about_Classes_Methods

Article
11/13/2023

Short description

Describes how to define methods for PowerShell classes.

Long description

Methods define the actions that a class can perform. Methods can take parameters that specify input data. Methods always define an output type. If a method doesn't return any output, it must have the Void output type. If a method doesn't explicitly define an output type, the method's output type is Void.

In class methods, no objects get sent to the pipeline except those specified in the return statement. There's no accidental output to the pipeline from the code.

Note

This is fundamentally different from how PowerShell functions handle output, where everything goes to the pipeline.

Nonterminating errors written to the error stream from inside a class method aren't passed through. You must use throw to surface a terminating error. Using the Write-* cmdlets, you can still write to PowerShell's output streams from within a class method. The cmdlets respect the preference variables in the calling scope. However, you should avoid using the Write-* cmdlets so that the method only outputs objects using the return statement.

Class methods can reference the current instance of the class object by using the $this automatic variable to access properties and other methods defined in the current class. The $this automatic variable isn't available in static methods.

Class methods can have any number of attributes, including the hidden and static attributes.

Syntax

Class methods use the following syntaxes:

One-line syntax

[[<attribute>]...] [hidden] [static] [<output-type>] <method-name> ([<method-parameters>]) { <body> }

Multiline syntax

[[<attribute>]...]
[hidden]
[static]
[<output-type>] <method-name> ([<method-parameters>]) {
  <body>
}

Examples

Example 1 - Minimal method definition

The GetVolume() method of the ExampleCube1 class returns the volume of the cube. It defines the output type as a floating number and returns the result of multiplying the Height, Length, and Width properties of the instance.

class ExampleCube1 {
    [float]   $Height
    [float]   $Length
    [float]   $Width

    [float] GetVolume() { return $this.Height * $this.Length * $this.Width }
}

$box = [ExampleCube1]@{
    Height = 2
    Length = 2
    Width  = 3
}

$box.GetVolume()

Example 2 - Method with parameters

The GeWeight() method takes a floating number input for the density of the cube and returns the weight of the cube, calculated as volume multiplied by density.

class ExampleCube2 {
    [float]   $Height
    [float]   $Length
    [float]   $Width

    [float] GetVolume() { return $this.Height * $this.Length * $this.Width }
    [float] GetWeight([float]$Density) {
        return $this.GetVolume() * $Density
    }
}

$cube = [ExampleCube2]@{
    Height = 2
    Length = 2
    Width  = 3
}

$cube.GetWeight(2.5)

Example 3 - Method without output

This example defines the Validate() method with the output type as System.Void. This method returns no output. Instead, if the validation fails, it throws an error. The GetVolume() method calls Validate() before calculating the volume of the cube. If validation fails, the method terminates before the calculation.

class ExampleCube3 {
    [float]   $Height
    [float]   $Length
    [float]   $Width

    [float] GetVolume() {
        $this.Validate()

        return $this.Height * $this.Length * $this.Width
    }

    [void] Validate() {
        $InvalidProperties = @()
        foreach ($Property in @('Height', 'Length', 'Width')) {
            if ($this.$Property -le 0) {
                $InvalidProperties += $Property
            }
        }

        if ($InvalidProperties.Count -gt 0) {
            $Message = @(
                'Invalid cube properties'
                "('$($InvalidProperties -join "', '")'):"
                "Cube dimensions must all be positive numbers."
            ) -join ' '
            throw $Message
        }
    }
}

$Cube = [ExampleCube3]@{ Length = 1 ; Width = -1 }
$Cube

$Cube.GetVolume()

Height Length Width
------ ------ -----
  0.00   1.00 -1.00

Exception:
Line |
  20 |              throw $Message
     |              ~~~~~~~~~~~~~~
     | Invalid cube properties ('Height', 'Width'): Cube dimensions must
     | all be positive numbers.

The method throws an exception because the Height and Width properties are invalid, preventing the class from calculating the current volume.

Example 4 - Static method with overloads

The ExampleCube4 class defines the static method GetVolume() with two overloads. The first overload has parameters for the dimensions of the cube and a flag to indicate whether the method should validate the input.

The second overload only includes the numeric inputs. It calls the first overload with $Static as $true. The second overload gives users a way to call the method without always having to define whether to strictly validate the input.

The class also defines GetVolume() as an instance (nonstatic) method. This method calls the second static overload, ensuring that the instance GetVolume() method always validates the cube's dimensions before returning the output value.

class ExampleCube4 {
    [float]   $Height
    [float]   $Length
    [float]   $Width

    static [float] GetVolume(
        [float]$Height,
        [float]$Length,
        [float]$Width,
        [boolean]$Strict
    ) {
        $Signature = "[ExampleCube4]::GetVolume({0}, {1}, {2}, {3})"
        $Signature = $Signature -f $Height, $Length, $Width, $Strict
        Write-Verbose "Called $Signature"

        if ($Strict) {
            [ValidateScript({$_ -gt 0 })]$Height = $Height
            [ValidateScript({$_ -gt 0 })]$Length = $Length
            [ValidateScript({$_ -gt 0 })]$Width  = $Width
        }

        return $Height * $Length * $Width
    }

    static [float] GetVolume([float]$Height, [float]$Length, [float]$Width) {
        $Signature = "[ExampleCube4]::GetVolume($Height, $Length, $Width)"
        Write-Verbose "Called $Signature"

        return [ExampleCube4]::GetVolume($Height, $Length, $Width, $true)
    }

    [float] GetVolume() {
        Write-Verbose "Called `$this.GetVolume()"
        return [ExampleCube4]::GetVolume(
            $this.Height,
            $this.Length,
            $this.Width
        )
    }
}

$VerbosePreference = 'Continue'
$Cube = [ExampleCube4]@{ Height = 2 ; Length = 2 }
$Cube.GetVolume()

VERBOSE: Called $this.GetVolume()
VERBOSE: Called [ExampleCube4]::GetVolume(2, 2, 0)
VERBOSE: Called [ExampleCube4]::GetVolume(2, 2, 0, True)

MetadataError:
Line |
  19 |              [ValidateScript({$_ -gt 0 })]$Width  = $Width
     |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | The variable cannot be validated because the value 0 is not a valid
     | value for the Width variable.

The verbose messages in the method definitions show how the initial call to $this.GetVolume() calls the static method.

Calling the static method directly with the Strict parameter as $false returns 0 for the volume.

[ExampleCube4]::GetVolume($Cube.Height, $Cube.Length, $Cube.Width, $false)

VERBOSE: Called [ExampleCube4]::GetVolume(2, 2, 0, False)
0

Method signatures and overloads

Every class method has a unique signature that defines how to call the method. The method's output type, name, and parameters define the method signature.

When a class defines more than one method with the same name, the definitions of that method are overloads. Overloads for a method must have different parameters. A method can't define two implementations with the same parameters, even if the output types are different.

The following class defines two methods, Shuffle() and Deal(). The Deal() method defines two overloads, one without any parameters and the other with the Count parameter.

class CardDeck {
    [string[]]$Cards  = @()
    hidden [string[]]$Dealt  = @()
    hidden [string[]]$Suits  = @('Clubs', 'Diamonds', 'Hearts', 'Spades')
    hidden [string[]]$Values = 2..10 + @('Jack', 'Queen', 'King', 'Ace')

    CardDeck() {
        foreach($Suit in $this.Suits) {
            foreach($Value in $this.Values) {
                $this.Cards += "$Value of $Suit"
            }
        }
        $this.Shuffle()
    }

    [void] Shuffle() {
        $this.Cards = $this.Cards + $this.Dealt | Where-Object -FilterScript {
             -not [string]::IsNullOrEmpty($_)
        } | Get-Random -Count $this.Cards.Count
    }

    [string] Deal() {
        if ($this.Cards.Count -eq 0) { throw "There are no cards left." }

        $Card        = $this.Cards[0]
        $this.Cards  = $this.Cards[1..$this.Cards.Count]
        $this.Dealt += $Card

        return $Card
    }

    [string[]] Deal([int]$Count) {
        if ($Count -gt $this.Cards.Count) {
            throw "There are only $($this.Cards.Count) cards left."
        } elseif ($Count -lt 1) {
            throw "You must deal at least 1 card."
        }

        return (1..$Count | ForEach-Object { $this.Deal() })
    }
}

Method output

By default, methods don't have any output. If a method signature includes an explicit output type other than Void, the method must return an object of that type. Methods don't emit any output except when the return keyword explicitly returns an object.

Method parameters

Class methods can define input parameters to use in the method body. Method parameters are enclosed in parentheses and are separated by commas. Empty parentheses indicate that the method requires no parameters.

Parameters can be defined on a single line or multiple lines. The following blocks show the syntax for method parameters.

([[<parameter-type>]]$<parameter-name>[, [[<parameter-type>]]$<parameter-name>])

(
    [[<parameter-type>]]$<parameter-name>[,
    [[<parameter-type>]]$<parameter-name>]
)

Method parameters can be strongly typed. If a parameter isn't typed, the method accepts any object for that parameter. If the parameter is typed, the method tries to convert the value for that parameter to the correct type, throwing an exception if the input can't be converted.

Method parameters can't define default values. All method parameters are mandatory.

Method parameters can't have any other attributes. This prevents methods from using parameters with the Validate* attributes. For more information about the validation attributes, see about_Functions_Advanced_Parameters.

You can use one of the following patterns to add validation to method parameters:

Reassign the parameters to the same variables with the required validation attributes. This works for both static and instance methods. For an example of this pattern, see Example 4.
Use Update-TypeData to define a ScriptMethod that uses validation attributes on the parameters directly. This only works for instance methods. For more information, see the Defining instance methods with Update-TypeData section.

Automatic variables in methods

Not all automatic variables are available in methods. The following list includes automatic variables and suggestions for whether and how to use them in PowerShell class methods. Automatic variables not included in the list aren't available to class methods.

$? - Access as normal.
$_ - Access as normal.
$args - Use the explicit parameter variables instead.
$ConsoleFileName - Access as $Script:ConsoleFileName instead.
$Error - Access as normal.
$EnabledExperimentalFeatures - Access as $Script:EnabledExperimentalFeatures instead.
$Event - Access as normal.
$EventArgs - Access as normal.
$EventSubscriber - Access as normal.
$ExecutionContext - Access as $Script:ExecutionContext instead.
$false - Access as normal.
$foreach - Access as normal.
$HOME - Access as $Script:HOME instead.
$Host - Access as $Script:Host instead.
$input - Use the explicit parameter variables instead.
$IsCoreCLR - Access as $Script:IsCoreCLR instead.
$IsLinux - Access as $Script:IsLinux instead.
$IsMacOS - Access as $Script:IsMacOS instead.
$IsWindows - Access as $Script:IsWindows instead.
$LASTEXITCODE - Access as normal.
$Matches - Access as normal.
$MyInvocation - Access as normal.
$NestedPromptLevel - Access as normal.
$null - Access as normal.
$PID - Access as $Script:PID instead.
$PROFILE - Access as $Script:PROFILE instead.
$PSBoundParameters - Don't use this variable. It's intended for cmdlets and functions. Using it in a class may have unexpected side effects.
$PSCmdlet - Don't use this variable. It's intended for cmdlets and functions. Using it in a class may have unexpected side effects.
$PSCommandPath - Access as normal.
$PSCulture - Access as $Script:PSCulture instead.
$PSEdition - Access as $Script:PSEdition instead.
$PSHOME - Access as $Script:PSHOME instead.
$PSItem - Access as normal.
$PSScriptRoot - Access as normal.
$PSSenderInfo - Access as $Script:PSSenderInfo instead.
$PSUICulture - Access as $Script:PSUICulture instead.
$PSVersionTable - Access as $Script:PSVersionTable instead.
$PWD - Access as normal.
$Sender - Access as normal.
$ShellId - Access as $Script:ShellId instead.
$StackTrace - Access as normal.
$switch - Access as normal.
$this - Access as normal. In a class method, $this is always the current instance of the class. You can access the class properties and methods with it. It's not available in static methods.
$true - Access as normal.

For more information about automatic variables, see about_Automatic_Variables.

Hidden methods

You can hide methods of a class by declaring them with the hidden keyword. Hidden class methods are:

Not included in the list of class members returned by the Get-Member cmdlet. To show hidden methods with Get-Member, use the Force parameter.
Not displayed in tab completion or IntelliSense unless the completion occurs in the class that defines the hidden method.
Public members of the class. They can be called and inherited. Hiding a method doesn't make it private. It only hides the method as described in the previous points.

Note

When you hide any overload for a method, that method is removed from IntelliSense, completion results, and the default output for Get-Member.

For more information about the hidden keyword, see about_Hidden.

Static methods

You can define a method as belonging to the class itself instead of instances of the class by declaring the method with the static keyword. Static class methods:

Are always available, independent of class instantiation.
Are shared across all instances of the class.
Are always available.
Can't access instance properties of the class. They can only access static properties.
Live for the entire session span.

Derived class methods

When a class derives from a base class, it inherits the methods of the base class and their overloads. Any method overloads defined on the base class, including hidden methods, are available on the derived class.

A derived class can override an inherited method overload by redefining it in the class definition. To override the overload, the parameter types must be the same as for the base class. The output type for the overload can be different.

Unlike constructors, methods can't use the : base(<parameters>) syntax to invoke a base class overload for the method. The redefined overload on the derived class completely replaces the overload defined by the base class.

The following example shows the behavior for static and instance methods on derived classes.

The base class defines:

The static methods Now() for returning the current time and DaysAgo() for returning a date in the past.
The instance property TimeStamp and a ToString() instance method that returns the string representation of that property. This ensures that when an instance is used in a string it converts to the datetime string instead of the class name.
The instance method SetTimeStamp() with two overloads. When the method is called without parameters, it sets the TimeStamp to the current time. When the method is called with a DateTime, it sets the TimeStamp to that value.

class BaseClass {
    static [datetime] Now() {
        return Get-Date
    }
    static [datetime] DaysAgo([int]$Count) {
        return [BaseClass]::Now().AddDays(-$Count)
    }

    [datetime] $TimeStamp = [BaseClass]::Now()

    [string] ToString() {
        return $this.TimeStamp.ToString()
    }

    [void] SetTimeStamp([datetime]$TimeStamp) {
        $this.TimeStamp = $TimeStamp
    }
    [void] SetTimeStamp() {
        $this.TimeStamp = [BaseClass]::Now()
    }
}

The next block defines classes derived from BaseClass:

DerivedClassA inherits from BaseClass without any overrides.
DerivedClassB overrides the DaysAgo() static method to return a string representation instead of the DateTime object. It also overrides the ToString() instance method to return the timestamp as an ISO8601 date string.
DerivedClassC overrides the parameterless overload of the SetTimeStamp() method so that setting the timestamp without parameters sets the date to 10 days before the current date.

class DerivedClassA : BaseClass     {}
class DerivedClassB : BaseClass     {
    static [string] DaysAgo([int]$Count) {
        return [BaseClass]::DaysAgo($Count).ToString('yyyy-MM-dd')
    }
    [string] ToString() {
        return $this.TimeStamp.ToString('yyyy-MM-dd')
    }
}
class DerivedClassC : BaseClass {
    [void] SetTimeStamp() {
        $this.SetTimeStamp([BaseClass]::Now().AddDays(-10))
    }
}

The following block shows the output of the static Now() method for the defined classes. The output is the same for every class, because the derived classes don't override the base class implementation of the method.

"[BaseClass]::Now()     => $([BaseClass]::Now())"
"[DerivedClassA]::Now() => $([DerivedClassA]::Now())"
"[DerivedClassB]::Now() => $([DerivedClassB]::Now())"
"[DerivedClassC]::Now() => $([DerivedClassC]::Now())"

[BaseClass]::Now()     => 11/06/2023 09:41:23
[DerivedClassA]::Now() => 11/06/2023 09:41:23
[DerivedClassB]::Now() => 11/06/2023 09:41:23
[DerivedClassC]::Now() => 11/06/2023 09:41:23

The next block calls the DaysAgo() static method of each class. Only the output for DerivedClassB is different, because it overrode the base implementation.

"[BaseClass]::DaysAgo(3)     => $([BaseClass]::DaysAgo(3))"
"[DerivedClassA]::DaysAgo(3) => $([DerivedClassA]::DaysAgo(3))"
"[DerivedClassB]::DaysAgo(3) => $([DerivedClassB]::DaysAgo(3))"
"[DerivedClassC]::DaysAgo(3) => $([DerivedClassC]::DaysAgo(3))"

[BaseClass]::DaysAgo(3)     => 11/03/2023 09:41:38
[DerivedClassA]::DaysAgo(3) => 11/03/2023 09:41:38
[DerivedClassB]::DaysAgo(3) => 2023-11-03
[DerivedClassC]::DaysAgo(3) => 11/03/2023 09:41:38

The following block shows the string presentation of a new instance for each class. The representation for DerivedClassB is different because it overrode the ToString() instance method.

"`$base = [BaseClass]::New()     => $($base = [BaseClass]::New(); $base)"
"`$a    = [DerivedClassA]::New() => $($a = [DerivedClassA]::New(); $a)"
"`$b    = [DerivedClassB]::New() => $($b = [DerivedClassB]::New(); $b)"
"`$c    = [DerivedClassC]::New() => $($c = [DerivedClassC]::New(); $c)"

$base = [BaseClass]::New()     => 11/6/2023 9:44:57 AM
$a    = [DerivedClassA]::New() => 11/6/2023 9:44:57 AM
$b    = [DerivedClassB]::New() => 2023-11-06
$c    = [DerivedClassC]::New() => 11/6/2023 9:44:57 AM

The next block calls the SetTimeStamp() instance method for each instance, setting the TimeStamp property to a specific date. Each instance has the same date, because none of the derived classes override the parameterized overload for the method.

[datetime]$Stamp = '2024-10-31'
"`$base.SetTimeStamp(`$Stamp) => $($base.SetTimeStamp($Stamp) ; $base)"
"`$a.SetTimeStamp(`$Stamp)    => $($a.SetTimeStamp($Stamp); $a)"
"`$b.SetTimeStamp(`$Stamp)    => $($b.SetTimeStamp($Stamp); $b)"
"`$c.SetTimeStamp(`$Stamp)    => $($c.SetTimeStamp($Stamp); $c)"

$base.SetTimeStamp($Stamp) => 10/31/2024 12:00:00 AM
$a.SetTimeStamp($Stamp)    => 10/31/2024 12:00:00 AM
$b.SetTimeStamp($Stamp)    => 2024-10-31
$c.SetTimeStamp($Stamp)    => 10/31/2024 12:00:00 AM

The last block calls SetTimeStamp() without any parameters. The output shows that the value for the DerivedClassC instance is set to 10 days before the others.

"`$base.SetTimeStamp() => $($base.SetTimeStamp() ; $base)"
"`$a.SetTimeStamp()    => $($a.SetTimeStamp(); $a)"
"`$b.SetTimeStamp()    => $($b.SetTimeStamp(); $b)"
"`$c.SetTimeStamp()    => $($c.SetTimeStamp(); $c)"

$base.SetTimeStamp() => 11/6/2023 9:53:58 AM
$a.SetTimeStamp()    => 11/6/2023 9:53:58 AM
$b.SetTimeStamp()    => 2023-11-06
$c.SetTimeStamp()    => 10/27/2023 9:53:58 AM

Defining instance methods with Update-TypeData

Beyond declaring methods directly in the class definition, you can define methods for instances of a class in the static constructor using the Update-TypeData cmdlet.

Use this snippet as a starting point for the pattern. Replace the placeholder text in angle brackets as needed.

class <ClassName> {
    static [hashtable[]] $MemberDefinitions = @(
        @{
            MemberName = '<MethodName>'
            MemberType = 'ScriptMethod'
            Value      = {
              param(<method-parameters>)

              <method-body>
            }
        }
    )

    static <ClassName>() {
        $TypeName = [<ClassName>].Name
        foreach ($Definition in [<ClassName>]::MemberDefinitions) {
            Update-TypeData -TypeName $TypeName @Definition
        }
    }
}

Tip

The Add-Member cmdlet can add properties and methods to a class in non-static constructors, but the cmdlet runs every time the constructor is called. Using Update-TypeData in the static constructor ensures that the code for adding the members to the class only needs to run once in a session.

Defining methods with default parameter values and validation attributes

Methods defined directly in a class declaration can't define default values or validation attributes on the method parameters. To define class methods with default values or validation attributes, they must be defined as ScriptMethod members.

In this example, the CardDeck class defines a Draw() method that uses both a validation attribute and a default value for the Count parameter.

class CookieJar {
    [int] $Cookies = 12

    static [hashtable[]] $MemberDefinitions = @(
        @{
            MemberName = 'Eat'
            MemberType = 'ScriptMethod'
            Value      = {
                param(
                    [ValidateScript({ $_ -ge 1 -and $_ -le $this.Cookies })]
                    [int] $Count = 1
                )

                $this.Cookies -= $Count
                if ($Count -eq 1) {
                    "You ate 1 cookie. There are $($this.Cookies) left."
                } else {
                    "You ate $Count cookies. There are $($this.Cookies) left."
                }
            }
        }
    )

    static CookieJar() {
        $TypeName = [CookieJar].Name
        foreach ($Definition in [CookieJar]::MemberDefinitions) {
            Update-TypeData -TypeName $TypeName @Definition
        }
    }
}

$Jar = [CookieJar]::new()
$Jar.Eat(1)
$Jar.Eat()
$Jar.Eat(20)
$Jar.Eat(6)

You ate 1 cookie. There are 11 left.

You ate 1 cookie. There are 10 left.

MethodInvocationException:
Line |
  36 |  $Jar.Eat(20)
     |  ~~~~~~~~~~~~
     | Exception calling "Eat" with "1" argument(s): "The attribute
     | cannot be added because variable Count with value 20 would no
     | longer be valid."

You ate 6 cookies. There are 4 left.

Note

While this pattern works for validation attributes, notice that the exception is misleading, referencing an inability to add an attribute. It might be a better user experience to explicitly check the value for the parameter and raise a meaningful error instead. That way, users can understand why they're seeing the error and what to do about it.

Limitations

PowerShell class methods have the following limitations:

Method parameters can't use any attributes, including validation attributes.

Workaround: Reassign the parameters in the method body with the validation attribute or define the method in the static constructor with the Update-TypeData cmdlet.
Method parameters can't define default values. The parameters are always mandatory.

Workaround: Define the method in the static constructor with the Update-TypeData cmdlet.
Methods are always public, even when they're hidden. They can be overridden when the class is inherited.

Workaround: None.
If any overload of a method is hidden, every overload for that method is treated as hidden too.

Workaround: None.

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about_Classes_Methods

Short description

Long description

Syntax

One-line syntax

Multiline syntax

Examples

Example 1 - Minimal method definition

Example 2 - Method with parameters

Example 3 - Method without output

Example 4 - Static method with overloads

Method signatures and overloads

Method output

Method parameters

Automatic variables in methods

Hidden methods

Static methods

Derived class methods

Defining instance methods with Update-TypeData

Defining methods with default parameter values and validation attributes

Limitations

See also

Additional resources