About calculated properties

Short Description

PowerShell provides the ability to dynamically add new properties and alter the formatting of objects output to the pipeline.

Long Description

A number of PowerShell cmdlets transform, aggregate, or process input objects into output objects using parameters that allow the addition of new properties to those output objects. These parameters can be used to generate new, calculated properties on output objects based on the values of input objects. The calculated property is defined by a hashtable containing key-value pairs that specify the name of the new property, an expression to calculate the value, and optional formatting information.

Supported cmdlets

The following cmdlets support calculated property values for the Property parameter. The Format-* cmdlets also support calculated values for the GroupBy parameter.

The following list itemizes the cmdlets that support calculated properties and the key-value pairs that are supported by each cmdlet.

  • Compare-Object

    • expression
  • ConvertTo-Html

    • name/label - optional (added in PowerShell 6.x)
    • expression
    • width - optional
    • alignment - optional
  • Format-Custom

    • expression
    • depth - optional
  • Format-List

    • name/label - optional
    • expression
    • formatstring - optional

    This same set of key-value pairs also apply to calculated property values passed to the GroupBy parameter for all Format-* cmdlets.

  • Format-Table

    • name/label - optional
    • expression
    • formatstring - optional
    • width - optional
    • alignment - optional
  • Format-Wide

    • expression
    • formatstring - optional
  • Group-Object

    • expression
  • Measure-Object

    • Only supports a script block for the expression, not a hashtable.
    • Not supported in PowerShell 5.1 and older.
  • Select-Object

    • name/label - optional
    • expression
  • Sort-Object

    • expression
    • ascending/descending - optional

Note

The value of the expression can be a script block instead of a hashtable. For more information, see the Notes section.

Hashtable key definitions

  • name/label - Specifies the name of the property being created. You can use name or its alias, label, interchangeably.
  • expression - A script block used to calculate the value of the new property.
  • alignment - Used by cmdlets that produce tabular output to define how the values are displayed in a column. The value must be 'left', 'center', or 'right'.
  • formatstring - Specifies a format string that defines how the value is formatted for output. For more information about format strings, see Format types in .NET.
  • width - Specifies the maximum width column in a table when the value is displayed. The value must be greater than 0.
  • depth - The Depth parameter of Format-Custom specifies the depth of expansion for all properties. The depth key allows you to specify the depth of expansion per property.
  • ascending / descending - Allows you to specify the order of sorting for one or more properties. These are boolean values.

The hashtable keys need not be spelled out as long as the specified name prefix is unambiguous. For example, n can be used in lieu of Name and e can be used in lieu of Expression.

Examples

Compare-Object

With calculated properties, you can control how the properties of the input objects are compared. In this example, rather than comparing the values directly, the values are compared to the result of the arithmetic operation (modulus of 2).

Compare-Object @{p=1} @{p=2} -property @{ Expression = { $_.p % 2 } }
 $_.p % 2  SideIndicator
---------- -------------
         0 =>
         1 <=

ConvertTo-Html

ConvertTo-Html can convert a collection of objects to an HTML table. Calculated properties allow you to control how the table is presented.

Get-Alias |
  ConvertTo-Html Name,
                 Definition,
                 @{
                    name='ParameterCount'
                    expr={$_.Parameters.Keys.Count}
                    align='center'
                 } |
    Out-File .\aliases.htm -Force

This example creates an HTML table containing a list of PowerShell aliases and the number parameters for each aliased command. The values of ParameterCount column are centered.

Format-Custom

Format-Custom provides a custom view of an object in a format similar to a class definition. More complex objects can contain members that are deeply nested with complex types. The Depth parameter of Format-Custom specifies the depth of expansion for all properties. The depth key allows you to specify the depth of expansion per property.

In this example, the depth key simplifies the custom output for the Get-Date cmdlet. Get-Date returns a DateTime object. The Date property of this object is also a DateTime object, so the object is nested.

Get-Date | Format-Custom @{expr={$_.Date};depth=1},TimeOfDay
class DateTime
{
  $_.Date =
    class DateTime
    {
      Date = 8/7/2020 12:00:00 AM
      Day = 7
      DayOfWeek = Friday
      DayOfYear = 220
      Hour = 0
      Kind = Local
      Millisecond = 0
      Minute = 0
      Month = 8
      Second = 0
      Ticks = 637323552000000000
      TimeOfDay = 00:00:00
      Year = 2020
      DateTime = Friday, August 07, 2020 12:00:00 AM
    }
  TimeOfDay =
    class TimeSpan
    {
      Ticks = 435031592302
      Days = 0
      Hours = 12
      Milliseconds = 159
      Minutes = 5
      Seconds = 3
      TotalDays = 0.503508787386574
      TotalHours = 12.0842108972778
      TotalMilliseconds = 43503159.2302
      TotalMinutes = 725.052653836667
      TotalSeconds = 43503.1592302
    }
}

Format-List

In this example, we use calculated properties to change the name and format of the output from Get-ChildItem.

Get-ChildItem *.json -File |
  Format-List Fullname,
              @{
                 name='Modified'
                 expression={$_.LastWriteTime}
                 formatstring='O'
              },
              @{
                 name='Size'
                 expression={$_.Length/1KB}
                 formatstring='N2'
              }
FullName : C:\Git\PS-Docs\PowerShell-Docs\.markdownlint.json
Modified : 2020-07-23T10:26:28.4092457-07:00
Size     : 2.40

FullName : C:\Git\PS-Docs\PowerShell-Docs\.openpublishing.publish.config.json
Modified : 2020-07-23T10:26:28.4092457-07:00
Size     : 2.25

FullName : C:\Git\PS-Docs\PowerShell-Docs\.openpublishing.redirection.json
Modified : 2020-07-27T13:05:24.3887629-07:00
Size     : 324.60

Format-Table

In this example, the calculated property adds a Type property used to classify the files by the content type.

Get-ChildItem -File |
  Sort-Object extension |
    Format-Table Name, Length -GroupBy @{
      name='Type'
      expression={
        switch ($_.extension) {
          '.md'   {'Content'}
          ''      {'Metacontent'}
          '.ps1'  {'Automation'}
          '.yml'  {'Automation'}
          default {'Configuration'}
        }
      }
    }
   Type: Metacontent

Name              Length
----              ------
ThirdPartyNotices   1229
LICENSE-CODE        1106
LICENSE            19047

   Type: Configuration

Name                                Length
----                                ------
.editorconfig                          183
.gitattributes                         419
.gitignore                             228
.markdownlint.json                    2456
.openpublishing.publish.config.json   2306
.openpublishing.redirection.json    332394
.localization-config                   232

   Type: Content

Name            Length
----            ------
README.md         3355
CONTRIBUTING.md    247

   Type: Automation

Name                      Length
----                      ------
.openpublishing.build.ps1    796
build.ps1                   7495
ci.yml                       645
ci-steps.yml                2035
daily.yml                   1271

Format-Wide

The Format-Wide cmdlet allows you to display the value of one property for objects in a collection as a multi-column list.

For this example, we want to see the filename and the size (in kilobytes) as a wide listing. Since Format-Wide does not display more than one property, we use a calculated property to combine the value of two properties into a single value.

Get-ChildItem -File |
  Format-Wide -Property @{e={'{0} ({1:N2}kb)' -f $_.name,($_.length/1kb)}}
.editorconfig (0.18kb)                          .gitattributes (0.41kb)
.gitignore (0.22kb)                             .localization-config (0.23kb)
.markdownlint.json (2.40kb)                     .openpublishing.build.ps1 (0.78kb)
.openpublishing.publish.config.json (2.25kb)    .openpublishing.redirection.json (324.60kb)
build.ps1 (7.32kb)                              ci.yml (0.63kb)
ci-steps.yml (1.99kb)                           CONTRIBUTING.md (0.24kb)
daily.yml (1.24kb)                              LICENSE (18.60kb)
LICENSE-CODE (1.08kb)                           README.md (3.28kb)
ThirdPartyNotices (1.20kb)

Group-Object

The Group-Object cmdlet displays objects in groups based on the value of a specified property. In this example, the calculated property counts the number of files of each content type.

Get-ChildItem -File |
  Sort-Object extension |
    Group-Object -NoElement -Property @{
      expression={
        switch ($_.extension) {
          '.md'   {'Content'}
          ''      {'Metacontent'}
          '.ps1'  {'Automation'}
          '.yml'  {'Automation'}
          default {'Configuration'}
        }
      }
    }
Count Name
----- ----
    5 Automation
    7 Configuration
    2 Content
    3 Metacontent

Measure-Object

The Measure-Object cmdlet calculates the numeric properties of objects. In this example, we use a calculated property to get the count (Sum) of the numbers, between 1 and 10, that are evenly divisible by 3.

1..10 | Measure-Object -Property {($_ % 3) -eq 0} -Sum
Count             : 10
Average           :
Sum               : 3
Maximum           :
Minimum           :
StandardDeviation :
Property          : ($_ % 3) -eq 0

Note

Unlike the other cmdlets, Measure-Object does not accept a hashtable for calculated properties. You must use a script block.

Select-Object

You can use calculated properties to add additional members to the objects output with the Select-Object cmdlet. In this example, we are listing the PowerShell aliases that begin with the letter C. Using Select-Object, we output the alias, the cmdlet it's mapped to, and a count for the number of parameters defined for the cmdlet. Using a calculated property, we can create the ParameterCount property.

$aliases = Get-Alias c* |
  Select-Object Name,
                Definition,
                @{
                    name='ParameterCount'
                    expr={$_.Parameters.Keys.Count}
                }
$aliases | Get-Member
$aliases
   TypeName: Selected.System.Management.Automation.AliasInfo

Name           MemberType   Definition
----           ----------   ----------
Equals         Method       bool Equals(System.Object obj)
GetHashCode    Method       int GetHashCode()
GetType        Method       type GetType()
ToString       Method       string ToString()
Definition     NoteProperty string Definition=Get-Content
Name           NoteProperty string Name=cat
ParameterCount NoteProperty System.Int32 ParameterCount=21

Name    Definition         ParameterCount
----    ----------         --------------
cat     Get-Content                    21
cd      Set-Location                   15
cdd     Push-MyLocation                 1
chdir   Set-Location                   15
clc     Clear-Content                  20
clear   Clear-Host                      0
clhy    Clear-History                  17
cli     Clear-Item                     20
clp     Clear-ItemProperty             22
cls     Clear-Host                      0
clv     Clear-Variable                 19
cnsn    Connect-PSSession              29
compare Compare-Object                 20
copy    Copy-Item                      24
cp      Copy-Item                      24
cpi     Copy-Item                      24
cpp     Copy-ItemProperty              23
cvpa    Convert-Path                   13

Sort-Object

Using the calculated properties, you can sort data in different orders per property. This example sorts data from a CSV file in ascending order by Date. But within each date, it sorts the rows in descending order by UnitsSold.

Import-Csv C:\temp\sales-data.csv |
  Sort-Object Date, @{expr={$_.UnitsSold}; desc=$true}, Salesperson  |
    Select-Object Date, Salesperson, UnitsSold
Date       Salesperson UnitsSold
----       ----------- ---------
2020-08-01 Sally       3
2020-08-01 Anne        2
2020-08-01 Fred        1
2020-08-02 Anne        6
2020-08-02 Fred        2
2020-08-02 Sally       0
2020-08-03 Anne        5
2020-08-03 Sally       3
2020-08-03 Fred        1
2020-08-04 Anne        2
2020-08-04 Fred        2
2020-08-04 Sally       2

Notes

  • You may specify the expression script block directly, as an argument, rather than specifying it as the Expression entry in a hashtable. For example:

    '1', '10', '2' | Sort-Object { [int] $_ }
    

    This example is convenient for cmdlets that do not require (or support) naming a property via the Name key, such as Sort-Object, Group-Object, and Measure-Object.

    For cmdlets that support naming the property, the script block is converted to a string and used as the name of the property in the output.

  • Expression script blocks run in child scopes, meaning that the caller's variables cannot be directly modified.

  • Pipeline logic is applied to the output from Expression script blocks. This means that outputting a single-element array causes that array to be unwrapped.

  • For most cmdlets, errors inside expression script blocks are quietly ignored. For Sort-Object, statement-terminating and script-terminating errors are output but they do not terminate the statement.

See Also

about_Hash_Tables

Compare-Object

ConvertTo-Html

Format-Custom

Format-List

Format-Table

Format-Wide

Group-Object

Measure-Object

Select-Object

Sort-Object

Format types in .NET