[csharp] Generated enumerations as query parameters not compiling

Created by: rubms

Description

When specifying query parameters as enumerations, the autogenerated code uses the enumeration type, instead of making it Nullable. This C# code fails compile when those query parameters are assigned null because C# enumerations are not nullable (they are not objects, as Java enumerations).

The expected behavior is to generate nullable enumerations: EnumClass? varName.

openapi-generator version

This is reproduced with openapi-generator v3.3.3 (current master).

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: "This spec is for testing the wrong generation of enumerations in query parameters"
  version: 1.0.0
  title: Enumerations test case
paths:
  /enum_test/:
    get:
      tags:
        - Enums
      summary: Get with enumeration query params.
      description: Get with enumeration query params.
      operationId: getEnumTest
      parameters:
      - name: testEnum
        in: query
        schema:
          $ref: "#/components/schemas/EnumClass"
      responses:
        '200':
          description: successful operation
components:
  schemas:
    EnumClass:
      type: string
      enum:
      - Value1
      - Value2
      - Value3

Command line used for generation

java -jar openapi-generator-cli-3.3.3.jar generate -g csharp -i test.yml -o test

Steps to reproduce

Run the openapi-generator using the csharp generator and the attached OpenApi specification. The generated C# code will not compile because enumerations in C# are not nullable.

Suggest a fix/enhancement

Enumerations should be generated nullable instead.