Error when modifying object that is not being enumerated - loops

Question

I have the following script:
$serverList = #{
"Server1Name" = #{ "WindowsService1" = "Status"; "WindowsService2" = "Status" };
"Server2Name" = #{ "WindowsService1" = "Status"; "WindowsService2" = "Status" };
"Server3Name" = #{ "WindowsService1" = "Status" };
"Server4Name" = #{ "WindowsService1" = "Status" };
"Server5Name" = #{ "WindowsService1" = "Status" };
"Server6Name" = #{ "WindowsService1" = "Status" }
}
$copy = $serverList.Clone()
foreach ($server in $copy.Keys) {
foreach ($service in $copy[$server].Keys) {
$serviceInfo = Get-Service -ComputerName $server -Name $service
$serverList[$server][$service] = $serviceInfo.Status
}
}
I made sure that I am not modifying the hashtable that is being enumerated, but yet I still get this error when I run the script:
Collection was modified; enumeration operation may not execute.At line:14 char:14
+ foreach ($service in $copy[$server].Keys) {
+ ~~~~~~~~
+ CategoryInfo : OperationStopped: (:) [], InvalidOperationException
+ FullyQualifiedErrorId : System.InvalidOperationException
I read up on this here: http://blog.matticus.net/2013/11/powershell-clearing-values-within.html. If I copy the code form there, it executes without error for me.
Could my problem have something to do with nested foreach loops? Is there a mistake in my code? Can anyone shed any light on this?

Answer 1

Powershell does not like that you are modifying the collection which you are iterating over.
In the beginning you made a clone called $copy to avoid this problem. The clone() is a "shallow copy", thus the objects being refered to for each key are the same in your copy.
On this line:
$serverList[$server][$service] = $serviceInfo.Status
You modify the inner collection - which you are currently iterating over.
In fact, the outter collection is never modified, only referred to, so the outter clone() call is unneccessary. Instead, you should clone the inner collection.
Something like this (untested):
$serverList = #{
"Server1Name" = #{ "WindowsService1" = "Status"; "WindowsService2" = "Status" };
"Server2Name" = #{ "WindowsService1" = "Status"; "WindowsService2" = "Status" };
"Server3Name" = #{ "WindowsService1" = "Status" };
"Server4Name" = #{ "WindowsService1" = "Status" };
"Server5Name" = #{ "WindowsService1" = "Status" };
"Server6Name" = #{ "WindowsService1" = "Status" }
}
foreach ($server in $serverList.Keys) {
$copy = $serverList[$server].clone();
foreach ($service in $copy.Keys) {
$serviceInfo = Get-Service -ComputerName $server -Name $service
$serverList[$server][$service] = $serviceInfo.Status
}
}

Answer 2

I was surprised that the .Clone() method just creates a new reference to the same object, it does not create a new object with the same properties. I couldn't find an easy way to actually copy an entire hashtable, rather than cloning it. So I wrote a function to do this:
Function Copy-HashTable($HashTable) {
$newHash = #{}
$HashTable.GetEnumerator() | ForEach-Object {
if ($_.Value -is "Hashtable") {
$newHash[$_.Key] = Copy-HashTable $_.Value
} else {
$newHash[$_.Key] = $_.Value
}
}
$newHash
}
Applying this to your code, you would just need to replace the line
$copy = $serverList.Clone()
with
$copy = Copy-HashTable $ServerList