PowerShell classes – – overloading constructors

You can usually create a .NET object from a class in one of several ways – no parameters i.e. and empty object up to an object with all of its properties populated as you create it. When you define a class you define the various ways in which it can be created – these are known as its constructors. When you have multiple constructors they are known as overloaded constructors i.e. a number of different ways to create the instance.

 

You have seen the default constructor in the previous post:

 

$obj = [LastBoot]::new()

 

You just use new() without any arguments to create the empty object which you then populate. If you know some of the values before you create the object you can use one of the overloads on the constructor to create the object:

 

$obj2 = [LastBoot]::new($comp, $lbtime)

 

Here you’re supplying the computername and last boot time as you create the object.  The class definition looks like this:

 

class LastBoot {
[string]$ComputerName
[string]$LastBootime

LastBoot(){}

LastBoot([string]$computer, [string]$lbt) {
  $ComputerName = $computer
  $LastBootime = $lbt
}

}

$comp = $env:COMPUTERNAME
$lbtime = Get-CimInstance -ClassName Win32_OperatingSystem -ComputerName $comp |
    select -ExpandProperty LastBootUpTime

 

$obj = [LastBoot]::new()

$obj.ComputerName = $comp
$obj.LastBootime = $lbtime

$obj

 

$obj2 = [LastBoot]::new($comp, $lbtime)
$obj2

 

Define the two properties as before. You have to explicitly define the default constructor when you are overloading the constructor:

 

LastBoot(){}

 

That says create a new instance of the LastBoot class but don’t set any properties.

 

Our overload:

LastBoot([string]$computer, [string]$lbt) {
  $ComputerName = $computer
  $LastBootime = $lbt
}

 

says create an instance of LAstBoot and I’m going to give you the computer name and last boot time. Map what you’re given to the ComputerName and LastBootime properties as shown.

 

Use the default constructor like this:

$obj = [LastBoot]::new()

 

And the overload like this

$obj2 = [LastBoot]::new($comp, $lbtime)

 

In case you’re wondering why this will useful to know – the simple answer is that you can create DSC resources using PowerShell classes which makes them much easier to write. But before we get to that you need to fully understand how PowerShell classes work.

 

Warning – PowerShell 5.0 is still in preview stage so this could change

Advertisements
This entry was posted in PowerShell v5. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s