[BUG][csharp] Error with using enum and default

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?
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Description

I have a model with a property. The property is an enum with a default. If I try to instantiate the model I get an argument error (see below). If I change the constructor on the generated model so the default for the argument is null then the error is resolved.

Here's the relevant input:
https://github.com/jason-o-matic/docraptor-csharp/blob/852a3895792292b4483eb43d17be07fc02556503/docraptor.yaml#L391-L398

Here's the code that produces the error:
https://github.com/jason-o-matic/docraptor-csharp/blob/852a3895792292b4483eb43d17be07fc02556503/test/prince_options.cs#L43-L45

Here's the error message I get when running that code:

test/prince_options.cs(43,22): error CS1729: The type `DocRaptor.Model.PrinceOptions' does not contain a constructor that takes `24' arguments
/app/bin/DocRaptorWithDependencies.dll (Location of the symbol related to previous error)
Compilation failed: 1 error(s), 0 warnings

Here's the commit where you can see the change that fixes the issue:
https://github.com/jason-o-matic/docraptor-csharp/commit/0a3f87bd2a52afa2e002da8c1b0284cd948a555e

That line is pretty long, but the important part is that

InputEnum? input = InputEnum.Html

changes to

InputEnum? input = null

I'm not super familiar C# so I'm not really sure why this is erroring in the first place. It seems like using an enum as an argument default should be fine. It seems like switching to null is reasonable because there's https://github.com/jason-o-matic/docraptor-csharp/blob/852a3895792292b4483eb43d17be07fc02556503/src/DocRaptor/Model/PrinceOptions.cs#L134-L141 in the constructor to set the default.

openapi-generator version

I got the same error on 6.6.0 and several previous versions.

OpenAPI declaration file content or url

https://raw.githubusercontent.com/jason-o-matic/docraptor-csharp/852a3895792292b4483eb43d17be07fc02556503/docraptor.yaml

Generation Details

This was used in the above repo to generate the client:

docker run -it -w /local --rm -v /Users/jason/projects/docraptor/docraptor-csharp:/local openapitools/openapi-generator-cli:v6.6.0 generate -i docraptor.yaml -g csharp -c generator-config.json

Running script/test in the above repo produced the error. It basically uses several different docker mono versions to run the files in the test directory.

Steps to reproduce

1. Generate the client where a model has an enum parameter with a default
2. Run code to instantiate that model

Related issues/PRs

I looked through https://github.com/OpenAPITools/openapi-generator/issues?q=is%3Aissue+is%3Aopen+enum+default+csharp but didn't see anything I recognized as being the same.

Suggest a fix

It seems that switching the default value of the argument to null fixes the issue, but I'm not sure if that's the best solution. I noticed the other arguments used default() around the default argument, so maybe that's another option, but perhaps using the enum inside default() would still cause an issue?

Start with the linked PrinceOptions.cs and the provided OpenAPI declaration, then reproduce using the documented openapi-generator CLI command and the C# generator. Compare the generated enum parameter default with the constructor's default-setting logic and run the generated project's test command. Done means the generated client compiles and model instantiation succeeds with an enum default.