about_Classes_Inheritance
Short description
Describes how you can define classes that extend other types.
Long description
PowerShell classes support inheritance, which allows you to define a child class that reuses (inherits), extends, or modifies the behavior of a parent class. The class whose members are inherited is called the base class. The class that inherits the members of the base class is called the derived class.
PowerShell supports single inheritance only. A class can only inherit from a single class. However, inheritance is transitive, which allows you to define an inheritance hierarchy for a set of types. In other words, type D can inherit from type C, which inherits from type B, which inherits from the base class type A. Because inheritance is transitive, the members of type A are available to type D.
Derived classes don't inherit all members of the base class. The following members aren't inherited:
- Static constructors, which initialize the static data of a class.
- Instance constructors, which you call to create a new instance of the class. Each class must define its own constructors.
You can extend a class by creating a new class that derives from an existing class. The derived class inherits the properties and methods of the base class. You can add or override the base class members as required.
Classes can also inherit from interfaces, which define a contract. A class that inherits from an interface must implement that contract. When it does, the class is usable like any other class implementing that interface. If a class inherits from an interface but doesn't implement the interface, PowerShell raises a parsing error for the class.
Some PowerShell operators depend on a class implementing a specific interface.
For example, the -eq operator only checks for reference equality unless the
class implements the System.IEquatable interface. The -le, -lt, -ge,
and -gt operators only work on classes that implement the
System.IComparable interface.
A derived class uses the : syntax to extend a base class or implement
interfaces. The derived class should always be leftmost in the class
declaration.
This example shows the basic PowerShell class inheritance syntax.
Class Derived : Base {...}
This example shows inheritance with an interface declaration coming after the base class.
Class Derived : Base, Interface {...}
Syntax
Class inheritance uses the following syntaxes:
One line syntax
class <derived-class-name> : <base-class-or-interface-name>[, <interface-name>...] {
<derived-class-body>
}
For example:
# Base class only
class Derived : Base {...}
# Interface only
class Derived : System.IComparable {...}
# Base class and interface
class Derived : Base, System.IComparable {...}
Multiline syntax
class <derived-class-name> : <base-class-or-interface-name>[,
<interface-name>...] {
<derived-class-body>
}
For example:
class Derived : Base,
System.IComparable,
System.IFormattable,
System.IConvertible {
# Derived class definition
}
Examples
Example 1 - Inheriting and overriding from a base class
The following example shows the behavior of inherited properties with and without overriding. Run the code blocks in order after reading their description.
Defining the base class
The first code block defines PublishedWork as a base class. It has two
static properties, List and Artists. Next, it defines the static
RegisterWork() method to add works to the static List property and the
artists to the Artists property, writing a message for each new entry in
the lists.
The class defines three instance properties that describe a published work.
Finally, it defines the Register() and ToString() instance methods.
class PublishedWork {
static [PublishedWork[]] $List = @()
static [string[]] $Artists = @()
static [void] RegisterWork([PublishedWork]$Work) {
$wName = $Work.Name
$wArtist = $Work.Artist
if ($Work -notin [PublishedWork]::List) {
Write-Verbose "Adding work '$wName' to works list"
[PublishedWork]::List += $Work
} else {
Write-Verbose "Work '$wName' already registered."
}
if ($wArtist -notin [PublishedWork]::Artists) {
Write-Verbose "Adding artist '$wArtist' to artists list"
[PublishedWork]::Artists += $wArtist
} else {
Write-Verbose "Artist '$wArtist' already registered."
}
}
static [void] ClearRegistry() {
Write-Verbose "Clearing PublishedWork registry"
[PublishedWork]::List = @()
[PublishedWork]::Artists = @()
}
[string] $Name
[string] $Artist
[string] $Category
[void] Init([string]$WorkType) {
if ([string]::IsNullOrEmpty($this.Category)) {
$this.Category = "${WorkType}s"
}
}
PublishedWork() {
$WorkType = $this.GetType().FullName
$this.Init($WorkType)
Write-Verbose "Defined a published work of type [$WorkType]"
}
PublishedWork([string]$Name, [string]$Artist) {
$WorkType = $this.GetType().FullName
$this.Name = $Name
$this.Artist = $Artist
$this.Init($WorkType)
Write-Verbose "Defined '$Name' by $Artist as a published work of type [$WorkType]"
}
PublishedWork([string]$Name, [string]$Artist, [string]$Category) {
$WorkType = $this.GetType().FullName
$this.Name = $Name
$this.Artist = $Artist
$this.Init($WorkType)
Write-Verbose "Defined '$Name' by $Artist ($Category) as a published work of type [$WorkType]"
}
[void] Register() { [PublishedWork]::RegisterWork($this) }
[string] ToString() { return "$($this.Name) by $($this.Artist)" }
}
Defining a derived class without overrides
The first derived class is Album. It doesn't override any properties or methods. It adds a new instance property, Genres, that doesn't exist on the base class.
class Album : PublishedWork {
[string[]] $Genres = @()
}
The following code block shows the behavior of the derived Album class.
First, it sets the $VerbosePreference so that the messages from the class
methods emit to the console. It creates three instances of the class, shows
them in a table, and then registers them with the inherited static
RegisterWork() method. It then calls the same static method on the base class
directly.
$VerbosePreference = 'Continue'
$Albums = @(
[Album]@{
Name = 'The Dark Side of the Moon'
Artist = 'Pink Floyd'
Genres = 'Progressive rock', 'Psychedelic rock'
}
[Album]@{
Name = 'The Wall'
Artist = 'Pink Floyd'
Genres = 'Progressive rock', 'Art rock'
}
[Album]@{
Name = '36 Chambers'
Artist = 'Wu-Tang Clan'
Genres = 'Hip hop'
}
)
$Albums | Format-Table
$Albums | ForEach-Object { [Album]::RegisterWork($_) }
$Albums | ForEach-Object { [PublishedWork]::RegisterWork($_) }
VERBOSE: Defined a published work of type [Album]
VERBOSE: Defined a published work of type [Album]
VERBOSE: Defined a published work of type [Album]
Genres Name Artist Category
------ ---- ------ --------
{Progressive rock, Psychedelic rock} The Dark Side of the Moon Pink Floyd Albums
{Progressive rock, Art rock} The Wall Pink Floyd Albums
{Hip hop} 36 Chambers Wu-Tang Clan Albums
VERBOSE: Adding work 'The Dark Side of the Moon' to works list
VERBOSE: Adding artist 'Pink Floyd' to artists list
VERBOSE: Adding work 'The Wall' to works list
VERBOSE: Artist 'Pink Floyd' already registered.
VERBOSE: Adding work '36 Chambers' to works list
VERBOSE: Adding artist 'Wu-Tang Clan' to artists list
VERBOSE: Work 'The Dark Side of the Moon' already registered.
VERBOSE: Artist 'Pink Floyd' already registered.
VERBOSE: Work 'The Wall' already registered.
VERBOSE: Artist 'Pink Floyd' already registered.
VERBOSE: Work '36 Chambers' already registered.
VERBOSE: Artist 'Wu-Tang Clan' already registered.
Notice that even though the Album class didn't define a value for Category or any constructors, the property was defined by the default constructor of the base class.
In the verbose messaging, the second call to the RegisterWork() method
reports that the works and artists are already registered. Even though the
first call to RegisterWork() was for the derived Album class, it used the
inherited static method from the base PublishedWork class. That method
updated the static List and Artist properties on the base class, which
the derived class didn't override.
The next code block clears the registry and calls the Register() instance
method on the Album objects.
[PublishedWork]::ClearRegistry()
$Albums.Register()
VERBOSE: Clearing PublishedWork registry
VERBOSE: Adding work 'The Dark Side of the Moon' to works list
VERBOSE: Adding artist 'Pink Floyd' to artists list
VERBOSE: Adding work 'The Wall' to works list
VERBOSE: Artist 'Pink Floyd' already registered.
VERBOSE: Adding work '36 Chambers' to works list
VERBOSE: Adding artist 'Wu-Tang Clan' to artists list
The instance method on the Album objects has the same effect as calling the static method on the derived or base class.
The following code block compares the static properties for the base class and the derived class, showing that they're the same.
[pscustomobject]@{
'[PublishedWork]::List' = [PublishedWork]::List -join ",`n"
'[Album]::List' = [Album]::List -join ",`n"
'[PublishedWork]::Artists' = [PublishedWork]::Artists -join ",`n"
'[Album]::Artists' = [Album]::Artists -join ",`n"
'IsSame::List' = (
[PublishedWork]::List.Count -eq [Album]::List.Count -and
[PublishedWork]::List.ToString() -eq [Album]::List.ToString()
)
'IsSame::Artists' = (
[PublishedWork]::Artists.Count -eq [Album]::Artists.Count -and
[PublishedWork]::Artists.ToString() -eq [Album]::Artists.ToString()
)
} | Format-List
[PublishedWork]::List : The Dark Side of the Moon by Pink Floyd,
The Wall by Pink Floyd,
36 Chambers by Wu-Tang Clan
[Album]::List : The Dark Side of the Moon by Pink Floyd,
The Wall by Pink Floyd,
36 Chambers by Wu-Tang Clan
[PublishedWork]::Artists : Pink Floyd,
Wu-Tang Clan
[Album]::Artists : Pink Floyd,
Wu-Tang Clan
IsSame::List : True
IsSame::Artists : True
Defining a derived class with overrides
The next code block defines the Illustration class inheriting from the base
PublishedWork class. The new class extends the base class by defining the
Medium instance property with a default value of Unknown.
Unlike the derived Album class, Illustration overrides the following properties and methods:
- It overrides the static Artists property. The definition is the same, but the Illustration class declares it directly.
- It overrides the Category instance property, setting the default value to
Illustrations. - It overrides the
ToString()instance method so the string representation of an illustration includes the medium it was created with.
The class also defines the static RegisterIllustration() method to first call
the base class RegisterWork() method and then add the artist to the
overridden Artists static property on the derived class.
Finally, the class overrides all three constructors:
- The default constructor is empty except for a verbose message indicating it created an illustration.
- The next constructor takes two string values for the name and artist that created the illustration. Instead of implementing the logic for setting the Name and Artist properties, the constructor calls the appropriate constructor from the base class.
- The last constructor takes three string values for the name, artist, and medium of the illustration. Both constructors write a verbose message indicating that they created an illustration.
class Illustration : PublishedWork {
static [string[]] $Artists = @()
static [void] RegisterIllustration([Illustration]$Work) {
$wArtist = $Work.Artist
[PublishedWork]::RegisterWork($Work)
if ($wArtist -notin [Illustration]::Artists) {
Write-Verbose "Adding illustrator '$wArtist' to artists list"
[Illustration]::Artists += $wArtist
} else {
Write-Verbose "Illustrator '$wArtist' already registered."
}
}
[string] $Category = 'Illustrations'
[string] $Medium = 'Unknown'
[string] ToString() {
return "$($this.Name) by $($this.Artist) ($($this.Medium))"
}
Illustration() {
Write-Verbose 'Defined an illustration'
}
Illustration([string]$Name, [string]$Artist) : base($Name, $Artist) {
Write-Verbose "Defined '$Name' by $Artist ($($this.Medium)) as an illustration"
}
Illustration([string]$Name, [string]$Artist, [string]$Medium) {
$this.Name = $Name
$this.Artist = $Artist
$this.Medium = $Medium
Write-Verbose "Defined '$Name' by $Artist ($Medium) as an illustration"
}
}
The following code block shows the behavior of the derived Illustration
class. It creates three instances of the class, shows them in a table, and then
registers them with the inherited static RegisterWork() method. It then calls
the same static method on the base class directly. Finally, it writes messages
showing the list of registered artists for the base class and the derived
class.
$Illustrations = @(
[Illustration]@{
Name = 'The Funny Thing'
Artist = 'Wanda Gág'
Medium = 'Lithography'
}
[Illustration]::new('Millions of Cats', 'Wanda Gág')
[Illustration]::new(
'The Lion and the Mouse',
'Jerry Pinkney',
'Watercolor'
)
)
$Illustrations | Format-Table
$Illustrations | ForEach-Object { [Illustration]::RegisterIllustration($_) }
$Illustrations | ForEach-Object { [PublishedWork]::RegisterWork($_) }
"Published work artists: $([PublishedWork]::Artists -join ', ')"
"Illustration artists: $([Illustration]::Artists -join ', ')"
VERBOSE: Defined a published work of type [Illustration]
VERBOSE: Defined an illustration
VERBOSE: Defined 'Millions of Cats' by Wanda Gág as a published work of type [Illustration]
VERBOSE: Defined 'Millions of Cats' by Wanda Gág (Unknown) as an illustration
VERBOSE: Defined a published work of type [Illustration]
VERBOSE: Defined 'The Lion and the Mouse' by Jerry Pinkney (Watercolor) as an illustration
Category Medium Name Artist
-------- ------ ---- ------
Illustrations Lithography The Funny Thing Wanda Gág
Illustrations Unknown Millions of Cats Wanda Gág
Illustrations Watercolor The Lion and the Mouse Jerry Pinkney
VERBOSE: Adding work 'The Funny Thing' to works list
VERBOSE: Adding artist 'Wanda Gág' to artists list
VERBOSE: Adding illustrator 'Wanda Gág' to artists list
VERBOSE: Adding work 'Millions of Cats' to works list
VERBOSE: Artist 'Wanda Gág' already registered.
VERBOSE: Illustrator 'Wanda Gág' already registered.
VERBOSE: Adding work 'The Lion and the Mouse' to works list
VERBOSE: Adding artist 'Jerry Pinkney' to artists list
VERBOSE: Adding illustrator 'Jerry Pinkney' to artists list
VERBOSE: Work 'The Funny Thing' already registered.
VERBOSE: Artist 'Wanda Gág' already registered.
VERBOSE: Work 'Millions of Cats' already registered.
VERBOSE: Artist 'Wanda Gág' already registered.
VERBOSE: Work 'The Lion and the Mouse' already registered.
VERBOSE: Artist 'Jerry Pinkney' already registered.
Published work artists: Pink Floyd, Wu-Tang Clan, Wanda Gág, Jerry Pinkney
Illustration artists: Wanda Gág, Jerry Pinkney
The verbose messaging from creating the instances shows that:
- When creating the first instance, the base class default constructor was called before the derived class default constructor.
- When creating the second instance, the explicitly inherited constructor was called for the base class before the derived class constructor.
- When creating the third instance, the base class default constructor was called before the derived class constructor.
The verbose messages from the RegisterWork() method indicate that the works
and artists were already registered. This is because the
RegisterIllustration() method called the RegisterWork() method internally.
However, when comparing the value of the static Artist property for both the base class and derived class, the values are different. The Artists property for the derived class only includes illustrators, not the album artists. Redefining the Artist property in the derived class prevents the class from returning the static property on the base class.
The final code block calls the ToString() method on the entries of the
static List property on the base class.
[PublishedWork]::List | ForEach-Object -Process { $_.ToString() }
The Dark Side of the Moon by Pink Floyd
The Wall by Pink Floyd
36 Chambers by Wu-Tang Clan
The Funny Thing by Wanda Gág (Lithography)
Millions of Cats by Wanda Gág (Unknown)
The Lion and the Mouse by Jerry Pinkney (Watercolor)
The Album instances only return the name and artist in their string. The
Illustration instances also included the medium in parentheses, because
that class overrode the ToString() method.
Example 2 - Implementing interfaces
The following example shows how a class can implement one or more interfaces. The example extends the definition of a Temperature class to support more operations and behaviors.
Initial class definition
Before implementing any interfaces, the Temperature class is defined with two properties, Degrees and Scale. It defines constructors and three instance methods for returning the instance as degrees of a particular scale.
The class defines the available scales with the TemperatureScale enumeration.
class Temperature {
[float] $Degrees
[TemperatureScale] $Scale
Temperature() {}
Temperature([float] $Degrees) { $this.Degrees = $Degrees }
Temperature([TemperatureScale] $Scale) { $this.Scale = $Scale }
Temperature([float] $Degrees, [TemperatureScale] $Scale) {
$this.Degrees = $Degrees
$this.Scale = $Scale
}
[float] ToKelvin() {
switch ($this.Scale) {
Celsius { return $this.Degrees + 273.15 }
Fahrenheit { return ($this.Degrees + 459.67) * 5/9 }
}
return $this.Degrees
}
[float] ToCelsius() {
switch ($this.Scale) {
Fahrenheit { return ($this.Degrees - 32) * 5/9 }
Kelvin { return $this.Degrees - 273.15 }
}
return $this.Degrees
}
[float] ToFahrenheit() {
switch ($this.Scale) {
Celsius { return $this.Degrees * 9/5 + 32 }
Kelvin { return $this.Degrees * 9/5 - 459.67 }
}
return $this.Degrees
}
}
enum TemperatureScale {
Celsius = 0
Fahrenheit = 1
Kelvin = 2
}
However, in this basic implementation, there's a few limitations as shown in the following example output:
$Celsius = [Temperature]::new()
$Fahrenheit = [Temperature]::new([TemperatureScale]::Fahrenheit)
$Kelvin = [Temperature]::new(0, 'Kelvin')
$Celsius, $Fahrenheit, $Kelvin
"The temperatures are: $Celsius, $Fahrenheit, $Kelvin"
[Temperature]::new() -eq $Celsius
$Celsius -gt $Kelvin
Degrees Scale
------- -----
0.00 Celsius
0.00 Fahrenheit
0.00 Kelvin
The temperatures are: Temperature, Temperature, Temperature
False
Cannot compare "Temperature" because it is not IComparable.
At line:1 char:1
+ $Celsius -gt $Kelvin
+ ~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : NotIcomparable
The output shows that instances of Temperature:
- Don't display correctly as strings.
- Can't be checked properly for equivalency.
- Can't be compared.
These three problems can be addressed by implementing interfaces for the class.
Implementing IFormattable
The first interface to implement for the Temperature class is
System.IFormattable. This interface enables formatting an instance of the
class as different strings. To implement the interface, the class needs to
inherit from System.IFormattable and define the ToString() instance
method.
The ToString() instance method needs to have the following signature:
[string] ToString(
[string]$Format,
[System.IFormatProvider]$FormatProvider
) {
# Implementation
}
The signature that the interface requires is listed in the reference documentation.
For Temperature, the class should support three formats: C to return the
instance in Celsius, F to return it in Fahrenheit, and K to return it in
Kelvin. For any other format, the method should throw a
System.FormatException.
[string] ToString(
[string]$Format,
[System.IFormatProvider]$FormatProvider
) {
# If format isn't specified, use the defined scale.
if ([string]::IsNullOrEmpty($Format)) {
$Format = switch ($this.Scale) {
Celsius { 'C' }
Fahrenheit { 'F' }
Kelvin { 'K' }
}
}
# If format provider isn't specified, use the current culture.
if ($null -eq $FormatProvider) {
$FormatProvider = [CultureInfo]::CurrentCulture
}
# Format the temperature.
switch ($Format) {
'C' {
return $this.ToCelsius().ToString('F2', $FormatProvider) + '°C'
}
'F' {
return $this.ToFahrenheit().ToString('F2', $FormatProvider) + '°F'
}
'K' {
return $this.ToKelvin().ToString('F2', $FormatProvider) + '°K'
}
}
# If we get here, the format is invalid.
throw [System.FormatException]::new(
"Unknown format: '$Format'. Valid Formats are 'C', 'F', and 'K'"
)
}
In this implementation, the method defaults to the instance scale for
format and the current culture when formatting the numerical degree value
itself. It uses the To<Scale>() instance methods to convert the degrees,
formats them to two-decimal places, and appends the appropriate degree symbol
to the string.
With the required signature implemented, the class can also define overloads to make it easier to return the formatted instance.
[string] ToString([string]$Format) {
return $this.ToString($Format, $null)
}
[string] ToString() {
return $this.ToString($null, $null)
}
The following code shows the updated definition for Temperature:
class Temperature : System.IFormattable {
[float] $Degrees
[TemperatureScale] $Scale
Temperature() {}
Temperature([float] $Degrees) { $this.Degrees = $Degrees }
Temperature([TemperatureScale] $Scale) { $this.Scale = $Scale }
Temperature([float] $Degrees, [TemperatureScale] $Scale) {
$this.Degrees = $Degrees
$this.Scale = $Scale
}
[float] ToKelvin() {
switch ($this.Scale) {
Celsius { return $this.Degrees + 273.15 }
Fahrenheit { return ($this.Degrees + 459.67) * 5 / 9 }
}
return $this.Degrees
}
[float] ToCelsius() {
switch ($this.Scale) {
Fahrenheit { return ($this.Degrees - 32) * 5 / 9 }
Kelvin { return $this.Degrees - 273.15 }
}
return $this.Degrees
}
[float] ToFahrenheit() {
switch ($this.Scale) {
Celsius { return $this.Degrees * 9 / 5 + 32 }
Kelvin { return $this.Degrees * 9 / 5 - 459.67 }
}
return $this.Degrees
}
[string] ToString(
[string]$Format,
[System.IFormatProvider]$FormatProvider
) {
# If format isn't specified, use the defined scale.
if ([string]::IsNullOrEmpty($Format)) {
$Format = switch ($this.Scale) {
Celsius { 'C' }
Fahrenheit { 'F' }
Kelvin { 'K' }
}
}
# If format provider isn't specified, use the current culture.
if ($null -eq $FormatProvider) {
$FormatProvider = [CultureInfo]::CurrentCulture
}
# Format the temperature.
switch ($Format) {
'C' {
return $this.ToCelsius().ToString('F2', $FormatProvider) + '°C'
}
'F' {
return $this.ToFahrenheit().ToString('F2', $FormatProvider) + '°F'
}
'K' {
return $this.ToKelvin().ToString('F2', $FormatProvider) + '°K'
}
}
# If we get here, the format is invalid.
throw [System.FormatException]::new(
"Unknown format: '$Format'. Valid Formats are 'C', 'F', and 'K'"
)
}
[string] ToString([string]$Format) {
return $this.ToString($Format, $null)
}
[string] ToString() {
return $this.ToString($null, $null)
}
}
enum TemperatureScale {
Celsius = 0
Fahrenheit = 1
Kelvin = 2
}
The output for the method overloads is shown in the following block.
$Temp = [Temperature]::new()
"The temperature is $Temp"
$Temp.ToString()
$Temp.ToString('K')
$Temp.ToString('F', $null)
The temperature is 0.00°C
0.00°C
273.15°K
32.00°F
Implementing IEquatable
Now that the Temperature class can be formatted for readability, users need be able to check whether two instances of the class are equal. To support this test, the class needs to implement the System.IEquatable interface.
To implement the interface, the class needs to inherit from
System.IEquatable and define the Equals() instance method. The Equals()
method needs to have the following signature:
[bool] Equals([object]$Other) {
# Implementation
}
The signature that the interface requires is listed in the reference documentation.
For Temperature, the class should only support comparing two instances of
the class. For any other value or type, including $null, it should return
$false. When comparing two temperatures, the method should convert both
values to Kelvin, since temperatures can be equivalent even with different
scales.
[bool] Equals([object]$Other) {
# If the other object is null, we can't compare it.
if ($null -eq $Other) {
return $false
}
# If the other object isn't a temperature, we can't compare it.
$OtherTemperature = $Other -as [Temperature]
if ($null -eq $OtherTemperature) {
return $false
}
# Compare the temperatures as Kelvin.
return $this.ToKelvin() -eq $OtherTemperature.ToKelvin()
}
With the interface method implemented, the updated definition for Temperature is:
class Temperature : System.IFormattable, System.IEquatable[object] {
[float] $Degrees
[TemperatureScale] $Scale
Temperature() {}
Temperature([float] $Degrees) { $this.Degrees = $Degrees }
Temperature([TemperatureScale] $Scale) { $this.Scale = $Scale }
Temperature([float] $Degrees, [TemperatureScale] $Scale) {
$this.Degrees = $Degrees
$this.Scale = $Scale
}
[float] ToKelvin() {
switch ($this.Scale) {
Celsius { return $this.Degrees + 273.15 }
Fahrenheit { return ($this.Degrees + 459.67) * 5 / 9 }
}
return $this.Degrees
}
[float] ToCelsius() {
switch ($this.Scale) {
Fahrenheit { return ($this.Degrees - 32) * 5 / 9 }
Kelvin { return $this.Degrees - 273.15 }
}
return $this.Degrees
}
[float] ToFahrenheit() {
switch ($this.Scale) {
Celsius { return $this.Degrees * 9 / 5 + 32 }
Kelvin { return $this.Degrees * 9 / 5 - 459.67 }
}
return $this.Degrees
}
[string] ToString(
[string]$Format,
[System.IFormatProvider]$FormatProvider
) {
# If format isn't specified, use the defined scale.
if ([string]::IsNullOrEmpty($Format)) {
$Format = switch ($this.Scale) {
Celsius { 'C' }
Fahrenheit { 'F' }
Kelvin { 'K' }
}
}
# If format provider isn't specified, use the current culture.
if ($null -eq $FormatProvider) {
$FormatProvider = [CultureInfo]::CurrentCulture
}
# Format the temperature.
switch ($Format) {
'C' {
return $this.ToCelsius().ToString('F2', $FormatProvider) + '°C'
}
'F' {
return $this.ToFahrenheit().ToString('F2', $FormatProvider) + '°F'
}
'K' {
return $this.ToKelvin().ToString('F2', $FormatProvider) + '°K'
}
}
# If we get here, the format is invalid.
throw [System.FormatException]::new(
"Unknown format: '$Format'. Valid Formats are 'C', 'F', and 'K'"
)
}
[string] ToString([string]$Format) {
return $this.ToString($Format, $null)
}
[string] ToString() {
return $this.ToString($null, $null)
}
[bool] Equals([object]$Other) {
# If the other object is null, we can't compare it.
if ($null -eq $Other) {
return $false
}
# If the other object isn't a temperature, we can't compare it.
$OtherTemperature = $Other -as [Temperature]
if ($null -eq $OtherTemperature) {
return $false
}
# Compare the temperatures as Kelvin.
return $this.ToKelvin() -eq $OtherTemperature.ToKelvin()
}
}
enum TemperatureScale {
Celsius = 0
Fahrenheit = 1
Kelvin = 2
}
The following block shows how the updated class behaves:
$Celsius = [Temperature]::new()
$Fahrenheit = [Temperature]::new(32, 'Fahrenheit')
$Kelvin = [Temperature]::new([TemperatureScale]::Kelvin)
@"
Temperatures are: $Celsius, $Fahrenheit, $Kelvin
`$Celsius.Equals(`$Fahrenheit) = $($Celsius.Equals($Fahrenheit))
`$Celsius -eq `$Fahrenheit = $($Celsius -eq $Fahrenheit)
`$Celsius -ne `$Kelvin = $($Celsius -ne $Kelvin)
"@
Temperatures are: 0.00°C, 32.00°F, 0.00°K
$Celsius.Equals($Fahrenheit) = True
$Celsius -eq $Fahrenheit = True
$Celsius -ne $Kelvin = True
Implementing IComparable
The last interface to implement for the Temperature class is
System.IComparable. When the class implements this interface, users can use
the -lt, -le, -gt, and -ge operators to compare instances of the class.
To implement the interface, the class needs to inherit from
System.IComparable and define the Equals() instance method. The Equals()
method needs to have the following signature:
[int] CompareTo([Object]$Other) {
# Implementation
}
The signature that the interface requires is listed in the reference documentation.
For Temperature, the class should only support comparing two instances of the class. Because the underlying type for the Degrees property, even when converted to a different scale, is a floating point number, the method can rely on the underlying type for the actual comparison.
[int] CompareTo([object]$Other) {
# If the other object's null, consider this instance "greater than" it
if ($null -eq $Other) {
return 1
}
# If the other object isn't a temperature, we can't compare it.
$OtherTemperature = $Other -as [Temperature]
if ($null -eq $OtherTemperature) {
throw [System.ArgumentException]::new(
"Object must be of type 'Temperature'."
)
}
# Compare the temperatures as Kelvin.
return $this.ToKelvin().CompareTo($OtherTemperature.ToKelvin())
}
The final definition for the Temperature class is:
class Temperature : System.IFormattable,
System.IComparable,
System.IEquatable[object] {
# Instance properties
[float] $Degrees
[TemperatureScale] $Scale
# Constructors
Temperature() {}
Temperature([float] $Degrees) { $this.Degrees = $Degrees }
Temperature([TemperatureScale] $Scale) { $this.Scale = $Scale }
Temperature([float] $Degrees, [TemperatureScale] $Scale) {
$this.Degrees = $Degrees
$this.Scale = $Scale
}
[float] ToKelvin() {
switch ($this.Scale) {
Celsius { return $this.Degrees + 273.15 }
Fahrenheit { return ($this.Degrees + 459.67) * 5 / 9 }
}
return $this.Degrees
}
[float] ToCelsius() {
switch ($this.Scale) {
Fahrenheit { return ($this.Degrees - 32) * 5 / 9 }
Kelvin { return $this.Degrees - 273.15 }
}
return $this.Degrees
}
[float] ToFahrenheit() {
switch ($this.Scale) {
Celsius { return $this.Degrees * 9 / 5 + 32 }
Kelvin { return $this.Degrees * 9 / 5 - 459.67 }
}
return $this.Degrees
}
[string] ToString(
[string]$Format,
[System.IFormatProvider]$FormatProvider
) {
# If format isn't specified, use the defined scale.
if ([string]::IsNullOrEmpty($Format)) {
$Format = switch ($this.Scale) {
Celsius { 'C' }
Fahrenheit { 'F' }
Kelvin { 'K' }
}
}
# If format provider isn't specified, use the current culture.
if ($null -eq $FormatProvider) {
$FormatProvider = [CultureInfo]::CurrentCulture
}
# Format the temperature.
switch ($Format) {
'C' {
return $this.ToCelsius().ToString('F2', $FormatProvider) + '°C'
}
'F' {
return $this.ToFahrenheit().ToString('F2', $FormatProvider) + '°F'
}
'K' {
return $this.ToKelvin().ToString('F2', $FormatProvider) + '°K'
}
}
# If we get here, the format is invalid.
throw [System.FormatException]::new(
"Unknown format: '$Format'. Valid Formats are 'C', 'F', and 'K'"
)
}
[string] ToString([string]$Format) {
return $this.ToString($Format, $null)
}
[string] ToString() {
return $this.ToString($null, $null)
}
[bool] Equals([object]$Other) {
# If the other object is null, we can't compare it.
if ($null -eq $Other) {
return $false
}
# If the other object isn't a temperature, we can't compare it.
$OtherTemperature = $Other -as [Temperature]
if ($null -eq $OtherTemperature) {
return $false
}
# Compare the temperatures as Kelvin.
return $this.ToKelvin() -eq $OtherTemperature.ToKelvin()
}
[int] CompareTo([object]$Other) {
# If the other object's null, consider this instance "greater than" it
if ($null -eq $Other) {
return 1
}
# If the other object isn't a temperature, we can't compare it.
$OtherTemperature = $Other -as [Temperature]
if ($null -eq $OtherTemperature) {
throw [System.ArgumentException]::new(
"Object must be of type 'Temperature'."
)
}
# Compare the temperatures as Kelvin.
return $this.ToKelvin().CompareTo($OtherTemperature.ToKelvin())
}
}
enum TemperatureScale {
Celsius = 0
Fahrenheit = 1
Kelvin = 2
}
With the full definition, users can format and compare instances of the class in PowerShell like any builtin type.
$Celsius = [Temperature]::new()
$Fahrenheit = [Temperature]::new(32, 'Fahrenheit')
$Kelvin = [Temperature]::new([TemperatureScale]::Kelvin)
@"
Temperatures are: $Celsius, $Fahrenheit, $Kelvin
`$Celsius.Equals(`$Fahrenheit) = $($Celsius.Equals($Fahrenheit))
`$Celsius.Equals(`$Kelvin) = $($Celsius.Equals($Kelvin))
`$Celsius.CompareTo(`$Fahrenheit) = $($Celsius.CompareTo($Fahrenheit))
`$Celsius.CompareTo(`$Kelvin) = $($Celsius.CompareTo($Kelvin))
`$Celsius -lt `$Fahrenheit = $($Celsius -lt $Fahrenheit)
`$Celsius -le `$Fahrenheit = $($Celsius -le $Fahrenheit)
`$Celsius -eq `$Fahrenheit = $($Celsius -eq $Fahrenheit)
`$Celsius -gt `$Kelvin = $($Celsius -gt $Kelvin)
"@
Temperatures are: 0.00°C, 32.00°F, 0.00°K
$Celsius.Equals($Fahrenheit) = True
$Celsius.Equals($Kelvin) = False
$Celsius.CompareTo($Fahrenheit) = 0
$Celsius.CompareTo($Kelvin) = 1
$Celsius -lt $Fahrenheit = False
$Celsius -le $Fahrenheit = True
$Celsius -eq $Fahrenheit = True
$Celsius -gt $Kelvin = True
Example 3 - Inheriting from a generic base class
This example shows how you can derive from a generic class like System.Collections.Generic.List.
Using a built-in class as the type parameter
Run the following code block. It shows how a new class can inherit from a generic type as long as the type parameter is already defined at parse time.
class ExampleStringList : System.Collections.Generic.List[string] {}
$List = [ExampleStringList]::New()
$List.AddRange([string[]]@('a','b','c'))
$List.GetType() | Format-List -Property Name, BaseType
$List
Name : ExampleStringList
BaseType : System.Collections.Generic.List`1[System.String]
a
b
c
Using a custom class as the type parameter
The next code block first defines a new class, ExampleItem,
with a single instance property and the ToString() method. Then it defines
the ExampleItemList class inheriting from the
System.Collections.Generic.List base class with ExampleItem as the type
parameter.
Copy the entire code block and run it as a single statement.
class ExampleItem {
[string] $Name
[string] ToString() { return $this.Name }
}
class ExampleItemList : System.Collections.Generic.List[ExampleItem] {}
An error occurred while creating the pipeline.
+ CategoryInfo : NotSpecified: (:) [], ParentContainsErrorRe
cordException
+ FullyQualifiedErrorId : RuntimeException
Running the entire code block raises an error because PowerShell hasn't loaded the ExampleItem class into the runtime yet. You can't use class name as the type parameter for the System.Collections.Generic.List base class yet.
Run the following code blocks in the order they're defined.
class ExampleItem {
[string] $Name
[string] ToString() { return $this.Name }
}
class ExampleItemList : System.Collections.Generic.List[ExampleItem] {}
This time, PowerShell doesn't raise any errors. Both classes are now defined. Run the following code block to view the behavior of the new class.
$List = [ExampleItemList]::New()
$List.AddRange([ExampleItem[]]@(
[ExampleItem]@{ Name = 'Foo' }
[ExampleItem]@{ Name = 'Bar' }
[ExampleItem]@{ Name = 'Baz' }
))
$List.GetType() | Format-List -Property Name, BaseType
$List
Name : ExampleItemList
BaseType : System.Collections.Generic.List`1[ExampleItem]
Name
----
Foo
Bar
Baz
Deriving a generic with a custom type parameter in a module
The following code blocks show how you can define a class that inherits from a generic base class that uses a custom type for the type parameter.
Save the following code block as GenericExample.psd1.
@{
RootModule = 'GenericExample.psm1'
ModuleVersion = '0.1.0'
GUID = '2779fa60-0b3b-4236-b592-9060c0661ac2'
}
Save the following code block as GenericExample.InventoryItem.psm1.
class InventoryItem {
[string] $Name
[int] $Count
InventoryItem() {}
InventoryItem([string]$Name) {
$this.Name = $Name
}
InventoryItem([string]$Name, [int]$Count) {
$this.Name = $Name
$this.Count = $Count
}
[string] ToString() {
return "$($this.Name) ($($this.Count))"
}
}
Save the following code block as GenericExample.psm1.
using namespace System.Collections.Generic
using module ./GenericExample.InventoryItem.psm1
class Inventory : List[InventoryItem] {}
# Define the types to export with type accelerators.
$ExportableTypes =@(
[InventoryItem]
[Inventory]
)
# Get the internal TypeAccelerators class to use its static methods.
$TypeAcceleratorsClass = [psobject].Assembly.GetType(
'System.Management.Automation.TypeAccelerators'
)
# Ensure none of the types would clobber an existing type accelerator.
# If a type accelerator with the same name exists, throw an exception.
$ExistingTypeAccelerators = $TypeAcceleratorsClass::Get
foreach ($Type in $ExportableTypes) {
if ($Type.FullName -in $ExistingTypeAccelerators.Keys) {
$Message = @(
"Unable to register type accelerator '$($Type.FullName)'"
'Accelerator already exists.'
) -join ' - '
throw [System.Management.Automation.ErrorRecord]::new(
[System.InvalidOperationException]::new($Message),
'TypeAcceleratorAlreadyExists',
[System.Management.Automation.ErrorCategory]::InvalidOperation,
$Type.FullName
)
}
}
# Add type accelerators for every exportable type.
foreach ($Type in $ExportableTypes) {
$TypeAcceleratorsClass::Add($Type.FullName, $Type)
}
# Remove type accelerators when the module is removed.
$MyInvocation.MyCommand.ScriptBlock.Module.OnRemove = {
foreach($Type in $ExportableTypes) {
$TypeAcceleratorsClass::Remove($Type.FullName)
}
}.GetNewClosure()
Tip
The root module adds the custom types to PowerShell's type accelerators. This
pattern enables module users to immediately access IntelliSense and
autocomplete for the custom types without needing to use the using module
statement first.
For more information about this pattern, see the "Exporting with type accelerators" section of about_Classes.
Import the module and verify the output.
Import-Module ./GenericExample.psd1
$Inventory = [Inventory]::new()
$Inventory.GetType() | Format-List -Property Name, BaseType
$Inventory.Add([InventoryItem]::new('Bucket', 2))
$Inventory.Add([InventoryItem]::new('Mop'))
$Inventory.Add([InventoryItem]@{ Name = 'Broom' ; Count = 4 })
$Inventory
Name : Inventory
BaseType : System.Collections.Generic.List`1[InventoryItem]
Name Count
---- -----
Bucket 2
Mop 0
Broom 4
The module loads without errors because the InventoryItem class is defined in a different module file than the Inventory class. Both classes are available to module users.
Inheriting a base class
When a class inherits from a base class, it inherits the properties and methods of the base class. It doesn't inherit the base class constructors directly, but it can call them.
When the base class is defined in .NET rather than PowerShell, note that:
- PowerShell classes can't inherit from sealed classes.
- When inheriting from a generic base class, the type parameter for the generic class can't be the derived class. Using the derived class as the type parameter raises a parse error.
To see how inheritance and overriding works for derived classes, see Example 1.
Derived class constructors
Derived classes don't directly inherit the constructors of the base class. If the base class defines a default constructor and the derived class doesn't define any constructors, new instances of the derived class use the base class default constructor. If the base class doesn't define a default constructor, derived class must explicitly define at least one constructor.
Derived class constructors can invoke a constructor from the base class with
the base keyword. If the derived class doesn't explicitly invoke a
constructor from the base class, it invokes the default constructor for the
base class instead.
To invoke a nondefault base constructor, add : base(<parameters>) after the
constructor parameters and before the body block.
class <derived-class> : <base-class> {
<derived-class>(<derived-parameters>) : <base-class>(<base-parameters>) {
# initialization code
}
}
When defining a constructor that calls a base class constructor, the parameters can be any of the following items:
- The variable of any parameter on the derived class constructor.
- Any static value.
- Any expression that evaluates to a value of the parameter type.
The Illustration class in Example 1 shows how a derived class can use the base class constructors.
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. To
call the base class method for an instance, cast the instance variable
($this) to the base class before calling the method.
The following snippet shows how a derived class can call the base class method.
class BaseClass {
[bool] IsTrue() { return $true }
}
class DerivedClass : BaseClass {
[bool] IsTrue() { return $false }
[bool] BaseIsTrue() { return ([BaseClass]$this).IsTrue() }
}
@"
[BaseClass]::new().IsTrue() = $([BaseClass]::new().IsTrue())
[DerivedClass]::new().IsTrue() = $([DerivedClass]::new().IsTrue())
[DerivedClass]::new().BaseIsTrue() = $([DerivedClass]::new().BaseIsTrue())
"@
[BaseClass]::new().IsTrue() = True
[DerivedClass]::new().IsTrue() = False
[DerivedClass]::new().BaseIsTrue() = True
For an extended sample showing how a derived class can override inherited methods, see the Illustration class in Example 1.
Derived class properties
When a class derives from a base class, it inherits the properties of the base class. Any properties defined on the base class, including hidden properties, are available on the derived class.
A derived class can override an inherited property by redefining it in the class definition. The property on the derived class uses the redefined type and default value, if any. If the inherited property defined a default value and the redefined property doesn't, the inherited property has no default value.
If a derived class doesn't override a static property, accessing the static property through the derived class accesses the static property of the base class. Modifying the property value through the derived class modifies the value on the base class. Any other derived class that doesn't override the static property also uses the value of the property on the base class. Updating the value of an inherited static property in a class that doesn't override the property might have unintended effects for classes derived from the same base class.
Example 1 shows how derived classes that inherit, extend, and override the base class properties.
Deriving from generics
When a class derives from a generic, the type parameter must already be defined before PowerShell parses the derived class. If the type parameter for the generic is a PowerShell class or enumeration defined in the same file or code block, PowerShell raises an error.
To derive a class from a generic base class with a custom type as the type
parameter, define the class or enumeration for the type parameter in a
different file or module and use the using module statement to load the type
definition.
For an example showing how to inherit from a generic base class, see Example 3.
Useful classes to inherit
There are a few classes that can be useful to inherit when authoring PowerShell modules. This section lists a few base classes and what a class derived from them can be used for.
- System.Attribute - Derive classes to define attributes that can be used for variables, parameters, class and enumeration definitions, and more.
- System.Management.Automation.ArgumentTransformationAttribute - Derive classes to handle converting input for a variable or parameter into a specific data type.
- System.Management.Automation.ValidateArgumentsAttribute - Derive classes to apply custom validation to variables, parameters, and class properties.
- System.Collections.Generic.List - Derive classes to make creating and managing lists of a specific data type easier.
- System.Exception - Derive classes to define custom errors.
Implementing interfaces
A PowerShell class that implements an interface must implement all the members of that interface. Omitting the implementation interface members causes a parse-time error in the script.
Note
PowerShell doesn't support declaring new interfaces in PowerShell script.
Instead, interfaces must be declared in .NET code and added to the session
with the Add-Type cmdlet or the using assembly statement.
When a class implements an interface, it can be used like any other class that implements that interface. Some commands and operations limit their supported types to classes that implement a specific interface.
To review a sample implementation of interfaces, see Example 2.
Useful interfaces to implement
There are a few interface classes that can be useful to inherit when authoring PowerShell modules. This section lists a few base classes and what a class derived from them can be used for.
- System.IEquatable - This interface enables users to compare two instances of the class. When a class doesn't implement this interface, PowerShell checks for equivalency between two instances using reference equality. In other words, an instance of the class only equals itself, even if the property values on two instances are the same.
- System.IComparable - This interface enables users to compare instances of
the class with the
-le,-lt,-ge, and-gtcomparison operators. When a class doesn't implement this interface, those operators raise an error. - System.IFormattable - This interface enables users to format instances of the class into different strings. This is useful for classes that have more than one standard string representation, like budget items, bibliographies, and temperatures.
- System.IConvertible - This interface enables users to convert instances of the class to other runtime types. This is useful for classes that have an underlying numerical value or can be converted to one.
Limitations
PowerShell doesn't support defining interfaces in script code.
Workaround: Define interfaces in C# and reference the assembly that defines the interfaces.
PowerShell classes can only inherit from one base class.
Workaround: Class inheritance is transitive. A derived class can inherit from another derived class to get the properties and methods of a base class.
When inheriting from a generic class or interface, the type parameter for the generic must already be defined. A class can't define itself as the type parameter for a class or interface.
Workaround: To derive from a generic base class or interface, define the custom type in a different
.psm1file and use theusing modulestatement to load the type. There's no workaround for a custom type to use itself as the type parameter when inheriting from a generic.
