and finally

If you’re old enough and seen UK TV you’ll recognise the title but this post is about using try – catch blocks.

Using try-catch this is a fairly normal construction

try {
Get-CimInstance -ClassName Win32_LogicalDisk  -ErrorAction Stop
}
catch {
Throw "something went wrong"
}

The number of commands within the try block should be minimised so that you stand a chance of knowing what you are going to catch.

if you introduce an error – for instance using the wrong classname

£> try {
Get-CimInstance -ClassName Win32_LogicalDrive  -ErrorAction Stop
}
catch {
Throw "something went wrong"
}
something went wrong
At line:5 char:2
+  Throw "something went wrong"
+  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (something went wrong:String) [], RuntimeException
    + FullyQualifiedErrorId : something went wrong

 

You can control what happens and even recover.

I don’t see the third element of a try-catch block being used much – that’s the finally block. The finally block will run irrespectively of whether the try block works or not. It’s your clean up block

Consider

£> try {
$cs = New-CimSession -ComputerName $env:COMPUTERNAME
Get-CimInstance -ClassName Win32_LogicalDrive  -CimSession $cs -ErrorAction Stop
}
catch {
Throw "something went wrong"
}
something went wrong
At line:6 char:2
+  Throw "something went wrong"
+  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (something went wrong:String) [], RuntimeException
    + FullyQualifiedErrorId : something went wrong
 

£> Get-CimSession


Id           : 1
Name         : CimSession1
InstanceId   : 68a4f534-d222-4e52-b6f9-ba65b1d57b3b
ComputerName : RSSURFACEPRO2
Protocol     : WSMAN

 

This try block fails but it leaves a CIM session hanging about. Not good practice. I know it will get cleaned up when you close the powershell session but that could be hours away. Better to clean up now.

£> try {
$cs = New-CimSession -ComputerName $env:COMPUTERNAME
Get-CimInstance -ClassName Win32_LogicalDrive  -CimSession $cs -ErrorAction Stop
}
catch {
Throw "something went wrong"
}
finally {
Remove-CimSession -CimSession $cs
}

Get-CimSession
something went wrong
At line:6 char:2
+  Throw "something went wrong"
+  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (something went wrong:String) [], RuntimeException
    + FullyQualifiedErrorId : something went wrong
 

no sign of a CIM session even if you try specifically.

 

£> Get-CimSession

£>

if you correct the code

try {
$cs = New-CimSession -ComputerName $env:COMPUTERNAME
Get-CimInstance -ClassName Win32_LogicalDisk  -CimSession $cs -ErrorAction Stop
}
catch {
Throw "something went wrong"
}
finally {
Remove-CimSession -CimSession $cs
}

Get-CimSession

 

You’ll find the the CIM session is still removed. A finally block will always run

When you are using try blocks think about your environment and use a finally block to clean up.

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