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.

Practice

1. What is the main purpose of parameter validation in a PowerShell script?

easy

A. To speed up the script execution

B. To format the output of the script

C. To check input values before the script runs to avoid errors

D. To create new variables automatically

Solution

1. Step 1: Understand parameter validation role

   Parameter validation ensures inputs meet certain rules before the script runs.

2. Step 2: Identify the main benefit

   This prevents errors caused by bad inputs and makes scripts safer.

3. Final Answer:

   To check input values before the script runs to avoid errors -> Option C

4. Quick Check:

   Parameter validation = input checking [OK]

Hint: Validation checks inputs early to prevent errors [OK]

Common Mistakes:

• Thinking validation speeds up script
• Confusing validation with output formatting
• Assuming validation creates variables

2. Which of the following is the correct syntax to validate that a parameter only accepts values from a fixed set: 'Red', 'Green', or 'Blue'?

easy

A. [ValidateSet('Red','Green','Blue')] param([string]$Color)

B. [ValidateRange('Red','Green','Blue')] param([string]$Color)

C. [ValidateNotNullOrEmpty('Red','Green','Blue')] param([string]$Color)

D. [ValidatePattern('Red|Green|Blue')] param([string]$Color)

Solution

1. Step 1: Identify correct validation attribute for fixed sets

   [ValidateSet] restricts input to specific allowed values.

2. Step 2: Match syntax with attribute usage

   The syntax [ValidateSet('Red','Green','Blue')] is correct for this purpose.

3. Final Answer:

   [ValidateSet('Red','Green','Blue')] param([string]$Color) -> Option A

4. Quick Check:

   Fixed set validation = ValidateSet [OK]

Hint: Use ValidateSet for fixed allowed values [OK]

Common Mistakes:

• Using ValidateRange for strings
• Confusing ValidateNotNullOrEmpty with value restriction
• Using ValidatePattern incorrectly for sets

3. What will be the output of this script if called with -Age 25?

function Test-Age {
  param(
    [ValidateRange(18,30)]
    [int]$Age
  )
  "Age is $Age"
}
Test-Age -Age 25

medium

A. "Age is 18"

B. Error: Parameter validation failed

C. No output

D. "Age is 25"

Solution

1. Step 1: Check parameter validation range

   The parameter $Age must be between 18 and 30 inclusive.

2. Step 2: Verify input against range

   Input 25 is within the range, so validation passes and script runs.

3. Final Answer:

   "Age is 25" -> Option D

4. Quick Check:

   25 in 18-30 range = valid input [OK]

Hint: Check if input fits ValidateRange limits [OK]

Common Mistakes:

• Assuming validation fails for valid input
• Confusing output with error message
• Ignoring the inclusive range

4. Identify the error in this parameter validation code:

function Set-Name {
  param(
    [ValidateNotNullOrEmpty()]
    [string]$Name
  )
  "Name set to $Name"
}
Set-Name -Name $null

medium

A. The parameter type should be [int] instead of [string]

B. The function will throw a validation error because $Name is null

C. The ValidateNotNullOrEmpty attribute is used incorrectly without parameters

D. The function will accept null and print 'Name set to '

Solution

1. Step 1: Understand ValidateNotNullOrEmpty behavior

   This attribute rejects null or empty string inputs.

2. Step 2: Analyze input value

   The input is $null, which violates the validation rule, causing an error.

3. Final Answer:

   The function will throw a validation error because $Name is null -> Option B

4. Quick Check:

   Null input with ValidateNotNullOrEmpty = error [OK]

Hint: Null or empty inputs fail ValidateNotNullOrEmpty [OK]

Common Mistakes:

• Thinking null is accepted
• Assuming ValidateNotNullOrEmpty needs parameters
• Confusing parameter type requirements

5. You want to create a script parameter $Port that only accepts integers between 1024 and 65535 and cannot be empty. Which parameter validation attributes should you use together?

hard

A. [ValidateRange(1024,65535)][ValidateNotNullOrEmpty()]

B. [ValidateSet(1024,65535)][ValidateNotNullOrEmpty()]

C. [ValidatePattern('^[0-9]{4,5}$')]

D. [ValidateNotNullOrEmpty()][ValidateLength(4,5)]

Solution

1. Step 1: Choose attribute for numeric range

   [ValidateRange(1024,65535)] ensures the integer is within the port number range.

2. Step 2: Ensure parameter is not empty

   [ValidateNotNullOrEmpty()] prevents null or empty input.

3. Step 3: Combine both for full validation

   Using both attributes together enforces the correct range and non-empty input.

4. Final Answer:

   [ValidateRange(1024,65535)][ValidateNotNullOrEmpty()] -> Option A

5. Quick Check:

   Range + NotNullOrEmpty = correct port validation [OK]

Hint: Combine ValidateRange and ValidateNotNullOrEmpty for numeric required input [OK]

Common Mistakes:

• Using ValidateSet for numeric ranges
• Relying on ValidatePattern for numeric range checks
• Missing non-empty validation