Understanding Classes in PowerShell – Part 02 – Adding Constructors

This blog post is continuation of series of posts on understanding classes in PowerShell. Part 01 of series can be read at here.

What is an Constructor

Constructor is an type of method or function which is called only when the object is created. It is never called after that no matter how many times you called the method or function.

The most common use of the constructor is to initialize properties related to the Object. As in .NET, the constructor must use the same name as the class.

For example, building on our previous example of class Dog, we can build a constructor as:

Class Dog{
 # Define properties
 [String] $Name
 [String] $Breed
 [String] $OwnerName
 [String] $OwnerAddress
 [DateTime] $RegistrationDate
 [long] $RegistrationNumber

# Define static properties 
 static [int] $NumberOfLegs = 4

#Defines Constructor
 Dog([String] $Name, [String] $Breed, [String] $OwnerName, [String]$OwnerAddress, [DateTime]$RegistrationDate)
 {
 $this.Breed = $Breed
 $this.Name = $Name
 $this.OwnerName = $OwnerName
 $this.OwnerAddress = $OwnerAddress
 $this.RegistrationDate = $RegistrationDate
 }

}

We can create the object like this:

# Using New-Object. Parameters for Argument list are positional and required by the constructor.
$myDog = New-Object -TypeName Dog -ArgumentList "Lucy","English Springer Spaniel","Cynthia G. Dumond","3105 Valley Lane, Austin, TX 78701","2017-12-12"

Or we can also use the static new() method:

$myDog2 = [Dog]::new("Lucy","English Springer Spaniel","Cynthia G. Dumond","3105 Valley Lane, Austin, TX 78701","2017-12-12")

In both cases, the object will be initialized and you can see the values as below:

PS C:\Windows\system32> $mydog2

Name : Lucy
Breed : English Springer Spaniel
OwnerName : Cynthia G. Dumond
OwnerAddress : 3105 Valley Lane, Austin, TX 78701
RegistrationDate : 12/12/2017 12:00:00 AM
RegistrationNumber : 0

PS C:\Windows\system32> $myDog

Name : Lucy
Breed : English Springer Spaniel
OwnerName : Cynthia G. Dumond
OwnerAddress : 3105 Valley Lane, Austin, TX 78701
RegistrationDate : 12/12/2017 12:00:00 AM
RegistrationNumber : 0

Just like .NET again, a constructor that takes no parameters is called a default constructor. Default constructors are invoked whenever an object is instantiated by using the new operator and no arguments are provided to new. For example, consider the below class:

Class Dog{
 # Define properties
 [String] $Name
 [String] $Breed
 [String] $OwnerName
 [String] $OwnerAddress
 [DateTime] $RegistrationDate
 [long] $RegistrationNumber

# Define static properties 
 static [int] $NumberOfLegs = 4

}

Since we have not defined any constructor explicitly, a default constructor will be automatically created. The job of this constructor is to initialize object properties to their default values. In such a case, we can create object using:

$myDog = [Dog]::new()
# or
$myDog = New-Object -TypeName Dog

This is what we have seen in the previous post.

Do note that if we have defined a constructor explicitly, it will not go ahead and create a default constructor. Considering our first example of this blog post, $myDog3 = [Dog]::new() will fail with following error:

Cannot find an overload for "new" and the argument count: "0".
At line:1 char:1
+ $myDog3 = [Dog]::new()
+ ~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [], MethodException
+ FullyQualifiedErrorId : MethodCountCouldNotFindBest

To overcome, We then need to define it explicitly like below:

Class Dog{
    # Define properties
    [String] $Name
    [String] $Breed
    [String] $OwnerName
    [String] $OwnerAddress
    [DateTime] $RegistrationDate
    [long] $RegistrationNumber

    # Define static properties 
    static [int] $NumberOfLegs = 4

    #Defines Constructor
    Dog([String] $Name, [String] $Breed, [String] $OwnerName, [String]$OwnerAddress, [DateTime]$RegistrationDate)
    {
      $this.Breed = $Breed
      $this.Name = $Name
      $this.OwnerName = $OwnerName
      $this.OwnerAddress = $OwnerAddress
      $this.RegistrationDate = $RegistrationDate
    }

    #Defines default Constructor
    Dog()
    {
     $this.Breed = ""
     $this.Name = ""
     $this.OwnerName = ""
     $this.OwnerAddress = ""
     $this.RegistrationDate = "01/01/0001 12:00:00 AM"
     $this.RegistrationNumber = 0
    }

}

$myDog3 = [Dog]::new()

$myDog3

This will run fine.

What is $this

The $this variable describes the current instance of the object. It is thought of like $_ for classes.

  1. If a property is not static, the syntax $this.PropertyName is used to reference the instance property.
  2. To refer to methods simply use the method name or $this.MethodName().

Do note that a static method or property cannot use $this.

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