PowerShell consumes GB of RAM when returning large byte array?

Question

I'm having a bit of trouble understanding exactly what is going and why it is happening. I have a small function grabbing all the bytes of a large 133MB PNG image file, storing it in a byte array, and returning it. There is either some behavior I don't understand, or perhaps a bug in PowerShell?

$TestFile = 'C:\test.png'

function GetByteArray($InputFile) {
    $ByteArray = [IO.File]::ReadAllBytes($InputFile)

    Write-Host ( 'Type: ' + $ByteArray.GetType() )
    Write-Host ( 'Size: ' + $ByteArray.Length )

    return $ByteArray
}
$NewArray = GetByteArray -InputFile $TestFile

Write-Host ( 'Type: ' + $NewArray.GetType() )
Write-Host ( 'Size: ' + $NewArray.Length )

pause

I am expecting the function to return a [Byte[]] about 133MB in size, but it does not. Instead PowerShell consumes about 5GB RAM, prints out the error message below, and returns a [System.Object[]].

Type: byte[]
Size: 140151164
Array dimensions exceeded supported range.
At F:\test.ps1:10 char:10
+   return $ByteArray
+          ~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (:) [], OutOfMemoryException
    + FullyQualifiedErrorId : System.OutOfMemoryException

Type: System.Object[]
Size: 134217728
Press Enter to continue...:

What am I not understanding? Why is the byte array being converted to an object? Why is eating almost all my RAM?

You are not returning byte array, but individual bytes.

user4003407
– user4003407

2018-09-17 08:32:33 +00:00
Commented Sep 17, 2018 at 8:32 — user4003407
– user4003407, Commented Sep 17, 2018 at 8:32

Ansgar Wiechers · Accepted Answer · 2018-10-01 22:46:53Z

4

PetSerAl is correct (as always), but perhaps a little more explanation is in order. When returning an array from a function PowerShell unrolls that array and returns the individual elements to the caller. There they are gathered into a regular array (System.Object[]).

To prevent this you need to wrap the array result in another array when returning it. PowerShell will only unroll the outer array and pass the nested array to the caller as a single element, thus preserving the type. Think of it as applying a kind of "transport encoding". Use the unary array construction operator (,) to do that:

return ,$ByteArray

edited Oct 1, 2018 at 22:46

answered Sep 17, 2018 at 8:48

Ansgar Wiechers

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1 Comment

Bighead Over a year ago

Thank you so much, this indeed correctly returns the byte array!

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