Dfyjkt

i'm trying to output to eventlog to the correct Entry Type (Information,Warning,Error) based on the stream that is coming out of my cmdlet, something like this:

function myfunction 
 Param(
 [switch]$stream1,
 [switch]$stream2
)
 if ($stream1) write-output 'stream 1 msg'
 if ($stream2) write-error 'stream 2 msg'


$eventlogparams = @'logname'='application';'source'='myapp';'eventid'='1'

myfunction -stream1 -stream2 `
 1> write-eventlog @eventlogparams -entrytype information -message $_ `
 2> write-eventlog @eventlogparams -entrytype error -message $_

does anyone have an idea of how to accomplish this?

You can merge the error stream and others into the success stream and distinguish between the origin streams by the data type of each pipeline object:

myfunction -channel1 -channel2 *>&1 | ForEach-Object 
 $entryType = switch ($_.GetType().FullName) 
 'System.Management.Automation.ErrorRecord' 'error'; break 
 'System.Management.Automation.WarningRecord' 'warning'; break 
 default 'information'
 
 write-eventlog @eventlogparams -entrytype $entryType -message $_

Redirection *>&1 sends the output from all (*) streams to (&) the success stream (1), so that all output, irrespective of what stream it came from, is sent through the pipeline.

The above only deals with errors and warnings specifically, and reports everything else, including the success output, as information, but it's easy to extend the approach - see bottom.

See about_Redirections for an overview of all 6 output streams available in PowerShell (as of v6).

To illustrate the technique with a simpler example:

& Write-Output good; Write-Error bad; Write-Warning problematic *>&1 | ForEach-Object 
 $entryType = switch ($_.GetType().FullName) 
 'System.Management.Automation.ErrorRecord' 'error'; break 
 'System.Management.Automation.WarningRecord' 'warning'; break 
 default 'information'
 
 '[0] 1' -f $entryType, $_

The above yields:

[information] good
[error] bad
[warning] problematic

The list of data types output by the various streams:

Stream Type
------ ----
#1 (Success) (whatever input type is provided).
#2 (Error) [System.Management.Automation.ErrorRecord]
#3 (Warning) [System.Management.Automation.WarningRecord]
#4 (Verbose) [System.Management.Automation.VerboseRecord]
#5 (Debug) [System.Management.Automation.DebugRecord]
#6 (Information) [System.Management.Automation.InformationRecord]

The following code was used to produce the above list (except for the first data row):

& 
 $VerbosePreference = $DebugPreference = $InformationPreference = 'Continue'
 $ndx = 2
 "Write-Error", "Write-Warning", "Write-Verbose", "Write-Debug", "Write-Information" 

As @Lee_Dailey rightly pointed , you need the event source to exist.Even after that,Your snippet might throw error like (checked in PS v5)

The process cannot access the file
'C:UsersusernameDesktopwrite-eventlog' because it is being used by
another process.

because redirection operator expects a file to redirect not a cmdlet or function that's the reason for the above error.

you can try modifying the code so that redirection operator stores the data in files and then push that into event log:

myfunction -channel1 -channel2 > output.txt 2> error.txt 

write-eventlog @eventlogparams -entrytype error -message ((get-content error.txt) -join "")

write-eventlog @eventlogparams -entrytype information -message ((get-content output.txt) -join "")

Another method is using outvariable and errorvariable , for this to work the function must be advanced function (I have added cmdletbinding for that):

function myfunction 
[CmdletBinding()]
 Param(
 [switch]$channel1,
 [switch]$channel2
)
 if ($channel1) write-output 'channel 1 msg'
 if ($channel2) write-error 'channel 2 msg'


$eventlogparams = @'logname'='application';'source'='myapp';'eventid'='1'

myfunction -channel1 -channel2 -OutVariable output -ErrorVariable errordata
write-eventlog @eventlogparams -entrytype error -message ($errordata -join "")

write-eventlog @eventlogparams -entrytype information -message ($output -join "")