I ran across this line the other day that a student wrote in a beginning PowerShell class. I can’t figure out how it works since there is no size property for the Win32_QuickFixEngineering object type (which is the object type passed by the Get-Hotfix cmdlet). What is size referring to that makes this command work?
get-hotfix | where {$.Description -eq ‘Security update’ -and $.size -lt 5MB}
Wow … funny. Obviously it does not matter what property you specify. At least on my system for example
get-hotfix | where {$.Description -eq ‘Security update’ -and $.Surname -lt ‘RogerRabbit*’} produces the same output like your original command line. 
Why is nobody using Strict Mode ? Why is it not enabled by default ? (Rethoric questions)
Use
Set-StrictMode -Version Latest
Get-Hotfix ... your Where-Object {}
With Strict Mode, you’ll get an error when accessing a property that does not exist (Get-Help Set-StrictMode).
You’ll immediatley see and fix your mistakes, saving lots of time and writing safer code.
Without it, you get $null for each inexistent property.
“AnyText”.Lenght is another nice example…
Another question is why ($null -lt 5MB) and not ($null -gt 5MB)…
www . powershelltraining . eu
this is because of the type casting and comparison. IMO it compares the length if the $_.size (which is a known existing property hence length is 0) to the ‘5MB’ which has length greater than empty.
the output will be different if you use -gt instead of -lt.
below example proves it
1..5 | Where-Object -FilterScript {$_.f -lt 1}
1..5 | Where-Object -FilterScript {$_.f -ge 1}
1..5 | Where-Object -FilterScript {$_.f -ge $null}
Another way to look at it:
get-hotfix | foreach {$_.size -eq $null}
True
True
True
True
True
True
True
True
True
True
True
True
Thank you. Excellent answers. Indeed it does give different results with -gt. And I was unaware of the “null” response for unknown property values. This has been very excellent help and cleared a lot of things up.
Or just do this, which works with or without Strict mode set.
Set-StrictMode -Version Latest get-hotfix | where Description -eq 'Security Update' Source Description HotFixID InstalledBy InstalledOn ------ ----------- -------- ----------- ----------- Lab01 Security Update KB4457146 NT AUTHORITY\SYSTEM 9/12/2018 12:00:00 AM Lab01 Security Update KB4465663 NT AUTHORITY\SYSTEM 11/14/2018 12:00:00 AM Lab01 Security Update KB4467694 NT AUTHORITY\SYSTEM 11/14/2018 12:00:00 AM Lab01 Security Update KB4467702 NT AUTHORITY\SYSTEM 11/14/2018 12:00:00 AM
Also the reason for your ‘-and’ failure, is that there is no property called ‘size’, so combining the will fail of course.
(get-hotfix)[0] | Get-Member | ft -a
TypeName: System.Management.ManagementObject#root\cimv2\Win32_QuickFixEngineering
Name MemberType Definition
---- ---------- ----------
PSComputerName AliasProperty PSComputerName = __SERVER
Caption Property string Caption {get;set;}
CSName Property string CSName {get;set;}
Description Property string Description {get;set;}
FixComments Property string FixComments {get;set;}
HotFixID Property string HotFixID {get;set;}
InstallDate Property string InstallDate {get;set;}
InstalledBy Property string InstalledBy {get;set;}
Name Property string Name {get;set;}
ServicePackInEffect Property string ServicePackInEffect {get;set;}
Status Property string Status {get;set;}
__CLASS Property string __CLASS {get;set;}
__DERIVATION Property string[] __DERIVATION {get;set;}
__DYNASTY Property string __DYNASTY {get;set;}
__GENUS Property int __GENUS {get;set;}
__NAMESPACE Property string __NAMESPACE {get;set;}
__PATH Property string __PATH {get;set;}
__PROPERTY_COUNT Property int __PROPERTY_COUNT {get;set;}
__RELPATH Property string __RELPATH {get;set;}
__SERVER Property string __SERVER {get;set;}
__SUPERCLASS Property string __SUPERCLASS {get;set;}
PSStatus PropertySet PSStatus {__PATH, Status}
ConvertFromDateTime ScriptMethod System.Object ConvertFromDateTime();
ConvertToDateTime ScriptMethod System.Object ConvertToDateTime();
InstalledOn ScriptProperty System.Object InstalledOn {get=if ([environment]::osversion.version.build -ge 7000)...
Even if you look at the extended properties
(get-hotfix)[0] | Select * | Format-List -Force
I usually check the properties like this:
cmdlet | fl *
Just “| fl” might use a format file and not show everything.
But this will, including some not show any other way, still, no size property, which contributes to the OP’s error.
(get-hotfix)[0] | Get-Member -Force Name MemberType ---- ---------- PSComputerName AliasProperty pstypenames CodeProperty psadapted MemberSet psbase MemberSet psextended MemberSet psobject MemberSet PSStandardMembers MemberSet Caption Property CSName Property Description Property FixComments Property HotFixID Property InstallDate Property InstalledBy Property Name Property ServicePackInEffect Property Status Property __CLASS Property __DERIVATION Property __DYNASTY Property __GENUS Property __NAMESPACE Property __PATH Property __PROPERTY_COUNT Property __RELPATH Property __SERVER Property __SUPERCLASS Property PSStatus PropertySet ConvertFromDateTime ScriptMethod ConvertToDateTime ScriptMethod InstalledOn ScriptProperty
this is working because, if some property is not present for an object Powershell will create a new custom property. have a look at the below code.
PS C:\windows\system32> get-hotfix | select size
It won’t give any error instead it will create a new property called size. As size does not have any value it will hold $null.Now if you compare by using (-lt) it will always return true as ($null -lt any value) is true. Have a look at the below code,
PS C:\windows\system32> get-hotfix | for each {$_.size -lt 5}
True
True
True
True
True
True
True
True
True
now if you change the compare operator to (-gt )it will always give false. As $null -gt any value is false.
Foreach doesn’t behaves like Select-Object. Here comparison happens with the length of the non-existing property, hence its true.
Hmm, but $null isn’t equal to 0?
PS C:\users\admin> $null -lt 1 True PS C:\users\admin> $null -lt -1 False PS C:\users\admin> $null -le 0 True PS C:\users\admin> $null -eq 0 False PS C:\users\ccfadmin> $null -lt 0 True
Well, $null is $null… it has no type. It is not equal to 0 by itself.
But sometimes, variables in PowerShell can be converted (Cast) to another type. $Null can be converted to [int], and it has to have a valid value. Both
[int]$Null
$Null -AS [int]
return 0, by convention.
Hmm, $null is in there somewhere close to 0!
PS C:\users\admin> $null -gt -.00000000000000000000000000000000000001 True PS C:\users\admin> $null -lt 0 True
$_.size -lt 5 converts $null to int and [int]$null is 0, hence it returns true
$_.size -lt ‘5’ converts $null to string and [string]$null length is 0, hence it returns true .
Usually the type is dictated by the left element in the comparison. $null seems to be a special case, since it has no type.
PS C:\Users\admin> '10' -gt 9 False PS C:\Users\admin> 10 -gt '9' True PS C:\Users\admin> [datetime]::today -eq '11/20' True
yes it is a special case, just had a chat with Bruce Payette and PFB his comments.
Bruce Payette:
$null in a numeric context (like comparing to a number) is converted to 0. And since 0 is less than 1, $null is special-cased by the runtime. It doesn't actually get converted, but is treated as a value less than 0 but greater that the smallest negative number. Likewise in a string context, it is treated as being 'smaller' than the smallest possible string i.e. ''.
This simple question has brought out a lot if interesting nuances and taught me, and hopefully others, a lot. I appreciate all the feedback.