How can I force Powershell to return an array when a call only returns one object?

Define the variable as an array in one of two ways...

Wrap your piped commands in parentheses with an @ at the beginning:

$serverIps = @(gwmi Win32_NetworkAdapterConfiguration 
    | Where { $_.IPAddress } 
    | Select -Expand IPAddress 
    | Where { $_ -like '*.*.*.*' } 
    | Sort)

Specify the data type of the variable as an array:

[array]$serverIps = gwmi Win32_NetworkAdapterConfiguration 
    | Where { $_.IPAddress } 
    | Select -Expand IPAddress 
    | Where { $_ -like '*.*.*.*' } 
    | Sort

Or, check the data type of the variable...

IF ($ServerIps -isnot [array])
{ <error message> }
ELSE
{ <proceed> }

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Force the result to an Array so you could have a Count property. Single objects (scalar) do not have a Count property. Strings have a length property so you might get false results, use the Count property:

if (@($serverIps).Count -le 1)...

By the way, instead of using a wildcard that can also match strings, use the -as operator:

[array]$serverIps = gwmi Win32_NetworkAdapterConfiguration -filter "IPEnabled=TRUE" | Select-Object -ExpandProperty IPAddress | Where-Object {($_ -as [ipaddress]).AddressFamily -eq 'InterNetwork'}

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You can either add a comma(,) before return list like return ,$list or cast it [Array] or [YourType[]] at where you tend to use the list.

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If you declare the variable as an array ahead of time, you can add elements to it - even if it is just one...

This should work...

$serverIps = @()

gwmi Win32_NetworkAdapterConfiguration 
    | Where { $_.IPAddress } 
    | Select -Expand IPAddress 
    | Where { $_ -like '*.*.*.*' } 
    | Sort | ForEach-Object{$serverIps += $_}

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You can use Measure-Object to get the actual object count, without resorting to an object's Count property.

$serverIps = gwmi Win32_NetworkAdapterConfiguration 
    | Where { $_.IPAddress } 
    | Select -Expand IPAddress 
    | Where { $_ -like '*.*.*.*' } 
    | Sort

if (($serverIps | Measure).Count -le 1) {
    Write-Host "You need at least 2 IP addresses for this to work!"
    exit
}

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Return as a referenced object, so it never converted while passing.

return @{ Value = @("single data") }

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I had this problem passing an array to an Azure deployment template. If there was one object, PowerShell "converted" it to a string. In the example below, $a is returned from a function that gets VM objected according to the value of a tag. I pass the $a to the New-AzureRmResourceGroupDeployment cmdlet by wrapping it in @(). Like so:

$TemplateParameterObject=@{
     VMObject=@($a)
}

New-AzureRmResourceGroupDeployment -ResourceGroupName $RG -Name "TestVmByRole" -Mode Incremental -DeploymentDebugLogLevel All -TemplateFile $templatePath -TemplateParameterObject $TemplateParameterObject -verbose

VMObject is one of the template's parameters.

Might not be the most technical / robust way to do it, but it's enough for Azure.

Update

Well the above did work. I've tried all the above and some, but the only way I have managed to pass $vmObject as an array, compatible with the deployment template, with one element is as follows (I expect MS have been playing again (this was a report and fixed bug in 2015)):

[void][System.Reflection.Assembly]::LoadWithPartialName("System.Web.Extensions")
    
    foreach($vmObject in $vmObjects)
    {
        #$vmTemplateObject = $vmObject 
        $asJson = (ConvertTo-Json -InputObject $vmObject -Depth 10 -Verbose) #-replace '\s',''
        $DeserializedJson = (New-Object -TypeName System.Web.Script.Serialization.JavaScriptSerializer -Property @{MaxJsonLength=67108864}).DeserializeObject($asJson)
    }

$vmObjects is the output of Get-AzureRmVM.

I pass $DeserializedJson to the deployment template' parameter (of type array).

For reference, the lovely error New-AzureRmResourceGroupDeployment throws is

"The template output '{output_name}' is not valid: The language expression property 'Microsoft.WindowsAzure.ResourceStack.Frontdoor.Expression.Expressions.JTokenExpression' 
can't be evaluated.."

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There is a way to deal with your situation. Leave most of you code as-is, just change the way to deal with the $serverIps object. This code can deal with $null, only one item, and many items.

$serverIps = gwmi Win32_NetworkAdapterConfiguration 
    | Where { $_.IPAddress } 
    | Select -Expand IPAddress 
    | Where { $_ -like '*.*.*.*' } 
    | Sort

# Always use ".Count" instead of ".Length".
# This works on $null, only one item, or many items.
if ($serverIps.Count -le 1) {
    Write-Host "You need at least 2 IP addresses for this to work!"
    exit
}

# Always use foreach on a array-possible object, so that
# you don't have deal with this issue anymore.
$serverIps | foreach {
    # The $serverIps could be $null. Even $null can loop once.
    # So we need to skip the $null condition.
    if ($_ -ne $null) {
        # Get the index of the array.
        # The @($serverIps) make sure it must be an array.
        $idx = @($serverIps).IndexOf($item)

        if ($idx -eq 0) { $primaryIp = $_ }
        if ($idx -eq 1) { $secondaryIp = $_ }
    }
}

In PowerShell Core, there is a .Count property exists on every objects. In Windows PowerShell, there are "almost" every object has an .Count property.

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