PowerShell compare CSV to AD Subnets and update / add if required - powershell

Question

I've got a CSV file with headers subnet, site, description. I want to import this into PoSH and then compare what is in AD Sites and Services. Now, this might be "null" or it might already contain subnet values. In either case, I need the subnets in AD compared to the CSV and updated/added as required.
I thought about If/Else, but that doesn't seem to handle the scenario where no subnets exist.
So I moved to try/catch and also Compare-Object.
Anyway, an example of what I've tried:
$csvSubnets = Import-Csv -Path C:\subnets.csv
$adSubnets = Get-ADReplicationSubnet -Filter *
Compare-Object -ReferenceObject #($csvSubnets | Select-Object) -DifferenceObject #($adSubnets | Select-Object) -Property Name | Where-Object {
$_.SideIndicator -eq '<='
} | ForEach-Object {
$csvSubnetName = $_.subnet
$csvSiteName = $_.site
$csvDescription = $_.description
$adSubnetName = $adSubnets.Where({$_.Name -eq $csvSubnetName})
New-ADReplicationSubnet -Site $_.site -Name $_.subnet -Description $_.description
}
This returns the following error:
New-ADReplicationSubnet : Cannot validate argument on parameter 'Site'. The argument is null
Would anyone have a suggestion as to how to fix this, or perhaps a more efficient way of achieving what I need? Doesn't need to be computationally efficient as there's probably never more than 10 sites (subnets)

Answer 1

The main thing I'd point out is you aren't passing the original objects coming out of the comparison to the ForEach-Object loop. Either reference the property $_.InputObject or preferably use the -PassThru parameter on the Compare-Object command:
Compare-Object -ReferenceObject #($csvSubnets | Select-Object) -DifferenceObject #($adSubnets | Select-Object) -Property Name -PassThru |
Where-Object { $_.SideIndicator -eq '<='} |
ForEach-Object {
$csvSubnetName = $_.subnet
$csvSiteName = $_.site
$csvDescription = $_.description
$adSubnetName = $adSubnets.Where({$_.Name -eq $csvSubnetName})
New-ADReplicationSubnet -Site $_.site -Name $_.subnet -Description $_.description
}
An aside:
The Compare-Object command is rather hard to read, given intermingled pipelines. It may be better to establish those as variables separately. However, you aren't specifying any properties in those Select-Object commands. I presume you are trying to level-set the typing to [PSCustomObject]'s in which case you should really only need to do that with the rich objects returned from Get-ADReplicationSubnet Import-Csv will always return [PSCustomObject]. So you could revise a little like:
$csvSubnets = Import-Csv -Path 'C:\subnets.csv'
$adSubnets = Get-ADReplicationSubnet -Filter * | Select-Object *
Compare-Object -ReferenceObject $csvSubnets -DifferenceObject $adSubnets -Property Name -PassThru |
Where-Object { $_.SideIndicator -eq '<='} |
ForEach-Object {
$csvSubnetName = $_.subnet
$csvSiteName = $_.site
$csvDescription = $_.description
$adSubnetName = $adSubnets.Where({$_.Name -eq $csvSubnetName})
New-ADReplicationSubnet -Site $_.site -Name $_.subnet -Description $_.description
}
I can't test your code but after solving the first problem you may have another with this line:
$adSubnetName = $adSubnets.Where({$_.Name -eq $csvSubnetName})
You aren't referencing the .Name property after you've isolated the AD Subnet you're interested in. I might be missing something, but if you do have that problem a quick fix might be something like:
$adSubnetName = ($adSubnets.Where({$_.Name -eq $csvSubnetName})).Name
Also, I like the .Where() method, but I'm a little iffy on it's return types, and on the readability of unnecessarily mixing and matching. You could replace with a regular Where{} clause:
$adSubnetName = ( $adSubnets | Where-Object{ $_.Name -eq $csvSubnetName } ).Name
Update:
It is true that if the difference object is null the Compare-Object command will fail. However, that approach stemmed from your original code. There are actually many ways to extract the difference between 2 lists. Here's a more concise example:
$csvSubnets = Import-Csv -Path 'C:\subnets.csv'
$adSubnets = (Get-ADReplicationSubnet -Filter *).Name
$csvSubnets |
Where-Object{ $_.Name -notin $adSubnets } |
ForEach-Object{
$SubnetParams = #{
Name = $_.subnet
Site = $_.site
Description = $_.description
}
New-ADReplicationSubnet #SubnetParams
}
In this case we use the -notin operator against a list of subnet names. $adSubnets isn't being used for anything else. We don't need it to be objects custom or otherwise. Now for each record in the CSV file just see if the name isn't on the list, and if not proceed to create the subnet.
Speed is not a likely concern in this case however you can actually make the above example a little more efficient using If logic in the loop and eliminating the Where{}.
$csvSubnets = Import-Csv -Path 'C:\subnets.csv'
$adSubnets = (Get-ADReplicationSubnet -Filter *).Name
$csvSubnets |
ForEach-Object{
If($_.Name -notin $adSubnets ) {
$SubnetParams = #{
Name = $_.subnet
Site = $_.site
Description = $_.description
}
New-ADReplicationSubnet #SubnetParams
}
}
In either example the key point is that -notin will work fine even if the right hand side of the comparison is null. It's simply return false and therefore the subnet will get created.

Answer 2

Firstly, thank you to the other responses here which lead me down a slightly different path to a solution.
$VerbosePreference = 'Continue'
$csvSubnets = Import-Csv -Delimiter "," -Path C:\subnets.csv
foreach ($subnet in $csvSubnets) {
$ADSubnetName = $subnet.subnet
if ($S = Get-ADReplicationSubnet -Filter {Name -eq $ADSubnetName}) {
Write-Verbose "Subnet $($ADSubnetName) already present"
Continue # to the next subnet
}
$SubnetParams = #{
Name = $subnet.subnet
Site = $subnet.site
Description = $subnet.description
}
Write-Verbose "Created subnet $($ADSubnetName)"
New-ADReplicationSubnet #SubnetParams
}