366 lines
12 KiB
Markdown
366 lines
12 KiB
Markdown
---
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title: 'PowerShell Read Only Class Properties'
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date: 2017-07-19T11:15:47+01:00
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draft: false
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# lastmod: 2021-12-23T11:15:47+01:00
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# showDateUpdated: true
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# custom overrides for pages
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# showDate: false
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# showAuthor: false
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# showWordCount: false
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# showReadingTime: false
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# showEdit: false
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# sharingLinks: [null]
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---
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{{< figure src="vader_cube.png" width="800" height="800">}}
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{{< note >}}
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If you're not familiar with Powershell classes I suggest you reading this awesome blog article from Michael Willis
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[Powershell v5 Classes & Concepts](https://xainey.github.io/2016/powershell-classes-and-concepts/). It covers
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everything your need to know about classes.
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{{< /note >}}
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## Background
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So lets start with some thoughts about the underlying situation. We assume there is a complex class structure and we
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don't want others to mess with our data. Or maybe we need to protect our data but still need to expose it.
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Therefore we can't define our class like the following example:
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```powershell
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Class DeathStar {
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[String]$Class = 'Space battle station'
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[Int]$Width = '160000'
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[String[]]$HyperDriveRating = @('Class 4', 'Class 20')
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$Crew = @{
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ImperialNavy = 342953
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Stormtroopers = 25984
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}
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[String]$Ready = $null
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DeathStar () {
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$this.Ready = $true
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}
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}
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```
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With this class definition we can easily create an instance and take a look at the default displayed properties:
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```console
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~> $DeathStarOne = [DeathStar]::New()
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~> $DeathStarOne
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Class : Space battle station
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Width : 160000
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HyperDriveRating : {Class 4, Class 20}
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Crew : {Stormtroopers, ImperialNavy}
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Ready : True
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```
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Furthermore the rebellion could manipulate sensitive data of our death star if we would publish the death start
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class like this. Let's do this with the `Ready` property and the available crew members:
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```console
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~> $DeathStarOne.Ready = $false
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~> $DeathStartOne
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Class : Space battle station
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Width : 160000
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HyperDriveRating : {Class 4, Class 20}
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Crew : {Stormtroopers, ImperialNavy}
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State : False
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~> $DeathStarOne.Crew.ImperialNavy = 0
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~> $DeathStarOne.Crew
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Name Value
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---- -----
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Stormtroopers 0
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ImperialNavy 342953
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```
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Although Powershell doesn't have real readonly class properties, we can mimic them in two elegant ways:
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- Class methods as getter and setter functions
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- Script properties with getter and setter functions.
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Both of them rely to the same concept of using C# like getter and setter functions. But reading and writing
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the values differs.
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## Using class methods
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To isolate our sensitive data we first need to mark the sensitive properties as `hidden`. Hidden properties won't
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get listed as object members unless we use the `Get-Member` function with the `-Force` switch.
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Additionally I like adding an underscore _(`_`)\_ as prefix to my variable names. We use the properties with
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underscores alter on.
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```powershell
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Class DeathStar {
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[String]$Class = 'Space battle station'
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[Int]$Width = '160000'
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[String[]]$HyperDriveRating = @('Class 4', 'Class 20')
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hidden $_Crew = @{
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ImperialNavy = 342953
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Stormtroopers = 25984
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}
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hidden [String]$_Ready = $null
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DeathStar () {
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$this._Ready = $true
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}
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}
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```
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With the customized class definition we check again our default output and members:
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```console
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~> $DeathStarOne = [DeathStar]::New()
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~> $DeathStarOne | Format-List
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Class : Space battle station
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Width : 160000
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HyperDriveRating : {Class 4, Class 20}
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```
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```console
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~> $DeathStarOne | Get-Member
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TypeName: DeathStar
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Name MemberType Definition
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---- ---------- ----------
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Equals Method bool Equals(System.Object obj)
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GetHashCode Method int GetHashCode()
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GetType Method type GetType()
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ToString Method string ToString()
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Class Property string Class {get;set;}
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HyperDriveRating Property string[] HyperDriveRating {get;set;}
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Width Property int Width {get;set;}
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```
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```console
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~> $DeathStarOne | Get-Member -Force
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TypeName: DeathStar
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Name MemberType Definition
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---- ---------- ----------
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Equals Method bool Equals(System.Object obj)
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GetHashCode Method int GetHashCode()
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GetType Method type GetType()
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ToString Method string ToString()
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Class Property string Class {get;set;}
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HyperDriveRating Property string[] HyperDriveRating {get;set;}
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Width Property int Width {get;set;}
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pstypenames CodeProperty System.Collections.ObjectModel.Collection`1[[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]] pstypenames{...
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psadapted MemberSet psadapted {Class, Width, HyperDriveRating, get_Class, set_Class, get_Width, set_Width, get_HyperDriveRating, set_HyperDriveRating, get__Crew, set__Cre...
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psbase MemberSet psbase {Class, Width, HyperDriveRating, get_Class, set_Class, get_Width, set_Width, get_HyperDriveRating, set_HyperDriveRating, get__Crew, set__Crew, ...
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psextended MemberSet psextended {}
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psobject MemberSet psobject {BaseObject, Members, Properties, Methods, ImmediateBaseObject, TypeNames, get_Members, get_Properties, get_Methods, get_ImmediateBaseObject,...
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Equals Method bool Equals(System.Object obj)
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GetHashCode Method int GetHashCode()
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GetType Method type GetType()
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get_Class Method string get_Class()
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get_HyperDriveRating Method string[] get_HyperDriveRating()
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get_Width Method int get_Width()
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get__Crew Method System.Object get__Crew()
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get__Ready Method string get__Ready()
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set_Class Method void set_Class(string )
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set_HyperDriveRating Method void set_HyperDriveRating(string[] )
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set_Width Method void set_Width(int )
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set__Crew Method void set__Crew(System.Object )
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set__Ready Method void set__Ready(string )
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ToString Method string ToString()
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Class Property string Class {get;set;}
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HyperDriveRating Property string[] HyperDriveRating {get;set;}
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Width Property int Width {get;set;}
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_Crew Property System.Object _Crew {get;set;}
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_Ready Property string _Ready {get;set;}
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```
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Next step is to add individual getter and setter methods to access the hidden variable. In our scenario we don't
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need the setter part, because that's the action we want to restrict. But to show you the syntax I added the `SetCrew`
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method.
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```powershell
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Class DeathStar {
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[String]$Class = 'Space battle station'
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[Int]$Width = '160000'
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[String[]]$HyperDriveRating = @('Class 4', 'Class 20')
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hidden $_Crew = @{
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ImperialNavy = 342953
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Stormtroopers = 25984
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}
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hidden [string]$_Ready = $null
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DeathStar () {
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$this._Ready = $true
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}
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[string] GetReady () {
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return $this._Ready
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}
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[string] GetCrew ([string]$Type) {
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return $this._Crew.$Type
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}
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SetCrew ([string]$Type, [Int]$Value) {
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Write-Warning 'Apology accepted, Captain Needa.'
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$this._Crew.$Type = $Value
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}
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}
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```
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If you take a look at the members again, you get now a list off all _public_ properties and methods we've created.
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```console
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~> $DeathStarOne | Get-Member
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TypeName: DeathStar
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Name MemberType Definition
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---- ---------- ----------
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Equals Method bool Equals(System.Object obj)
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GetCrew Method string GetCrew(string Type)
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GetHashCode Method int GetHashCode()
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GetReady Method string GetReady()
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GetType Method type GetType()
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SetCrew Method void SetCrew(string Type, int Value)
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ToString Method string ToString()
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Class Property string Class {get;set;}
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HyperDriveRating Property string[] HyperDriveRating {get;set;}
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Width Property int Width {get;set;}
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```
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That's it. Now you can access to your data while using the methods `GetCrew()`, `SetCrew()` and `GetReady()`. Keep
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in mind you can always isolate properties with methods like this as an kind of interface.
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{: .box-warning}
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<i class="fa fa-bolt" aria-hidden="true"></i> **WARNING:** If you work with complex classes containing a huge amount of public and hidden
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properties, this could get a problem. Because the way how you get and set values depends now on either it's a
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property or methods. So you could loose track of all members and how to work with them.
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## Using Script Properties
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Script properties are special class members which executes `get` or `set` methods depending on the action.
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You can add script properties to the well known _PSCustomObject_ or classes. Therefore you can use the
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`Add-Member` function as well.
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Here is a simple example with a _PSCustomObject_ to show the syntax:
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```powershell
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$MyXWing = [PSCustomObject]@{
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'Model' = 'T-65'
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'Class' = 'Starfighter'
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'Crew' = @{
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'Pilot' = 1
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'Astromech Droid' = 1
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}
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'_Ready' = $true
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}
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$MyXWing | Add-Member -Name 'Ready' -MemberType ScriptProperty -Value {
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# Getter
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return $this._Ready
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} -SecondValue {
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# Setter
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Write-Warning 'This is a readonly property!'
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}
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```
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```console
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~> $MyXWing | Format-List
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Model : T-65
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Class : Starfighter
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Crew : {Pilot, Astromech Droid}
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_Ready : True
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Ready : True
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~> $MyXWing.Ready
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True
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~> $MyXWing.Ready = $false
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WARNING: This is a readonly property!
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```
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If we want to use the script property in a class we have to create it in the class constructor.
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The class constructor knows the keyword `$this` which refers to te current object.
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```powershell
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Class DeathStar {
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[String]$Class = 'Space battle station'
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[Int]$Width = '160000'
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[String[]]$HyperDriveRating = @('Class 4', 'Class 20')
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$Crew = @{
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ImperialNavy = 342953
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Stormtroopers = 25984
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}
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hidden [String]$_Ready = $null
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DeathStar () {
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$this._Ready = $true
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$this | Add-Member -MemberType ScriptProperty -Name 'Ready' -Value {
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# Getter
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return $this._Ready
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} -SecondValue {
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# Setter
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Write-Warning 'This is a readonly property!'
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}
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}
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}
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```
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```console
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~> $DeathStarOne = [DeathStar]::New()
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~> $DeathStarOne
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Ready : True
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Class : Space battle station
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Width : 160000
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HyperDriveRating : {Class 4, Class 20}
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Crew : {Stormtroopers, ImperialNavy}
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~> $DeathStarOne | Get-Member | Format-Table
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TypeName: DeathStar
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Name MemberType Definition
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---- ---------- ----------
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Equals Method bool Equals(System.Object obj)
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GetHashCode Method int GetHashCode()
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GetType Method type GetType()
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ToString Method string ToString()
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Class Property string Class {get;set;}
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Crew Property System.Object Crew {get;set;}
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HyperDriveRating Property string[] HyperDriveRating {get;set;}
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Width Property int Width {get;set;}
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Ready ScriptProperty System.Object Ready {get=...
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```
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And that's exactly what we wanted. We have hidden the the property `_Ready` and exposed a script property called
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`Ready`. We can now get or set the values of to the script property like we would do it with a _normal_ property.
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## Final Conclusion
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I personally like using _script properties_. But I take is on step further and create all public properties with a
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separate method:
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{% gist OCram85/d673764614438493afaa5cd413999436 AddPublicMember.ps1 %}
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This avoids making errors if I work with multiple constructors. Without a method like `AddPublicMember` you are
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forced to define each public property in every constructor method.
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All you have to do is to add the `AddPublicMember`method to your class definition and call it in every constructor.
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Finally our death start class looks like this:
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{{< gist OCram85 d673764614438493afaa5cd413999436>}}
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So what do you think? - Feel free to discuss this in the comment section down below.
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