Powershell Call Base Class Function From Overidden Function

Question

I want to make a call to the parent function from its overidden function, i isolated my problem in the following code:

class SomeClass{
  [type]GetType(){
    write-host 'hooked'
    return $BaseClass.GetType() # how do i call the BaseClass GetType function??
  }
}
SomeClass::new().GetType()

i am expecting an output like this:

hooked
IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     SomeClass                                System.Object

Answer 1

In oder to call a method on the base class, cast $this to it (([object] $this).GetType(), in your case, given that your class implicitly derives from [object](System.Object)):

class SomeClass  {
  [type]GetType(){
    Write-Verbose -Verbose hooked!
    # Casting to [object] calls the original .GetType() method.
    return ([object] $this).GetType()
  }
}

[SomeClass]::new().GetType()

As an aside re referring to the base class in PowerShell custom classes:

• PowerShell only allows you to reference the base class via the abstract base identifier in base-class constructor calls:

  class Foo { [int] $Num; Foo([int] $Num) { $this.Num = $Num } }
  class FooSub : Foo { FooSub() : base(42) { } } # Note the `: base(...)` part
  [FooSub]::new().Num # -> 42
  

• In methods, the only (non-reflection-based) way to refer to the base class is via a type literal (cast), which in essence requires you to hard-code the base-class name (as also shown with ([object] $this) above):

  class Foo { [string] Method() { return 'hi' } }
  # Note the need to name the base class explicitly, as [Foo].
  class FooSub : Foo { [string] Method() { return ([Foo] $this).Method() + '!' } }
  [FooSub]::new().Method() # -> 'hi!'
  

  • Caveat: There appears to be a bug in Windows PowerShell (no longer present in PowerShell (Core) 7+), where calling a base class' method fails if the base class is a generic type that is constructed with type argument [pscustomobject] aka [psobject]. Use a reflection-based workaround in that case; e.g.:

    class Foo : System.Collections.ObjectModel.Collection[pscustomobject] {
      [void] Add([pscustomobject] $item) {
        Write-Verbose -Verbose hooked!
        # Use reflection to call the base class' .Add() method.
        # Note: In *PowerShell Core*, this workaround isn't necessary, and the usual would work:
        #         ([System.Collections.ObjectModel.Collection[pscustomobject]] $this).Add($item)
        # Note the use of .GetMethod() and .Invoke()
        [System.Collections.ObjectModel.Collection[pscustomobject]].GetMethod('Add').Invoke($this, [object[]] $item)
      }
    }