Resource and Canonical Properties use of null

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest version
• Search the existing issues.

Summary

There are built-in resources and canonical properties (_inDesiredState) that use null in addition to actual type of the property. Null should NOT be a valid type unless the property actually accepts null as a valid use case. By allowing null it can confuse users when we have auto-complete in BICEP and weakens the strongly validated schema.

JSON schema null and undefined properties are different.

https://json-schema.org/understanding-json-schema/reference/null

Excerpt:

It's important to remember that in JSON, null isn't equivalent to something being absent. See Required Properties for an example.

The mechanism to indicate a property is required or optional is by the use of the required property.

I understand that we're using nullable types so we can return only the properties that are actually defined instead of empty strings or false bools but it is not best practice.

Resources with Nullable Properties:

1. Microsoft.DSC.Debug/Echo
   showSecrets: type: ["boolean", "null"]

2. Microsoft.DSC.Transitional/PowerShellScript
   getScript: type: ["string", "null"]
   setScript: type: ["string", "null"]
   testScript: type: ["string", "null"]
   input: type: ["string", "boolean", "integer", "object", "array", "null"]
   output: type: ["array", "null"]
   _inDesiredState: type: ["boolean", "null"]

3. Microsoft.DSC.Transitional/WindowsPowerShellScript
   getScript: type: ["string", "null"]
   setScript: type: ["string", "null"]
   testScript: type: ["string", "null"]
   input: type: ["string", "boolean", "integer", "object", "array", "null"]
   output: type: ["array", "null"]
   _inDesiredState: type: ["boolean", "null"]

4. Microsoft.OpenSSH.SSHD/Windows
   shell: type: ["string", "null"]
   cmdOption: type: ["string", "null"]
   escapeArguments: type: ["boolean", "null"]

5. Microsoft.Windows/RebootPending
   reasons: type: ["array", "null"]
   The entire object itself: type: ["object", "null"]

6. Microsoft.Windows/Registry
   _metadata: anyOf: [{"$ref": "#/$defs/Metadata"}, {"type": "null"}]
   valueName: type: ["string", "null"]
   valueData: anyOf: [{"$ref": "#/$defs/RegistryValueData"}, {"type": "null"}]
   _exist: type: ["boolean", "null"]

Start by reviewing the JSON schemas for the listed DSC resources and the canonical _inDesiredState property, using the issue's resource and property list as the scope. Check which properties genuinely accept null versus merely being optional, then validate the resulting schemas against the documented JSON Schema semantics and Bicep autocomplete expectations. Done means removing unjustified null types while preserving legitimate nullable values.