Parameter attributes (Mandatory, ValidateSet) in PowerShell

Introduction

Parameter attributes help control how users give input to your script. Mandatory makes sure a value is given. ValidateSet limits input to specific choices.

You want to make sure a user always provides a value for a script parameter.

You want to restrict input to a few allowed options to avoid mistakes.

You are writing a script that needs specific settings chosen from a list.

You want to guide users to enter only valid values without extra checks.

Syntax

PowerShell

param(
    [Parameter(Mandatory=$true)]
    [ValidateSet('Option1', 'Option2', 'Option3')]
    [string]$ParameterName
)

Mandatory=$true means the user must provide this parameter.

ValidateSet lists allowed values; PowerShell will error if input is outside this set.

Examples

This parameter $Name must be provided by the user.

PowerShell

param(
    [Parameter(Mandatory=$true)]
    [string]$Name
)

This parameter $Color only accepts 'Red', 'Green', or 'Blue'.

PowerShell

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

This parameter $Action is required and must be one of the three options.

PowerShell

param(
    [Parameter(Mandatory=$true)]
    [ValidateSet('Start', 'Stop', 'Restart')]
    [string]$Action
)

Sample Program

This script requires the user to provide an $Operation parameter. It must be 'Add', 'Remove', or 'Update'. The script then prints the chosen operation.

PowerShell

param(
    [Parameter(Mandatory=$true)]
    [ValidateSet('Add', 'Remove', 'Update')]
    [string]$Operation
)

Write-Output "You chose the operation: $Operation"

Important Notes

If a mandatory parameter is missing, PowerShell will prompt the user to enter it.

Using ValidateSet helps prevent typos and invalid inputs.

You can combine Mandatory and ValidateSet to make sure input is both required and valid.

Summary

Mandatory makes a parameter required.

ValidateSet limits input to specific allowed values.

Together, they help make scripts safer and easier to use.

Practice

1. What does the [Parameter(Mandatory=$true)] attribute do in a PowerShell script parameter?

easy

A. It limits the parameter to specific allowed values.

B. It sets a default value for the parameter.

C. It makes the parameter optional.

D. It makes the parameter required when running the script.

Solution

1. Step 1: Understand the Mandatory attribute

   The Mandatory=$true setting means the parameter must be provided by the user when running the script.

2. Step 2: Differentiate from other attributes

   Unlike ValidateSet, which restricts values, Mandatory only requires the parameter to be present.

3. Final Answer:

   It makes the parameter required when running the script. -> Option D

4. Quick Check:

   Mandatory = required parameter [OK]

Hint: Mandatory means you must provide the parameter value [OK]

Common Mistakes:

• Confusing Mandatory with ValidateSet
• Thinking Mandatory sets default values
• Assuming Mandatory makes parameter optional

2. Which of the following is the correct syntax to restrict a mandatory parameter to only accept the values 'Red', 'Green', or 'Blue' in PowerShell?

easy

A. [Parameter(Mandatory=$true)][ValidateSet('Red','Green','Blue')]$Color

B. [ValidateSet('Red','Green','Blue')][Parameter()]$Color

C. [Parameter()][ValidateSet('Red','Green','Blue')]$Color

D. [ValidateSet('Red','Green','Blue')]$Color

Solution

1. Step 1: Check attribute order and syntax

   Both Parameter and ValidateSet attributes can be used together. The order does not affect functionality, but the common style is to put Parameter first.

2. Step 2: Confirm Mandatory and ValidateSet usage

   [Parameter(Mandatory=$true)][ValidateSet('Red','Green','Blue')]$Color correctly uses Mandatory=$true and ValidateSet with the allowed values.

3. Final Answer:

   [Parameter(Mandatory=$true)][ValidateSet('Red','Green','Blue')]$Color -> Option A

4. Quick Check:

   Correct syntax = [Parameter(Mandatory=$true)][ValidateSet('Red','Green','Blue')]$Color [OK]

Hint: Use [Parameter()] before [ValidateSet()] for clarity [OK]

Common Mistakes:

• Swapping attribute order causing confusion
• Missing parentheses in ValidateSet
• Not setting Mandatory when required

3. Given the script snippet:

function Test-Color {
  param(
    [Parameter(Mandatory=$true)]
    [ValidateSet('Red','Green','Blue')]
    [string]$Color
  )
  Write-Output "Color chosen: $Color"
}

Test-Color -Color Yellow

What will happen when you run this script?

medium

A. It prompts the user to enter a valid color.

B. It outputs: Color chosen: Yellow

C. It throws a validation error because 'Yellow' is not in the ValidateSet.

D. It runs without error but ignores the color value.

Solution

1. Step 1: Understand ValidateSet behavior

   The ValidateSet attribute restricts input to the listed values only. 'Yellow' is not in the set 'Red', 'Green', 'Blue'.

2. Step 2: Predict script behavior on invalid input

   Passing 'Yellow' causes a validation error and stops the script before output.

3. Final Answer:

   It throws a validation error because 'Yellow' is not in the ValidateSet. -> Option C

4. Quick Check:

   ValidateSet blocks invalid values [OK]

Hint: ValidateSet blocks values not listed [OK]

Common Mistakes:

• Assuming script prompts instead of erroring
• Thinking it ignores invalid values
• Confusing Mandatory with ValidateSet behavior

4. You wrote this parameter declaration:

param(
  [Parameter(Mandatory=$true)]
  [ValidateSet('Small','Medium','Large')]
  [string]$Size
)

But when running the script without the -Size argument, it does not prompt for input and continues silently. What is the likely cause?

medium

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

B. The script is missing CmdletBinding() to enable parameter validation.

C. Mandatory parameters never prompt for input automatically.

D. The ValidateSet attribute is incorrect and disables Mandatory.

Solution

1. Step 1: Understand Mandatory prompt behavior

   Mandatory parameters prompt for input only if the function or script uses CmdletBinding() or is an advanced function.

2. Step 2: Identify missing CmdletBinding()

   Without CmdletBinding(), the script treats parameters as simple variables and does not enforce Mandatory prompts.

3. Final Answer:

   The script is missing CmdletBinding() to enable parameter validation. -> Option B

4. Quick Check:

   CmdletBinding() enables Mandatory prompts [OK]

Hint: Add CmdletBinding() to enable Mandatory prompts [OK]

Common Mistakes:

• Thinking ValidateSet disables Mandatory
• Assuming Mandatory never prompts
• Changing parameter type unnecessarily

5. You want to create a script parameter that is required and only accepts the values 'Start', 'Stop', or 'Restart'. Which of the following parameter declarations correctly enforces both requirements and will prompt the user if the parameter is missing?

hard

A. [CmdletBinding()]
param(
[Parameter(Mandatory=$true)]
[ValidateSet('Start','Stop','Restart')]
[string]$Action
)

B. param(
[Parameter()]
[ValidateSet('Start','Stop','Restart')]
[string]$Action = 'Start'
)

C. [CmdletBinding()]
param(
[Parameter()]
[ValidateSet('Start','Stop','Restart')]
[string]$Action
)

D. param(
[Parameter(Mandatory=$true)]
[string]$Action
)

Solution

1. Step 1: Ensure Mandatory prompts user

   To prompt the user when a parameter is missing, the script must use CmdletBinding() and set Mandatory=$true.

2. Step 2: Restrict values with ValidateSet

   Using ValidateSet limits input to the allowed values 'Start', 'Stop', or 'Restart'.

3. Step 3: Check all options

   [CmdletBinding()] param( [Parameter(Mandatory=$true)] [ValidateSet('Start','Stop','Restart')] [string]$Action ) includes CmdletBinding(), Mandatory=$true, and ValidateSet, fulfilling all requirements.

4. Final Answer:

   [CmdletBinding()] param( [Parameter(Mandatory=$true)] [ValidateSet('Start','Stop','Restart')] [string]$Action ) -> Option A

5. Quick Check:

   CmdletBinding + Mandatory + ValidateSet = [CmdletBinding()] param( [Parameter(Mandatory=$true)] [ValidateSet('Start','Stop','Restart')] [string]$Action ) [OK]

Hint: Use CmdletBinding with Mandatory and ValidateSet together [OK]

Common Mistakes:

• Omitting CmdletBinding() so no prompt occurs
• Setting default value disables Mandatory prompt
• Not using ValidateSet to restrict values