PowerShell type accelerators: PSObject vs PSCustomObject

Question

In PowerShell v3.0 PSCustomObject was introduced. It\'s like PSObject, but better. Among other improvements (e.g. property order being preserved),

Answer 1

[PSObject] and [PSCustomObject] are aliases for the same type - System.Management.Automation.PSObject. I can't say there's a good reason for it, but it is at least suggestive of two different purposes, and maybe that's reason enough.

System.Management.Automation.PSObject is used to wrap objects. It was introduced to provide a common reflection api over any object that PowerShell wraps - .Net, WMI, COM, ADSI, or simple property bags.

System.Management.Automation.PSCustomObject is just an implementation detail. When you create a PSObject, the PSObject must wrap something. For property bags, the object wrapped is System.Management.Automation.PSCustomObject.SelfInstance (an internal member.) This instance is hidden from normal use of PowerShell, the only way to observe it is with reflection.

Property bags are created in multiple ways in PowerShell:

$o1 = [pscustomobject]@{Prop1 = 42}
$o2 = new-object psobject -Property @{Prop1 = 42 }

Both $o1 and $o2 above will be an instance of PSObject, and the PSObject will wrap PSCustomObject.SelfInstance. PSCustomObject.SelfInstance is used internally in PowerShell to tell the difference between a simple property bag and wrapping any other object.

Answer 2

Looking at the static methods:

PS C:\> [PSCustomObject] | gm -Static -MemberType Method



   TypeName: System.Management.Automation.PSObject

Name            MemberType Definition                                                        
----            ---------- ----------                                                        
AsPSObject      Method     static psobject AsPSObject(System.Object obj)                     
Equals          Method     static bool Equals(System.Object objA, System.Object objB)        
new             Method     psobject new(), psobject new(System.Object obj)                   
ReferenceEquals Method     static bool ReferenceEquals(System.Object objA, System.Object o...



PS C:\> [System.Management.Automation.PSCustomObject] | gm -Static -MemberType Method



   TypeName: System.Management.Automation.PSCustomObject

Name            MemberType Definition                                                        
----            ---------- ----------                                                        
Equals          Method     static bool Equals(System.Object objA, System.Object objB)        
ReferenceEquals Method     static bool ReferenceEquals(System.Object objA, System.Object o...

The type accelerator has a couple of new static methods added. I suspect it's using one of those as the constructor.