PowerShell Advanced functions

Adding [CmdletBinding()] above param() turns a plain function into an advanced function. The single line gets you every common parameter for free: -Verbose, -Debug, -ErrorAction, -WarningAction, -ErrorVariable, -OutVariable and more.

Lesson 2 min read Windows PowerShell 5.1 and PowerShell 7

Documentation: Microsoft Learn

Written and tested by Dhrub Bharali · Updated 02 October 2026

Overview

That is what makes Write-Verbose useful. Without [CmdletBinding()] there is no -Verbose switch to turn the messages on. With it, your verbose lines stay silent by default and appear on demand, which is how every built-in cmdlet behaves.

[CmdletBinding(SupportsShouldProcess)] goes further and gives you -WhatIf and -Confirm. Guard the state-changing line with if ($PSCmdlet.ShouldProcess($target)) and callers get a dry run for free. Any function that deletes, stops or overwrites something should have this.

The cost of [CmdletBinding()] is that PowerShell becomes strict about arguments: unknown parameters are now errors rather than being swept into $args. That strictness is the point.

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function Get-BigProcess {
  [CmdletBinding()]
  param(
  [int] $MinimumMB = 200
)
  Write-Verbose "Filtering processes above $MinimumMB MB"
  Get-Process |
  Where-Object { $_.WorkingSet64 -gt ($MinimumMB * 1MB) } |
  Select-Object Name, Id, @{ Name = 'WS_MB'; Expression = { [math]::Round($_.WorkingSet64 / 1MB, 1) } } }
Get-BigProcess -MinimumMB 400 -Verbose

Stage 1function-MinimumMB -Verbose

What happens

[CmdletBinding()] on line 2 is what makes -Verbose on the last line work. Run the call without -Verbose and the Write-Verbose line produces nothing.

The Select-Object uses a calculated property: a hashtable with Name and Expression keys, where the expression converts bytes to megabytes. That is how you add a column that does not exist on the original object.

Playbook rule

Use -WhatIf before destructive cmdlets.

Add -WhatIf to preview what a destructive cmdlet would do — nothing on your system changes until you run it again without the switch.

PS> Remove-Item C:\temp\old.log -WhatIf
What if: Performing the operation "Remove File" on target "C:\temp\old.log".

Nothing was deleted. -WhatIf only describes the action it would take.

PS> Remove-Item C:\temp\old.log
(no output — the file is gone)

Without -WhatIf there is no preview and no undo. This panel is a simulation — nothing on your machine was actually removed.

Worth remembering

[CmdletBinding()] adds -Verbose, -ErrorAction and the other common parameters.
Write-Verbose only does anything once -Verbose exists.
SupportsShouldProcess adds -WhatIf and -Confirm.
Unknown parameters become errors instead of landing in $args.

Primary documentation

Where-Object reference · Select-Object reference · Get-Process reference

Try it

Illustrative output · values vary by environment

1PS> function Get-BigProcess {

2PS> [CmdletBinding()]

3PS> param(

4PS> [int] $MinimumMB = 200

5PS> )

6PS> Write-Verbose "Filtering processes above $MinimumMB MB"

7PS> Get-Process |

8PS> Where-Object { $_.WorkingSet64 -gt ($MinimumMB * 1MB) } |

9PS> Select-Object Name, Id, @{ Name = 'WS_MB'; Expression = { [math]::Round($_.WorkingSet64 / 1MB, 1) } } }

10PS> Get-BigProcess -MinimumMB 400 -Verbose

Example reader

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function Get-BigProcess {
  [CmdletBinding()]
  param(
  [int] $MinimumMB = 200
)
  Write-Verbose "Filtering processes above $MinimumMB MB"
  Get-Process |
  Where-Object { $_.WorkingSet64 -gt ($MinimumMB * 1MB) } |
  Select-Object Name, Id, @{ Name = 'WS_MB'; Expression = { [math]::Round($_.WorkingSet64 / 1MB, 1) } } }
Get-BigProcess -MinimumMB 400 -Verbose

Ready to inspect.

An advanced function using -Verbose and a calculated property.

Show reference output

Illustrative output · values vary by environment

VERBOSE: Filtering processes above 400 MB
Name     Id     WS_MB
----     --     -----
chrome   9184   464.0
msedge   7133   521.4

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