PowerShell pscmdlet.writedebug vs Write-Debug

In working on some script based cmdlets I noticed some inconsistent behavior related to the Preference variables.  The $pscmdlet automatic variable provides several methods for interacting with a pipeline, including it’s own WriteDebug function which performs similarly to the Write-Debug cmdlet.  Supposedly it respects the $DebugPreference variable the same way as Write-Debug does, but in my testing I discovered this not to be the case.

Given the following example code, If you run the script with a -debug arguement it should output two lines of debug output without halting the script for permission to continue (the default action when -debug is specified is ‘Inquire’).

DebugTest.ps1

1. [CmdletBinding()]
2. param()
3. begin {
4.       $DebugPreference = 'SilentlyContinue'
5.       Write-Debug "BEGIN: This is a write-debug statement and shouldn't print"
6. }
7. end {
8.       if($pscmdlet.myinvocation.boundparameters.containskey("Debug")) {
9.            $DebugPreference='Continue'
10.       }
11.       Set-Variable -Name DebugPreference -Value 'SilentlyContinue' -Scope Local
12.       Write-Host "Debug: $DebugPreference"
13.       Write-Host "Verbose: $VerbosePreference"
14.       Write-Verbose "This is a write-verbose statement"
15.       Write-Debug "PROCESS: This is a write-debug statement and should print"
16.       $pscmdlet.WriteDebug("PROCESS: This is a WriteDebug statement and should print")
17. }

Note that if you change a preference variable inside of a script block, it will reset when that script block is completed which is why you have to set the value in the BEGIN and PROCESS block.