Recently I came across a puzzling issue while writing a custom Powershell DSC resource - when I tried to use it more than once in a single configuration, I would get this error message;

Add-NodeKeys : The key properties combination ‘{$yourModuleCode}’ is duplicated for keys ‘GetScript,SetScript,TestScript’ of resource ‘Script’ in node ‘localhost’. Please make sure key properties are unique for each resource in a node

After a bit of googling, I found a very informative article by Brian Scholer that explained to me what was going on.

Basically, Powershell DSC doesn’t like it when you have exact duplicates of a Script resource in the same Configuration block, which will happen if you place a Script within a ForEach-Object loop.

Powershell doesn’t take the value of $variable values into account until the MOF file is actually applied to a machine, so a ForEach-Object loop will generate identical Script blocks.

ForEach ($number in 1,2,3){
    Script example {
        TestScript  = { return $False}
        SetScript   = { Write-Output $Using:number}
        GetScript   = { @{ Result = "Hardcoded"}}
    }
}

An example configuration that would trigger the error

The differences would only become apparent at runtime, but by this point the MOF checker has already thrown an Exception and halted the MOF generator.

This problem was sufficiently frustrating

A workaround

Brian described an interesting solution - using a custom function, he substituted the value of $Using:variables before the uniqueness check happened. This means that the MOF checker will treat each Script block as being slightly different, as they have been modified to include some uniqueness.

ForEach ($number in 1,2,3){
    $v = @{ Result = $number }
    Script MyScript
    {   
        ...
        GetScript = { $Using:v } | Replace-Using
    }
}

Piping the GetScript value into Replace-Using to introduce uniqueness

A simpler workaround with GUID strings

Building on top of Brian’s findings, I believe I have found a much simpler solution for introducing uniqueness into Script blocks.

[guid]::NewGuid()) will generate a random GUID string, that will look like this;

Guid
----
882fb2c8-e9cd-4c9f-83f4-27fe34a55934

Running the NewGuid() function should always generate random strings with a low chance of duplicate collision.

By generating a GUID value in the GetScript section of the Script block, a unique string being placed into the hashtable that it returns.

GetScript =  "@{ Result = $([guid]::NewGuid()) }"

If you ForEach-Object loop over the Script block multiple times now, the GetScript return value will be unique for each iteration. This should prevent the Please make sure key properties are unique for each resource in a node error.

GUID generation takes a fraction of a second, so should not affect the performance of your DSC configuration.