$Using: scope in DSC script resources

If you have spent any time working with DSC resources, you have found yourself needing to use the Script resource for some things.

Script Script_Resource

{

    GetScript = {}

    SetScript = {}

    TestScript = {}

    DependsOn = ""

    Credential = ""

}

It is easy enough to use. Define your Get, Set, and Test PowerShell commands and you are all set. Everything is good until you need to work with passing in variables into your script block. You will quickly find that it does not work like you would expect.

This command will always return false:

$path = "c:\windows\temp"

Script Script_Resource

{

    TestScript = {

        Test-Path $path

    }

}

Because $path is not defined within the scope of the TestScript to a value, the Test-Path will return false. Take a look at the mof file and you can see this. 

instance of MSFT_ScriptResource as $MSFT_ScriptResource1ref

{

 ResourceID = "[Script]Script_Resource";

 TestScript = "\n Test-Path '$path'\n ";

 SourceInfo = "::7::9::Script";

 ModuleName = "PSDesiredStateConfiguration";

 ModuleVersion = "1.0";

};

I have found two ways to deal with this issue. If you think about it, the TestScript is just a string that gets ran on the target node. If you look at the resource, TestScript is defined as a String.

$path = "c:\windows\temp"

Script Script_Resource

{

    TestScript ="Test-Path '$path'"

}

This works really well when the command is a very simple one line script. Take a look at the mof file now.

instance of MSFT_ScriptResource as $MSFT_ScriptResource1ref

{

 ResourceID = "[Script]Script_Resource";

 TestScript = "Test-Path 'c:\\windows\\temp'";

 SourceInfo = "::6::9::Script";

 ModuleName = "PSDesiredStateConfiguration";

 ModuleVersion = "1.0";

}; 

This could end up very messy if the script block gets more complicated. What if you have variables that you want to define in the script and use some from the parent scope. You end up escaping things with that horrible back tick. But there is a better way.

This is where the $using: scope comes to the rescue. As far as I can tell, this is undocumented for use in script resources. But using it in Invoke-Command script blocks will allow you to reference variables in the parent scope. It works for our script resource too.

$path = "c:\windows\temp"

Script Script_Resource

{

    TestScript = {

        Test-Path $using:path

    }

} 

Now when we dive into the mof file, we can see just how this magic works. Our $path gets inserted into the script as part of the script. 

instance of MSFT_ScriptResource as $MSFT_ScriptResource1ref

{

 ResourceID = "[Script]Script_Resource";

 TestScript = "$path='c:\\windows\\temp'\n Test-Path $path\n ";

 SourceInfo = "::7::9::Script";

 ModuleName = "PSDesiredStateConfiguration";

 ModuleVersion = "1.0";

};

The $using: scope is something I often overlook but this will be a very handy way to use it.

One final note about my examples. I did trim them down to minimize the code. If you want to recreate my tests, you will need to have the SetScript and GetScript properties defined for each script block.