DSC: Script Resource GetScript

If you look on the TechNet page for the Script Resource you will see

GetScript = { <# This must return a hash table #> }

Which is technically speaking, true...usually...right up until the point you try to run Get-DscConfiguration on a machine, in which case it will get to that script resource and die saying:

The PowerShell provider returned results that are not valid from Get-TargetResource. The <keyname> key is not a valid property in the corresponding provider schema file. The results from Get-TargetResource must be in a Hashtable format. The keys in the Hashtable must be the same as the properties in the corresponding provider schema file.

The consensus around the web is that the error is saying you have to return a hashtable with keys that match the properties of the schema, so in this case the schema for the Script resource is:

#pragma namespace("\\\\.\\root\\microsoft\\windows\\DesiredStateConfiguration")

[ClassVersion("1.0.0"),FriendlyName("Script")] 
class MSFT_ScriptResource : OMI_BaseResource
{
  [Key] string GetScript;
  [Key] string SetScript;
  [Key] string TestScript;
  [write,EmbeddedInstance("MSFT_Credential")] string Credential;
  [Read] string Result;
};

Which means in order for your Script resource to be compliant you need to return:

GetScript = {return @{ Result = ();GetScript=$GetScript;TestScript=$TestScript;SetScript=$SetScript}}

But when you think about it, this doesn't make a lot of sense. In every other resource I can think of it makes absolute sense, because the parameters in the schema determine the status of the resource you want to control, not how you control it and how you test for it.

It would be like Get-TargetResource for the Registry resource not returning the information about the key, its value, etc. but rather returning that AND returning the entire contents of MSFT_RegistryResource.psm1 which would make literally no sense. We don't care HOW you check or HOW you set, and returning a Get-Script with the contents of Get-Script is...batty...we care about the resource being controlled.

Luckily, the statement that "the keys need to match the parameters" can be interpreted to mean you need to match ALL of them, or it can be interpreted to mean "they just need to exist" and in the case of the Script resource Result does exist. And that is what we need to return.

GetScript = {return @{Result=''}}

They really need to update the TechNet page to say "GetScript needs to return a hash table with at least one key matching a parameter in the schema for the resource".

No need to return potentially hundreds of lines of code in some M.C. Escher-like construct containing itself. Just stick to returning information about the resource you are controlling. If your script sets the contents of a file, return the contents of that file. Not the contents of the file AND the script you used to set it AND the script you used to test it.