operator to add an element to an array. When you
use
it, Windows PowerShell actually creates a new array with the values of the
original array and the added value. For example, to add an element with a
value of 200 to the array in the $a variable, type:

    $a += 200

Source: about_Arrays

+= is an expensive operation, so when you need to add many items you should try to add them in as few operations as possible, ex:

$arr = 1..3    #Array
$arr += (4..5) #Combine with another array in a single write-operation

$arr.Count
5

If that’s not possible, consider using a more efficient collection like List or ArrayList (see the other answer).