Catching dynamic exception types in PowerShell

Question

I'm working on a PowerShell library that automates some network management operations. Some of these operations have arbitrary delays, and each can fail in unique ways. To handle these delays gracefully, I'm creating a generic retry function that has three main purposes:

1. Execute an arbitrary command (with parameters)
2. If it fails in a recognized way, try it again, up to some limit
3. If it fails in an unexpected way, bail and report

The problem is item #2. I want to be able to specify the expected exception type for the command. How can I do this in PowerShell?

Here's my function:

Function Retry-Command {
    [CmdletBinding()]
    Param(
        [Parameter(Mandatory=$True, Position=0)]
        [String] $name,

        [Parameter(Mandatory=$True, Position=1)]
        [String] $scriptBlock,

        [String[]] $argumentList,
        [Int] $maxAttempts=3,
        [Int] $retrySeconds=10,
        [System.Exception] $retryException=[System.Management.Automation.RuntimeException]
    )
    $attempts = 1
    $keepTrying = $True
    $cmd = [ScriptBlock]::Create($scriptblock)
    do {
        try {
            &$cmd @argumentList
            $keepTrying = $False
            Write-Verbose "Command [$commandName] succeeded after $attmpts attempts."
        } catch [$retryException] {
            $msg = "Command [$commandName] failed. Attempt $attempts of $maxAttempts."
            Write-Verbose $msg;
            if ($maxAttempts -gt $attempts) {
                Write-Debug "Sleeping $retrySeconds"
                Start-Sleep -Seconds $retrySeconds
            } else {
                $keepTrying = $False
                Write-Debug "Reached $attempts attempts. Re-raising exception."
                Throw $_.Exception
            }
        } catch [System.Exception] {
            $keepTrying = $False
            $msg = "Unexpected exception while executing command [$CommandName]: "
            Write-Error $msg + $_.Exception.ToString()
            Throw $_.Exception
        } finally {
            $attempts += 1
        }
    } while ($True -eq $keepTrying)
}

I call it like this:

$result = Retry-Command -Name = "Foo the bar" -ScriptBlock $cmd -ArgumentList $cmdArgs

But this is the result:

Retry-Command : Cannot process argument transformation on parameter 'retryException'. 
Cannot convert the "System.Management.Automation.RuntimeException" value of type "System.RuntimeType" to type "System.Exception".
At Foo.ps1:111 char:11
+ $result = Retry-Command <<<<  -Name "Foo the bar" -ScriptBlock $cmd -ArgumentList $cmdArgs
    + CategoryInfo          : InvalidData: (:) [Retry-Command], ParameterBindin...mationException
    + FullyQualifiedErrorId : ParameterArgumentTransformationError,Retry-Command

This seems to be saying that the type of [System.Management.Automation.RuntimeException] is not itself a [System.Exception], but is instead a [System.RuntimeType] which makes sense.

So, how do I specify the type of the exception to be caught?

Answer 1

It's not possible to use a variable as a catch criteria, it has to be a type-object (or something), everything else gives you an error. A workaround would be something like this:

#You can get the name of the exception using the following (or .Name for just the short name)
#PS > $myerr.Exception.GetType().Fullname
#System.UnauthorizedAccessException


function test {
    param(
    #Validate that specified name is a class that inherits from System.Exception base class
    [ValidateScript({[System.Exception].IsAssignableFrom([type]$_)})]
    $ExceptionType
    )

    try {
        #Test-script, Will throw UnauthorizedAccessException when not run as admin
        (Get-Content C:\test.txt) | % { $_ -replace 'test','lol' } | Set-Content C:\test.txt
    }
    catch [System.Exception] {
        #Check if exceptiontype is equal to the value specified in exceptiontype parameter
        if($_.Exception.GetType() -eq ([type]$ExceptionType)) {
            "Hello. You caught me"
        } else {
        "Uncaught stuff: $($_.Exception.Gettype())"
        }
    }
}

A few tests. One with non-existing type, then with non-exception type, and finally a working one

PS > test -ExceptionType system.unaut
test : Cannot validate argument on parameter 'ExceptionType'. Cannot convert the "system.unaut" val
ue of type "System.String" to type "System.Type".
At line:1 char:21
+ test -ExceptionType system.unaut
+                     ~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [test], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationError,test


PS > test -ExceptionType String
test : Cannot validate argument on parameter 'ExceptionType'. The "[System.Exception].IsAssignableF
rom([type]$_)" validation script for the argument with value "String" did not return true. Determin
e why the validation script failed and then try the command again.
At line:1 char:21
+ test -ExceptionType String
+                     ~~~~~~
    + CategoryInfo          : InvalidData: (:) [test], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationError,test


PS > test -ExceptionType UnauthorizedAccessException
Hello. You caught me