Accepting Parameters without Quotes

In the previous tip, we introduced a function that generates nicely centered headers and accepted a single string. Here is the function and its results:

function Show-Header($Text)
{
  $Width=80
  $padLeft = [int]($width / 2) + ($text.Length / 2)
  $text.PadLeft($padLeft, "=").PadRight($Width, "=")
}

 
PS> Show-Header Starting
====================================Starting====================================

PS> Show-Header "Processing Input Values"
=============================Processing Input Values============================
 

As you can see, the function works beautifully, but the user needs to quote the string once it contains spaces or other special characters. Wouldn’t it be nice if the function accepted strings without quotes as well, and treated all input as one argument?

That’s easily possible, provided:

• the parameter is declared a clear data type, i.e. [string], so PowerShell knows what to do with your arguments
• you are just using a single parameter, so PowerShell knows that all input needs to go into that parameter

Here is the revised function:

function Show-Header([Parameter(ValueFromRemainingArguments)][string]$Text)
{
  $Width=80
  $padLeft = [int]($width / 2) + ($text.Length / 2)
  $text.PadLeft($padLeft, "=").PadRight($Width, "=")
}

The magic is done by the ValueFromRemainingArguments attribute. Now, the user can simply type text and won’t need to use quotes:

 
PS> Show-Header Starting
====================================Starting====================================

PS> Show-Header Processing Input Values
=============================Processing Input Values============================ 
 

However, there is one caveat: any special characters such as parenthesis and quotes are interpreted and can still interfere. In these cases, you have to quote the string like before.

Your learning points:

• use the ValueFromRemainingArguments attribute to allow PowerShell to assign all unbound (extra) arguments to that parameter
• use distinct data types for parameters so PowerShell knows how to convert ambiguous data. Without a [string] data type, for example, PowerShell would have created a string array from your input values when a string contained spaces

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