[BUG][csharp][dotnet] Query parameters with explode: true not supported

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

In OpenAPI 3.0, we can wrap query params into one param by using style: form and explode: true. (Ref: https://swagger.io/docs/specification/serialization/)

And the generated C# code has only one param in this method.

[Expected] But the param cannot be converted into the correct HTTP query param, eg. in CURL:

curl -X 'GET' \
  'https://raw.githubusercontent.com/resource?lang=en&scope=mobile' \ # correct
  -H 'accept: application/json'

[Actual] The generated code will convert the object's root name into the query param, eg. in CURL:

curl -X 'GET' \
  'https://raw.githubusercontent.com/resource?filter=XXXParam' \ # wrong
  -H 'accept: application/json'

openapi-generator version

v7.3.0

OpenAPI declaration file content or url

info:
  title: Example
  version: 1.0.0
openapi: 3.0.3
paths:
  "/resource":
    parameters:
      - name: filters
        in: query
        required: true
        schema:
          type: object
          additionalProperties:
            type: string
          example:
            lang: en
            scope: mobile
        style: form
        explode: true
    get:
      responses:
        '200':
          description: Ok
          content:
            application/json:
              schema:
                properties:
                  id:
                    type: string
                type: object

Generation Details

openapi-generator-cli generate -g csharp --additional-properties=library=httpclient --additional-properties=targetFramework=net6.0 -i file.yaml 

Steps to reproduce

1. Generate the code
2. new ResourceGetFiltersParam() and put some value into it
3. Call await DefaultApi().resourceGet(param)
4. Check the result

Related issues/PRs

• #10438
• #15633
• #7973
• #16435

Suggest a fix

In the ClientUtils.mustache, the generated code will call the function ParameterToMultiMap and the process will go to this step
https://github.com/OpenAPITools/openapi-generator/blob/275ed9296dea4a1a894cc5b3402b5b8f3a61e7d7/modules/openapi-generator/src/main/resources/csharp/ClientUtils.mustache#L87-L90

Like the suggestion in #16435 , we can use the similar way to solve this problem:

                  {{#isExplode}}
                    {{#isModel}}
                      for (let key of Object.keys({{paramName}})) {
                        localVarQueryParameter[key] = ({{paramName}} as any)[key];
                      }

                    {{/isModel}}
                    {{#isPrimitiveType}}
                        localVarQueryParameter['{{baseName}}'] = {{paramName}};
                    {{/isPrimitiveType}}
                  {{/isExplode}}
                  {{^isExplode}}
                      localVarQueryParameter['{{baseName}}'] = {{paramName}};
                  {{/isExplode}}

[!NOTE]
I want to open a Pull Request to solve this issue. ✍🙏

Start with the C# ClientUtils.mustache section linked in the issue, then trace the generated call to ParameterToMultiMap. Reproduce with the provided OpenAPI YAML and csharp generation command, using the filters example to compare the generated request with the expected lang=en&scope=mobile query. Done means exploded object properties are serialized as separate query parameters.