Mandatory and default parameters of a function

Question

I have written a little PowerShell send-email-function for a special purpose (error-message) so that the from and to addresses are always the same!

  Function Send-EMail {
      Param (
          [Parameter(Mandatory=$true)]  [String]$EmailTo   = "[email protected]", # default
          [Parameter(Mandatory=$true)]  [String]$EmailFrom = "[email protected]", #default
          [Parameter(Mandatory=$true)]  [String]$Subject,
          [Parameter(Mandatory=$true)]  [String]$Body,
          [Parameter(mandatory=$false)] [String]$Attachment,
          [Parameter(mandatory=$true)]  [String]$Password
      )
          $SMTPServer = "smtp.gmail.com"
          $SMTPMessage = New-Object System.Net.Mail.MailMessage($EmailFrom,$EmailTo,$Subject,$Body)
          if ($attachment -ne $null) {
              $SMTPattachment = New-Object System.Net.Mail.Attachment($attachment)
              $SMTPMessage.Attachments.Add($STMPattachment)
          }
          $SMTPClient = New-Object Net.Mail.SmtpClient($SmtpServer, 587)
          $SMTPClient.EnableSsl = $true
          $SMTPClient.Credentials = New-Object System.Net.NetworkCredential($EmailFrom.Split("@")[0], $Password);
          $SMTPClient.Send($SMTPMessage)
          Remove-Variable -Name SMTPMessage
          Remove-Variable -Name SMTPClient
          Remove-Variable -Name Password
          Remove-Variable -Name Body
          Remove-Variable -Name Subject

  } #End Function Send-EMail
  ....
  $subj = "Subject"
  $body = @" Body-Text  "@
  Send-EMail -Subject $subj -Body $body -Password "myPWD" -Attachment $logFile

I expect now that I don't have to specify again the email address, but if run it line by line in the ISE debugger a little window is opened asking me for the EmailTo address:

What do I have to change so that I am not asked for the already given addresses?

Solution

The Mandatory parameter attribute flag:

[Parameter(Mandatory=$true)]

really means "It is mandatory for the caller to supply an argument to this parameter".

If you want a parameter to fall back to a default value that you provide in the param block, set the Mandatory flag to $false:

[Parameter(Mandatory=$false)]
[string]$EmailTo = "[email protected]",

---

This may seem a little counter-intuitive, but it allows you to detect when a user didn't supply a parameter that is needed:

if(-not($PSBoundParameters.ContainsKey('EmailTo')) -and $EmailTo)
{
    # User relied on default value
}