Using enumerations in PowerShell (2/2)

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Using enums (part 2)

Simple enums define a list of allowed values, and the user can pick exactly one. When enums are defined as a flag array, then the user can pick more than one value:

[Flags()]

enum Features

{

   BootLog

   RealTimeProtection

   Firewall

   PhysicalAccess

   Leash

}



[Features]$myFeatures = 'Firewall', 'Leash'

$myFeatures 

By adding the [Flags()] attribute to the enum, each enum entry now gets its own unique bit value, thus the user can combine as many enum items as needed. The type [Enum] can shed some light into this. Here it lists the defined enum names:

• PS C:\> [Enum]::GetNames([Features])
  

Next, it shows the assigned true numeric values:

• PS C:\> [Enum]::GetNames([Features]) | ForEach-Object {
     '{0} = {1}' -f $_, ([Features]::$_ -as [int])
  }
  

When you assign multiple values, bit values are added:

• PS C:\> [Features]$myFeatures = 'Firewall', 'Leash'
  PS C:\> $myFeatures
  Firewall, Leash 
  PS C:\> $myFeatures.value__
  

There is just one problem: the assigned numbers are consecutive and thus ambiguous – PhysicalAccess has ID 3, but RealTimeProtection+Firewall also adds to 3.

When you use [Flags()], you also need to manually assign unique values to the enum items:

[Flags()]

enum Features

{

   BootLog =  1

   RealTimeProtection = 2

   Firewall = 4

   PhysicalAccess = 8

   Leash = 16

}

Now each item has a unique ID:

• PS C:\> [Enum]::GetNames([Features]) | ForEach-Object {'{0} = {1}' -f $_,
  ([Features]::$_ -as [int])}
  

Now, with unique IDs, you can use enums like any other numbers to do binary arithmetic:

• PS C:\> [Features]'Bootlog,Leash'
  BootLog, Leash
  PS C:\> [Features]'Bootlog,Leash' -as [int]
  

Use the "+=" operator to add an element, and "-=" to remove it:

• PS C:\> [Features]$features = 'Bootlog'
  PS C:\> $features += 'Leash'
  PS C:\> $features += 'Firewall'
  PS C:\> $features
  BootLog, Firewall, Leash
  PS C:\> $features -= 'Leash'
  PS C:\> $features
  BootLog, Firewall
  

Likewise, when testing whether a given value is present in such a list, use binary operators like -band and -bor:

• PS C:\> $features
  BootLog, Firewall
  PS C:\> ($features -band 'Firewall') -eq 'Firewall'
  True
  PS C:\> ($features -band 'Leash') -eq 'Leash'
  False
   

Related links 

• Scriptember calendar
• Using enumerations in PowerShell part 1 is here
• Try out ScriptRunner here
• ScriptRunner: Book a demo with our product experts