PowerShell (QuickBytes) classes

A few months ago I came across this question on X.

‘What is the output of the last line if you run the code pasted below?’

1. Brown
2. SyntaxError
3. InvalidOperation
4. Nothing.

class Dog {

    [string]
    $Hair

    Dog() {
        $this.Color = 'Brown'
        $this.Hair = 'Long'
    }
}

([Dog]::new()).Color

In this case, the answer would be 3 InvalidOperation.

But why?

Table of contents

• Table of contents
  • Classes
  • Instantiation
  • Constructor
  • Properties
  • But why do we get an InvalidOperation error?
  • Methods
  • Wrapping Up

Classes

You can think of classes as a sort of blueprint that specifies how an object will look and behave.

Instantiation

Classes need to be instantiated before we can perform any actions with them. Instantiating means that we first have to create an object that holds an instance of our class. In other words, we create an object based on our blueprint. To instantiate our Dog class we simply type: [Dog]::new()

Constructor

Constructors are special methods that are called whenever a class is instantiated. They are optional, meaning you can define a class without a constructor. The constructor must always have the same name as the class. In our Dog class, it is the Dog(){} code block where the values of the properties Color and Hair are set.

    # Constructor											
    Dog() {
        $this.Color = 'Brown'
        $this.Hair = 'Long'
    }

TIP Methods are similar to functions. In class terminology, we always refer to a function as a method.

Properties

Our Dog() constructor are responsible for setting the value of two properties. Color and Hair. Both referred to by the automatic variable $this. $this is nothing more then a reference to the current instance of the property.

But why do we get an InvalidOperation error?

Looking at our class we’ve only defined the property Hair . The property Color is missing. Hence the InvalidOperation error.

    # Properties			
    [string]
    $Hair

Methods

Our Dog class currently does not contain any methods. Methods must always have a return type. If you’re method is returning a string, you’re class would be something like:

class Dog {

    [string]
    $Hair

    [string]
    $Color

    Dog() {
        $this.Color = 'Brown'
        $this.Hair = 'Long'
    }

    # Method Bark() that will return a string
    [string] Bark(){
		return 'Woof'
	}
}

([Dog]::new()).Bark()

Wrapping Up

Need more info on classes, check this awesome series: https://4sysops.com/archives/powershell-classes-part-1-objects/