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

The latest release (v7.4.0) can generate C# code that does not compile when generating for aspnetcore and setting nullableReferenceTypes=true.

It happens when a property is optional and declared with "nullable: true". See the problem property in the example OpenAPI document. In the model class it generates the property where the type is declared with double question marks (??) instead of a single question mark.

Using version v7.3.0 it works fine.

openapi-generator version

7.4.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: A bug report
  description: Example for bug report
  version: "1.0.0"
tags:
  - name: demo
    description: Demo tag

paths:
  /v1/example:
    get:
      tags:
        - demo
      responses:
        '200':
          description: Ok
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/demoResponse'
                
components:
  schemas:
    demoResponse:
      type: object
      required:
        - ok
      properties:
        problem:
          type: string
          nullable: true
          description: This generates code that can not compile. 
        ok:
          type: string
          nullable: true
          description: This genrates code that can compile.

Generation Details

Using docker image:
docker run --rm --user $(id -u) -v $(pwd)/Documentation:/local openapitools/openapi-generator-cli:latest-release generate -i /local/api.yaml -o /local/Generated -g aspnetcore -p nullableReferenceTypes=true

or cli
java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i ~/api.yaml -o ./output -g aspnetcore -p nullableReferenceTypes=true

Steps to reproduce

1. Run below command to generate code from OpenAPI document.
   java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i ~/api.yaml -o ./output -g aspnetcore -p nullableReferenceTypes=true
2. Run below command to build dotnet solution.
   dotnet build output/

Related issues/PRs

Suggest a fix