Prompting the User for Input with PowerShell

Sometimes you need to prompt the user to provide some value before you can complete your PowerShell script. You’ve got a few different options depending on what you’re asking the user to select from.

Parameters

Setting a parameter as mandatory without providing a value will prompt the user to enter one, like this:

function Invoke-AmazingPowerShellFunction {
  Param(
    [Parameter(Mandatory=$true)]
    [string]$ImportantParameter
  )
}

Setting the parameter type ([string] in this case) isn’t essential but will help validate that the input is at least of the right type. The trouble with users is that they can, and will, enter any old nonsense as the parameter value and you need to be able to handle it.

The ValidateSet attribute helps out where you have a fixed set of values that are the only valid ones.

function Invoke-AmazingPowerShellFunction {
  Param(
    [Parameter(Mandatory=$true)]
    [ValidateSet('This','Or This','Or Possibly This')]
    [string]$ImportantParameter
  )
} 

If you don’t know at design-time what the valid options are going to be then you need a different approach.

Out-GridView

Out-GridView has an OutputMode parameter which allows you to specify whether the user should be able to select a value and if so, a single value or multiple values. It also allows you to set a title for the window and provides a filter to help the user find the right value. Good for when there is a lot to choose from. We use it, for example, to choose a project from Azure DevOps.

In passing I’ve also found Out-GridView useful when working with complex types e.g. from a web service response and I just want to be able to browse the values in the object. You can pipe anything to it and it will render it into a nice grid.

Write-Host ("You selected {0}" -f ('1','2','3' | Out-GridView -OutputMode Single -Title 'Please select a value'))

Roll Your Own

Recently I wanted to prompt the user to make a selection between some options in the terminal. In my experience the Out-GridView window doesn’t always open in the foreground and if you’re using multiple monitors won’t necessary open near the window you’re executing the script in. I thought I’d try keeping the focus in the terminal window instead.

I couldn’t find anything already in PowerShell to print a list of options and prompt the user to choose one, so I wrote the below. I’d be interested to know if I’ve missed something obvious already built in though.

It takes a collection of strings that represent the options to choose between and some text that you want to prompt the user with. The options are printed with numbers next to them, waits for some input from the user with Read-Host and matches it to their selection.

0 is hard-coded as a cancel option and will return an empty string, otherwise the string of the user’s selection is returned.

function Get-SelectionFromUser {
    param (
        [Parameter(Mandatory=$true)]
        [string[]]$Options,
        [Parameter(Mandatory=$true)]
        [string]$Prompt        
    )
    
    [int]$Response = 0;
    [bool]$ValidResponse = $false    

    while (!($ValidResponse)) {            
        [int]$OptionNo = 0

        Write-Host $Prompt -ForegroundColor DarkYellow
        Write-Host "[0]: Cancel"

        foreach ($Option in $Options) {
            $OptionNo += 1
            Write-Host ("[$OptionNo]: {0}" -f $Option)
        }

        if ([Int]::TryParse((Read-Host), [ref]$Response)) {
            if ($Response -eq 0) {
                return ''
            }
            elseif($Response -le $OptionNo) {
                $ValidResponse = $true
            }
        }
    }

    return $Options.Get($Response - 1)
}