How to strip illegal characters before trying to save filenames?

Casting the character array to System.String actually seems to join the array elements with spaces, meaning that

[string][System.IO.Path]::GetInvalidFileNameChars()

does the same as

[System.IO.Path]::GetInvalidFileNameChars() -join ' '

when you actually want

[System.IO.Path]::GetInvalidFileNameChars() -join ''

As @mjolinor mentioned (+1), this is caused by the output field separator ($OFS).

Evidence:

PS C:\> [RegEx]::Escape([string][IO.Path]::GetInvalidFileNameChars())
"\ \ \|\  \ ☺\ ☻\ ♥\ ♦\ ♣\ ♠\ \\ \t\ \n\ ♂\ \f\ \r\ ♫\ ☼\ ►\ ◄\ ↕\ ‼\ ¶\ §\ ▬\ ↨\ ↑\ ↓\ →\ ←\ ∟\ ↔\ ▲\ ▼\ :\ \*\ \?\ \\\ /
PS C:\> [RegEx]::Escape(([IO.Path]::GetInvalidFileNameChars() -join ' '))
"\ \ \|\  \ ☺\ ☻\ ♥\ ♦\ ♣\ ♠\ \\ \t\ \n\ ♂\ \f\ \r\ ♫\ ☼\ ►\ ◄\ ↕\ ‼\ ¶\ §\ ▬\ ↨\ ↑\ ↓\ →\ ←\ ∟\ ↔\ ▲\ ▼\ :\ \*\ \?\ \\\ /
PS C:\> [RegEx]::Escape(([IO.Path]::GetInvalidFileNameChars() -join ''))
"\| ☺☻♥♦\t\n♂\f\r♫☼►◄↕‼¶§▬↨↑↓→←∟↔▲▼:\*\?\\/
PS C:\> $OFS=''
PS C:\> [RegEx]::Escape([string][IO.Path]::GetInvalidFileNameChars())
"\| ☺☻♥♦\t\n♂\f\r♫☼►◄↕‼¶§▬↨↑↓→←∟↔▲▼:\*\?\\/

Change your function to something like this:

Function Remove-InvalidFileNameChars {
  param(
    [Parameter(Mandatory=$true,
      Position=0,
      ValueFromPipeline=$true,
      ValueFromPipelineByPropertyName=$true)]
    [String]$Name
  )

  $invalidChars = [IO.Path]::GetInvalidFileNameChars() -join ''
  $re = "[{0}]" -f [RegEx]::Escape($invalidChars)
  return ($Name -replace $re)
}

and it should do what you want.

My current favourite way to accomplish this is:

$Path.Split([IO.Path]::GetInvalidFileNameChars()) -join '_'

This replaces all invalid characters with _ and is very human readable, compared to alternatives such as:

$Path -replace "[$([RegEx]::Escape([string][IO.Path]::GetInvalidFileNameChars()))]+","_"