[BUG] [csharp-netcore] Inadequate constructor with oneOf constraint

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When oneOf is used along with a specific set of codegen options, we see this problem. In the following example, SomethingMydata.cs has two constructors: SomethingMydata() and SomethingMydata(string actualInstance). When the latter is used, it's impossible to know if the input is "Alpha" or "HelloWorld". If application code wants to initialize Something with "Alpha" there's no proper way to do this. Note the "Alpha" can contain two properties: "abc" and "def". Thus, Alpha can't be reduced to a single string.

openapi-generator version

openapi-generator-cli-6.5.0-20230329.141901-70.jar from https://oss.sonatype.org/content/repositories/snapshots/org/openapitools/openapi-generator-cli/6.5.0-SNAPSHOT/

Did not test with 6.5.0 which has just been released. However, I expect the same problem.

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  version: '1.0.0'
  title: 'oneOf constructor issue'
paths: {}
components:
  schemas:
    Something:
      type: object
      properties:
        mydata:
          oneOf:
            - $ref: '#/components/schemas/Alpha'
            - $ref: '#/components/schemas/HelloWorld'
      required:
        - mydata
    Alpha:
      type: object
      required:
        - abc
      properties:
        abc:
          $ref: '#/components/schemas/Abc'
        def:
          $ref: '#/components/schemas/Def'
    HelloWorld:
      type: string
      format: base64
      nullable: true
    Abc:
      type: string
      pattern: '^[A-Fa-f0-9]{32}$'
    Def:
      type: string
      pattern: '^[A-Fa-f0-9]{8,32}$'

Generation Details

java -jar lib/openapi-generator-cli-6.5.0-20230329.141901-70.jar generate -i OneOf.yaml -g csharp-netcore -o OneOf --additional-properties=targetFramework=net7.0,skipOneOfAnyOfGetter=true --additional-properties=packageName=Com.Example --skip-validate-spec --openapi-normalizer SIMPLIFY_BOOLEAN_ENUM=true,SIMPLIFY_ANYOF_STRING_AND_ENUM_STRING=true,REMOVE_ANYOF_ONEOF_AND_KEEP_PROPERTIES_ONLY=true,ADD_UNSIGNED_TO_INTEGER_WITH_INVALID_MAX_VALUE=true,SIMPLIFY_ONEOF_ANYOF=true

Steps to reproduce

As above

Related issues/PRs

1. https://github.com/OpenAPITools/openapi-generator/issues/14350
2. https://github.com/OpenAPITools/openapi-generator/issues/2791

Suggest a fix

No idea at the moment.

Start by reproducing the issue with the provided OpenAPI declaration and csharp-netcore generation command. Inspect the csharp-netcore generator's oneOf model and constructor handling; the fix is complete when generated SomethingMydata code provides an unambiguous way to initialize either Alpha or HelloWorld and the generated output is covered by an appropriate test.