Strange behaviour with strings and arrays

Question

Here I assign the value of an array to a variable, I then alter the variable, the array changes also.

$TestArray = @{ "ValueA" = "A" ; "ValueB" = "B" ; "Number" = "" }
$TestNumbers = 1..10

foreach ($number in $testnumbers) {

    $results = $TestArray
    $results.Number = $number
    Write-Host $TestArray.Number

}

I thought that $results = $TestArray would take a copy of $TestArray but this test shows that modifying $results also changes the corresponding value in $TestArray

Can anyone help me understand this behavior?

score 4 · Accepted Answer · 2015-05-27 15:08:28Z

4

Doing:

$results = $TestArray

will make $results a reference to the same object referred to by TestArray. So, if you change one, the other will also be affected because they are the same object.

To instead make $results a copy of $TestArray, you can use its Clone method:

$results = $TestArray.Clone()

Also, just for the record, $TestArray is not actually an array. It is a hashtable (also called a hashmap) where keys are paired with values. An array would be something like:

$TestArray = (1, 2, 3)

edited May 27, 2015 at 15:08

answered May 27, 2015 at 15:02

user2555451

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2 Comments

beehaus Over a year ago

Thanks, I am really surprised that I have only just noticed this behaviour. It makes sense. I am wondering is the reference behaviour applicable to all strings arrays hashtables and objects?

user2555451 Over a year ago

It depends on if the object in question can be mutated or not. If it is something like a hashtable, then yes, because you can add/remove items from it. If it is an immutable object such as a string, then no because strings cannot be changed (all operations on them produce new objects instead of modifying the old ones). Here is an explanation on the difference between mutable objects and immutable ones: en.wikipedia.org/wiki/Immutable_object.

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