Overview

The PSClasses project provides classes with inheritance to PowerShell versions earlier than 5.0.

PSClasses include important features of object-oriented languages, like following:

Feature Example

class inheritance

 
CreateClass "IntCounter" $null '[int] $value' @{ .. }

CreateClass "ModularCounter" -extends "IntCounter" '[int] $modulus' @{
    ..
}

overriding methods

 
CreateClass "IntCounter" $null '[int] $value' @{
    increment = { .. }
}

CreateClass "ModularCounter" -extends "IntCounter" '[int] $modulus' @{
    increment = { .. }
}

calling overridden superclass methods

 
CreateClass "IntCounter" $null '[int] $value' @{ increment = { .. } }

CreateClass "ModularCounter" -extends "IntCounter" '[int] $modulus' @{
    increment = {
        $this.IntCounter_increment()
        ..
    }
}

multiple constructors

 
CreateClass "ModularCounter" -extends "IntCounter" '[int] $modulus' @{
    init = { param([int] $value, [int] $modulus) .. }

    init0 = { param([int] $modulus) .. }
}

$modCounter = New "ModularCounter" { $self.init(6, 10) }
$modCounter2 = New "ModularCounter" { $self.init0(20) }

calling other constructors of same class

 
CreateClass "ModularCounter" -extends "IntCounter" '[int] $modulus' @{
    init = {
        param([int] $value, [int] $modulus)
        ..
    }

    init0 = {
        param([int] $modulus)
        $this.ModularCounter_init(0, $modulus)
    }
}

calling superclass constructors

 
CreateClass "IntCounter" $null '[int] $value' @{ .. }

CreateClass "BoundedCounter" -extends "IntCounter" '[int] $bound' @{
    init = {
        param([int] $value, [int] $bound)

        # Call generated constructor "init" of superclass
        $this.IntCounter_init($value)
        ..
    }
}

The library also contains an uncommon feature: automatic generation of the constructor which accepts arguments corresponding to all instance variables. This feature resembles case classes in Scala or data classes in Kotlin:

generated constructor

 
CreateClass "Config" $null '$drive, $certSubject, $certAccount'

# Constructor 'init' is automatically generated

$config = New "Config" { $self.init("C:", "CN=jsmith", "jsmith") }
The file ClassGenerator_Test.ps1 contains numerous examples in form of Pester tests.
The project includes a simple template engine used in implementation of some features.

Class creation

We can create a class by calling CreateClass function. The created class is represented by a class object. A variable holding the class object is automatically created. The variable name is formed like this: $<className>Class.

The CreateClass function accepts following arguments:

Argument Alias Mandatory

class name

-name

yes

superclass

-extends

no

instance variables

-variables

no

methods

-methods

no

Example: Person

CreateClass "Person" $null '[string] $name, [int] $age' @{

    getInfo = {
        "Name: $($this.name), age: $($this.age)"
    }
}

This call creates the class object $PersonClass:

> $PersonClass

className         : Person
super             :
allVariablesStr   : [string] $name, [int] $age
variables         : {name, age}
...

Instance variables

Instance variables are specified as a string containing a comma-separated variable list along with optional type specifiers. The string is parsed and the variable names extracted.

The string containing instance variables is also used for parameters of the generated constructor named init.

Syntax is the same as syntax of function parameters or script block parameters.

Examples

'[string] $name, [int] $age, [boolean] $male'
'[PSCustomObject] $successor, [string] $topic'
'$drive, $certSubject, $certAccount'
''

Methods

Methods are written as a dictionary of (name, script block) pairs. The methods dictionary can be an unordered hashtable or ordered dictionary.

Overriding methods

A derived class can override superclass methods.

Each overridden method is available in following form: <className>_<methodName>.

Example: counters

CreateClass "IntCounter" $null '[int] $value' @{

    increment = {
        $this.value += 1
    }

    reset = {
        $this.value = 0
    }
}

CreateClass "ModularCounter" -extends "IntCounter" '[int] $modulus' @{

    # Override superclass method
    increment = {
        If ($this.value -eq $this.modulus - 1) {
            # Call inherited method
            $this.reset()
        } Else {
            # Call superclass version
            $this.IntCounter_increment()
        }
    }
}

The ModularCounter class overrides increment method. The ModularCounter’s increment method calls the superclass version:

$this.IntCounter_increment()

Constructors

Constructors are special methods whose names start with init. A class can contain multiple constructors. Each constructor can call:

  • any other constructor of the same class, including the generated constructor

  • any superclass constructor

When a custom constructor calls other constructors it must use one of following forms:

  • <className>_<constructorName> (for calling other custom constructors)

  • <className>_gen_init (for calling the generated constructor)

Generated constructor

The constructor named init is generated automatically. It accepts arguments corresponding to all instance variables (including instance variables declared in superclasses) and just copies the arguments into instance variables.

The generated constructor can be overridden by a custom init constructor.

If overridden, the generated constructor is still available to be called from other constructors as a method with following name: <className>_gen_init.

Example: Person

Let’s look again at the above example which creates Person class:

CreateClass "Person" $null '[string] $name, [int] $age' @{

    # Constructor 'init' is automatically generated

    getInfo = {
        "Name: $($this.name), age: $($this.age)"
    }
}

The Person’s methods don’t include custom constructors. The generated constructor init is available after class creation. Its arguments correspond to instance variables:

  • $name

  • $age

We can immediately create new objects using the generated init constructor:

$person = New "Person" { $self.init("John Smith", 23) }

> $person

name       age
----       ---
John Smith  23

Calling other constructors

Let’s rewrite IntCounter and ModularCounter classes to include only the constructor methods:

CreateClass "IntCounter" $null '[int] $value' @{

    init0 = {
        $this.value = 0
    }
}

The IntCounter’s generated constructor init accepts [int] $value parameter. The class also includes a parameterless constructor init0.

CreateClass "ModularCounter" -extends "IntCounter" '[int] $modulus' @{

    # Same as generated constructor, but with argument checks
    init = {
        param([int] $value, [int] $modulus)

        # Call generated constructor
        $this.ModularCounter_gen_init($value, $modulus)

        If ($modulus -lt 1) {
            throw "ModularCounter: modulus bad"
        }
        If ($value -lt 0 -or $value -gt $modulus) {
            throw "ModularCounter: value bad"
        }
    }

    # A simplified constructor
    init0 = {
        param([int] $modulus)

        # Call another constructor
        $this.ModularCounter_init(0, $modulus)
    }
}

The ModularCounter’s generated constructor init which accepts the parameters [int] $value and [int] $modulus is overridden by the custom init constructor. The custom init constructor calls the generated init constructor:

$this.ModularCounter_gen_init($value, $modulus)

The class also includes a parameterless constructor init0 which calls the custom init constructor:

$this.ModularCounter_init(0, $modulus)

Calling superclass constructors

The following classes are a part of an example which illustrates the chain of responsibility design pattern. For simplicity we have excluded non-constructor methods.

CreateClass "HelpHandler" $null '[PSCustomObject] $successor, [string] $topic'

CreateClass "Widget" -extends "HelpHandler" '[PSCustomObject] $parent' @{

    init = {
        param([PSCustomObject] $parent, [string] $topic)

        # Widget's parent is HelpHandler's successor
        $this.HelpHandler_init($parent, $topic)
        $this.parent = $parent
    }
}

The Widget’s init constructor calls the generated constructor of the HelpHandler superclass:

$this.HelpHandler_init($parent, $topic)

Object creation

There are several ways to create new objects. The simplest way is to call the function New (or alternatively New_). Another way is to call the method new (or alternatively new_) on the class object).

Let’s illustrate creation of objects on the following simple class:

CreateClass "Point" $null '[double] $x, [double] $y' @{

    translate = {
        param([double] $x, [double] $y)

        $this.x += $x
        $this.y += $y
    }

    scale = {
        param([double] $factor)

        $this.x *= $factor
        $this.y *= $factor
    }
}

New

The function New accepts two arguments: a class name and a script block containing a constructor call, e.g.:

$point = New "Point" { $self.init(10, 20) }

Here we supply a parameterless script block. When function New is called it will create $self object and then perform the call $self.init(10, 20) on the object.

The function New expects the supplied script block to contain a constructor call on the object $self. If we use any other object it will not work.

The way of object creation with function New will not work correctly within closures. For example:

$script = {
    ...
    # This will not work
    $point = New "Point" { $self.init(-10, -50) }
    ...
}.GetNewClosure()

New_

The function New_ is similar to New but it expects a script block with a single parameter representing the object being created and initialized. The parameter name is irrelevant.

For example:

$point1 = New_ "Point" { param($self) $self.init(30, 50) }

$point2 = New_ "Point" { param($_) $_.init(25, 35) }
The way of object creation with function New_ will work correctly within closures.

Using class object methods

Instead of calling functions New or New_ we can create objects by applying methods new or new_ to the class object:

$point1 = $PointClass.new( { $self.init(10, 20) } )

$point2 = $PointClass.new_( { param($_) $_.init(5, 8) } )