[BUG] [csharp-netcore] When requestBody has OneOf, then it generates client with "UNKNOWN_BASE_TYPE"

[prajon84]

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 the requestBody has oneOf properties, then the client is generated with UNKNOWN_BASE_TYPE as a request parameter.

openapi-generator version

master @ e600f7d

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: C# Object with requestBody has oneOf
  version: v1
paths:
  '/hello':
    patch:
      responses:
        '200':
          description: returns compared object
          content:
            application/json:
              schema:
                type: string
      requestBody:
        required: true
        content:
          application/json:
            schema:
              oneOf:
                - type: string
                - type: integer

Generation Details

Steps to reproduce

>openapi-generator generate -g csharp-netcore -i openapi.yaml --library httpclient -o out/requestBody_with_oneOf/ --model-package Model --api-package Api

>cd out/requestBody_with_oneOf
>dotnet build --configuration Release

produces:

C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Api\DefaultApi.cs(299,34): error CS0246: The type or namespace name 'UNKNOWN_BASE_TYPE' could not be found (are you missing a using directive or an assembly reference?) [C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Org.OpenAPITools.csproj]
C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Api\DefaultApi.cs(36,27): error CS0246: The type or namespace name 'UNKNOWN_BASE_TYPE' could not be found (are you missing a using directive or an assembly reference?) [C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Org.OpenAPITools.csproj]
C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Api\DefaultApi.cs(311,83): error CS0246: The type or namespace name 'UNKNOWN_BASE_TYPE' could not be found (are you missing a using directive or an assembly reference?) [C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Org.OpenAPITools.csproj]
C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Api\DefaultApi.cs(67,61): error CS0246: The type or namespace name 'UNKNOWN_BASE_TYPE' could not be found (are you missing a using directive or an assembly reference?) [C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Org.OpenAPITools.csproj]
C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Api\DefaultApi.cs(47,52): error CS0246: The type or namespace name 'UNKNOWN_BASE_TYPE' could not be found (are you missing a using directive or an assembly reference?) [C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Org.OpenAPITools.csproj]
C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Api\DefaultApi.cs(79,86): error CS0246: The type or namespace name 'UNKNOWN_BASE_TYPE' could not be found (are you missing a using directive or an assembly reference?) [C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Org.OpenAPITools.csproj]
C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Api\DefaultApi.cs(356,74): error CS0246: The type or namespace name 'UNKNOWN_BASE_TYPE' could not be found (are you missing a using directive or an assembly reference?) [C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Org.OpenAPITools.csproj]
C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Api\DefaultApi.cs(369,123): error CS0246: The type or namespace name 'UNKNOWN_BASE_TYPE' could not be found (are you missing a using directive or an assembly reference?) [C:\Projects\ssh_clone\try-for-openapi-issue\requestBody_with_oneOf\src\Org.OpenAPITools\Org.OpenAPITools.csproj]

Related issues/PRs

None that I could find.

Suggest a fix

@wing328 @Blackclaws @shibayan @lucamazzanti @devhl-labs @jfeltesse-mdsol

[jfeltesse-mdsol]

Probably related to #5903

The root cause is in the generator core itself and as such affects multiple languages.

[wing328]

This should be fixed via #12353. Please pull the latest master to give it a try.

Happy to reopen this issue if needed.