Why is powershell so slow to validate huge byte[] arrays? - performance

Question

I've spent some time trying to find a bottleneck in a powershell application, without ever suspecting it was just a slow parameter validation. The sample code illustrates the problem:
function Test-ValidatePerformance
{
param(
[ValidateNotNullOrEmpty()]
[Byte[]]
$Data
)
$sw.Stop()
Write-Host "Executing after $([Math]::Round($sw.Elapsed.TotalMilliSeconds))ms"
}
function Test-NoValidatePerformance
{
param(
[Byte[]]
$Data
)
$sw.Stop()
Write-Host "Executing after $([Math]::Round($sw.Elapsed.TotalMilliSeconds))ms"
}
$buf = [IO.File]::ReadAllBytes('C:\17MB_FILE.bin')
Write-Host "Calling with validation..."
$sw = [Diagnostics.Stopwatch]::StartNew()
Test-ValidatePerformance $buf
Write-Host "`nCalling without validation..."
$sw = [Diagnostics.Stopwatch]::StartNew()
Test-NoValidatePerformance $buf
Output:
Calling with validation...
Executing after 1981ms
Calling without validation...
Executing after 3ms
My question is: Why is [ValidateNotNullOrEmpty()] so slow considering that (as its name states) it just checks for a null or empty parameter?

Answer 1

When you add (most) validation attributes to a collection, it is applied to each item in the collection; not the collection as a whole, so that validation will be run against every individual byte.
mklement0 brought up an open issue on GitHub about this very thing.
The easiest way to test that it's not empty is just to make the parameter mandatory; an empty array won't be accepted then:
function Test-ValidatePerformance
{
param(
[Parameter(Mandatory)]
[Byte[]]
$Data
)
$sw.Stop()
Write-Host "Executing after $([Math]::Round($sw.Elapsed.TotalMilliSeconds))ms"
}
Note: as the original poster pointed out, and Patrick Meinecke confirmed in this GitHub issue, there is a bug in Windows PowerShell (fixed in Core), regarding the performance issue with Mandatory parameters.
If you want the parameter to be optional, but if supplied it must not be empty, you can use [ValidateCount()] instead, which should be quick:
function Test-ValidatePerformance
{
param(
[ValidateCount(1,[int]::MaxValue)]
[Byte[]]
$Data
)
$sw.Stop()
Write-Host "Executing after $([Math]::Round($sw.Elapsed.TotalMilliSeconds))ms"
}
Or you can just do the check in code instead of using validation attributes.
function Test-ValidatePerformance
{
param(
[Byte[]]
$Data
)
if (-not $Data -and $PSBoundParameters.ContainsKey('Data')) {
throw [System.ArgumentException]'An empty array is not allowed'
}
$sw.Stop()
Write-Host "Executing after $([Math]::Round($sw.Elapsed.TotalMilliSeconds))ms"
}