Inheriting Classes in PowerShell 5 (part 1)

PowerShell 5 comes with built-in support for classes. You can use this new feature to enhance existing .NET classes. Here is an example: let’s create an enhanced process class with additional functionality.

Processes are typically represented by System.Diagnostics.Process objects. They have limited functionality, and for example provide no out-of-the-box way of gracefully closing an application. You can either kill it (losing unsaved data), or close it (the user can then abort the closing process).

Here is a new class called AppInstance that inherits from System.Diagnostics.Process. So it gets all the functionality already present in the process class, yet you can add additional properties and methods:

#requires -Version 5
class AppInstance : System.Diagnostics.Process
{
  # Constructor, being called when you instantiate a new object of
  # this class
  AppInstance([string]$Name) : base()
  {
    # launch the process, get a regular process object, and then
    # enhance it with additional functionality
    $this.StartInfo.FileName = $Name
    $this.Start()
    $this.WaitForInputIdle()
  }

  # for example, rename an existing method
  [void]Stop()
  {
    $this.Kill()
  }

  # or invent new functionality
  # Close() closes the window gracefully. Unlike Kill(),
  # the user gets the chance to save unsaved work for
  # a specified number of seconds before the process
  # is killed
  [void]Close([Int]$Timeout = 0)
  {
    # send close message
    $this.CloseMainWindow()
    # wait for success
    if ($Timeout -gt 0)
    {
      $null = $this.WaitForExit($Timeout * 1000)
    }
    # if process still runs (user aborted request), kill forcefully
    if ($this.HasExited -eq $false)
    {
      $this.Stop()
    }
  }

  # example of how to change a property like process priority
  [void]SetPriority([System.Diagnostics.ProcessPriorityClass] $Priority)
  {
    $this.PriorityClass = $Priority
  }

  [System.Diagnostics.ProcessPriorityClass]GetPriority()
  {
    if ($this.HasExited -eq $false)
    {
      return $this.PriorityClass
    }
    else
    {
      Throw "Process PID $($this.Id) does not run anymore."
    }
  }

  # add static methods, for example a way to list all processes
  # variant A: no arguments
  static [System.Diagnostics.Process[]] GetAllProcesses()
  {
    return [AppInstance]::GetAllProcesses($false)
  }
  # variant B: submit $false to see only processes that have a window
  static [System.Diagnostics.Process[]] GetAllProcesses([bool]$All)
  {
    if ($All)
    {
      return Get-Process
    }
    else
    {
      return Get-Process | Where-Object { $_.MainWindowHandle -ne 0 }
    }
  }
}

# you can always run static methods
[AppInstance]::GetAllProcesses($true) | Out-GridView -Title 'All Processes'
[AppInstance]::GetAllProcesses($false) | Out-GridView -Title 'Processes with Window'


# this is how you instantiate a new process and get back
# a new enhanced process object
# classic way:
# $notepad = New-Object -TypeName AppInstance('notepad')
# new (and faster) way in PowerShell 5 to instantiate new objects:
$notepad = [AppInstance]::new('notepad')

# set a different process priority
$notepad.SetPriority('BelowNormal')

# add some text to the editor to see the close message
Start-Sleep -Seconds 5

# close the application and offer to save changes for a maximum
# of 10 seconds
$notepad.Close(10)

As you see in the example, when you instantiate new objects from your class, it launches processes, and these processes expose the same properties and methods as usual. In addition, there are new methods such as SetPriority() and Close().

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