Add an ensurable DSC Resource interface with default implementation

[michaeltlombardi]

As an engineer developing and maintaining DSC Resources, I want to be able to inherit my implementation of an ensurable DSC Resource from a known implementation, overwriting defaults with my own custom handling only if needed. Having a defined interface would also help with developing ensurable DSC Resources, providing parse-time feedback on the implementation.

Ensurable resources are an extremely common pattern; any resource which can be added/set/removed should follow this pattern which is (at least somewhat) distinct from the requirements for a DSC Resource generally.

Examples

This is a short, non-exhaustive list of DSC Resources which are ensurable:

• JeaDsc: JeaRoleCapabilities and JeaSessionConfiguration
• DnsServerDsc: DnsRecordBase (which includes Ensure) is inherited by DnsRecordCname, DnsRecordPtr, DnsRecordA, DnsRecordAaaa, etc.
• AzureDevOpsDsc: AzDevOpsDscResourceBase (which includes Ensure) is inherited by AzDevOpsProject.

Proposal

Following on the proposals in #81 and #82, I propose the creation of an IEnsurableDscResource interface.

At a minimum, this interface should require:

• A GetEnsurableState method, which takes a hash table (or class instance) as input (the current state property hash) and returns an enum (not sure how feasible the return typing is, might have to be object or something instead)
• A TestEnsurableState method, which takes a hash table (or class instance) as input and returns a Boolean

With default implementations, IEnsurableDscResource could:

• Inherit from IDscResource interface (Define a base DSC Resource interface #81)
• Include the default DscBasicEnsure enum (Add a default Ensure enum to PSDesiredStateConfiguration #84) as the Ensure property
• Have the default implementation of GetEnsurableState return Present if any keys in the input are not null or empty, else Absent
• Have the default implementation of TestEnsurableState return $true if the value for the Ensure key in the input is equal to the value of the Ensure property on the class instance.
• Redefine the default implementation of Get as in Add a default implementation of a DSC Resource #82, but call GetEnsurableState to define the Ensure property of $CurrentState before returning it
• Redefine the default implementation of Test as in Add a default implementation of a DSC Resource #82, but call TestEnsurableState before comparing the rest of the resource properties directly, returning $false if any check fails and $true if they all pass.