Parameter validation in PowerShell

Introduction

Parameter validation helps check if the inputs to a script or function are correct before running it. This stops errors early and makes scripts safer.

When you want to make sure a user enters a number within a certain range.

When a script needs a file path and you want to check if the file exists.

When you want to allow only specific choices for a parameter.

When you want to ensure a parameter is not empty or null.

When you want to avoid running a script with wrong or missing inputs.

Syntax

PowerShell

function My-Function {
  param(
    [Parameter(Mandatory=$true)]
    [ValidateNotNullOrEmpty()]
    [string]$Name,

    [ValidateRange(1, 10)]
    [int]$Count
  )
  # Function code here
}

Use [Parameter(Mandatory=$true)] to require a parameter.

Validation attributes like [ValidateRange] or [ValidateNotNullOrEmpty] check input before the script runs.

Examples

This function only accepts ages between 0 and 120.

PowerShell

function Get-UserAge {
  param(
    [ValidateRange(0, 120)]
    [int]$Age
  )
  "User age is $Age"
}

This checks if the file path exists before continuing.

PowerShell

function Set-FilePath {
  param(
    [ValidateScript({Test-Path $_})]
    [string]$Path
  )
  "File path is valid: $Path"
}

This only allows the colors Red, Green, or Blue as input.

PowerShell

function Choose-Color {
  param(
    [ValidateSet('Red', 'Green', 'Blue')]
    [string]$Color
  )
  "You chose $Color"
}

Sample Program

This script greets a person by name a certain number of times. It checks that the name is not empty and the number of times is between 1 and 5.

PowerShell

function Greet-Person {
  param(
    [Parameter(Mandatory=$true)]
    [ValidateNotNullOrEmpty()]
    [string]$Name,

    [ValidateRange(1, 5)]
    [int]$Times = 1
  )

  for ($i = 1; $i -le $Times; $i++) {
    Write-Output "Hello, $Name!"
  }
}

# Call the function
Greet-Person -Name "Alice" -Times 3

Important Notes

If validation fails, PowerShell shows an error and stops the script.

You can combine multiple validation attributes for stronger checks.

Default values can be set for parameters to make them optional.

Summary

Parameter validation checks inputs before running code.

Use attributes like [ValidateRange], [ValidateSet], and [ValidateNotNullOrEmpty].

Validation helps avoid errors and makes scripts safer and easier to use.