about_Object_Creation

Άρθρο
12/13/2022

Short description

Explains how to create objects in PowerShell.

Long description

You can create objects in PowerShell and use the objects that you create in commands and scripts.

There are many ways to create objects, this list is not definitive:

New-Object: Creates an instance of a .NET Framework object or COM object.
Import-Csv/ConvertFrom-CSV: Creates custom objects (PSCustomObject) from the items defined as character separated values.
ConvertFrom-Json: Creates custom objects defined in JavaScript Object Notation (JSON).
ConvertFrom-StringData: Creates custom objects defined as key value pairs.
Add-Type: Allows you to define a class in your PowerShell session that you can instantiate with New-Object.
New-Module: The AsCustomObject parameter creates a custom object you define using script block.
Add-Member: Adds properties to existing objects. You can use Add-Member to create a custom object out of a simple type, like [System.Int32].
Select-Object: Selects properties on an object. You can use Select-Object to create custom and calculated properties on an already instantiated object.

The following additional methods are covered in this article:

By calling a type's constructor using a static new() method
By typecasting hash tables of property names and property values

Static new() method

All .NET types have a new() method that allows you to construct instances more easily. You can also see all the available constructors for a given type.

To see the constructors for a type, specify the new method name after the type name and press <ENTER>.

[System.Uri]::new

OverloadDefinitions
-------------------
uri new(string uriString)
uri new(string uriString, bool dontEscape)
uri new(uri baseUri, string relativeUri, bool dontEscape)
uri new(string uriString, System.UriKind uriKind)
uri new(uri baseUri, string relativeUri)
uri new(uri baseUri, uri relativeUri)

Now, you can create a System.Uri by specifying the appropriate constructor.

[System.Uri]::new("https://www.bing.com")

AbsolutePath   : /
AbsoluteUri    : https://www.bing.com/
LocalPath      : /
Authority      : www.bing.com
...

You can use the following sample to determine what .NET types are currently loaded for you to instantiate.

[AppDomain]::CurrentDomain.GetAssemblies() |
  ForEach-Object {
    $_.GetExportedTypes() |
      ForEach-Object { $_.FullName }
  }

Objects created using the new() method may not have the same properties as objects of the same type that are created by PowerShell cmdlets. PowerShell cmdlets, providers, and Extended Type System can add extra properties to the instance.

For example, the FileSystem provider in PowerShell adds six NoteProperty values to the DirectoryInfo object returned by Get-Item.

$PSDirInfo = Get-Item /
$PSDirInfo | Get-Member | Group-Object MemberType | Select-Object Count, Name

Count Name
----- ----
    4 CodeProperty
   13 Property
    6 NoteProperty
    1 ScriptProperty
   18 Method

When you create a DirectoryInfo object directly, it does not have those six NoteProperty values.

$NewDirInfo = [System.IO.DirectoryInfo]::new('/')
$NewDirInfo | Get-Member | Group-Object MemberType | Select-Object Count, Name

Count Name
----- ----
    4 CodeProperty
   13 Property
    1 ScriptProperty
   18 Method

For more information about the Extended Type System, see about_Types.ps1xml.

This feature was added in PowerShell 5.0

Create objects from hash tables

You can create an object from a hash table of properties and property values.

The syntax is as follows:

[<class-name>]@{
  <property-name>=<property-value>
  <property-name>=<property-value>
}

This method works only for classes that have a parameterless constructor. The object properties must be public and settable.

This feature was added in PowerShell version 3.0

Create custom objects from hash tables

Custom objects are very useful and are easy to create using the hash table method. The PSCustomObject class is designed specifically for this purpose.

Custom objects are an excellent way to return customized output from a function or script. This is more useful than returning formatted output that cannot be reformatted or piped to other commands.

The commands in the Test-Object function set some variable values and then use those values to create a custom object. You can see this object in use in the example section of the Update-Help cmdlet help topic.

function Test-Object {
  $ModuleName = "PSScheduledJob"
  $HelpCulture = "en-us"
  $HelpVersion = "3.1.0.0"
  [PSCustomObject]@{
    "ModuleName"=$ModuleName
    "UICulture"=$HelpCulture
    "Version"=$HelpVersion
  }
  $ModuleName = "PSWorkflow"
  $HelpCulture = "en-us"
  $HelpVersion = "3.0.0.0"
  [PSCustomObject]@{
    "ModuleName"=$ModuleName
    "UICulture"=$HelpCulture
    "Version"=$HelpVersion
  }
}
Test-Object

The output of this function is a collection of custom objects formatted as a table by default.

ModuleName        UICulture      Version
---------         ---------      -------
PSScheduledJob    en-us          3.1.0.0
PSWorkflow        en-us          3.0.0.0

Users can manage the properties of the custom objects just as they do with standard objects.

(Test-Object).ModuleName

 PSScheduledJob
 PSWorkflow

PSObject type objects maintain the list of members in the order that the members were added to the object. Even though Hashtable objects don't guarantee the order of the key-value pairs, casting a literal hashtable to [pscustomobject] maintains the order.

The hashtable must be a literal. If you wrap the hashtable in parentheses or if you cast a variable containing a hashtable, there is no guarantee that the order is preserved.

$hash = @{
    Name      = "Server30"
    System    = "Server Core"
    PSVersion = "4.0"
}
$Asset = [pscustomobject]$hash
$Asset

System      Name     PSVersion
------      ----     ---------
Server Core Server30 4.0

Create non-custom objects from hash tables

You can also use hash tables to create objects for non-custom classes. When you create an object for a non-custom class, the namespace-qualified type name is required, although you may omit any initial System namespace component.

For example, the following command creates a session option object.

[System.Management.Automation.Remoting.PSSessionOption]@{
  IdleTimeout=43200000
  SkipCnCheck=$True
}

The requirements of the hash table feature, especially the parameterless constructor requirement, eliminate many existing classes. However, most PowerShell option classes are designed to work with this feature, as well as other very useful classes, such as the ProcessStartInfo class.

[System.Diagnostics.ProcessStartInfo]@{
  CreateNoWindow="$true"
  Verb="run as"
}

Arguments               :
ArgumentList            : {}
CreateNoWindow          : True
EnvironmentVariables    : {OneDriveConsumer, PROCESSOR_ARCHITECTURE,
                           CommonProgramFiles(x86), APPDATA...}
Environment             : {[OneDriveConsumer, C:\Users\user1\OneDrive],
                           [PROCESSOR_ARCHITECTURE, AMD64],
                           [CommonProgramFiles(x86),
                           C:\Program Files (x86)\Common Files],
                           [APPDATA, C:\Users\user1\AppData\Roaming]...}
RedirectStandardInput   : False
RedirectStandardOutput  : False
RedirectStandardError   : False
...

You can also use the hash table feature when setting parameter values. For example, the value of the SessionOption parameter of the New-PSSession. cmdlet can be a hash table.

New-PSSession -ComputerName Server01 -SessionOption @{
  IdleTimeout=43200000
  SkipCnCheck=$True
}
Register-ScheduledJob Name Test -FilePath .\Get-Inventory.ps1 -Trigger @{
  Frequency="Daily"
  At="15:00"
}

Generic objects

You can also create generic objects in PowerShell. Generics are classes, structures, interfaces, and methods that have placeholders (type parameters) for one or more of the types that they store or use.

The following example creates a Dictionary object.

$dict = New-Object 'System.Collections.Generic.Dictionary[String,Int]'
$dict.Add("One", 1)
$dict

Key Value
--- -----
One     1

For more information on Generics, see Generics in .NET.

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