[BUG] [c-sharp] Inconsistency in default values discriminator

[tomelfring]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Using the C# generator I found some inconsistencies in the default values for classes with a discriminator with a mapping.

When the discriminator propertyName is in camelCase, I notice the following:

• JsonConverter uses a camelCase value
• The property of the discriminator has a DataMemberAttribute with a camelCase name
• In subclasses, the default value of the discriminator is default(string)

When the discriminator propertyname is in PascalCase, I notice the following:

• JsonConverter uses a camelCase value
• The property of the discriminator has a DataMemberAttribute with a PascalCase name
• In subclasses, the default value of the discriminator is matching the spec

Expected:

• The JsonConverter converterParameters and Datamember name to match
• The subclasses having meaningful default values for discriminator parameters when mapping is provided in the spec

openapi-generator version

openapi-generator-cli-4.3.1.jar

OpenAPI declaration file content or url

Two files, one with camelCase, one with PascalCase:
https://gist.github.com/thommy101/39b65b3a365a09d3738f26edc3e1682b

Command line used for generation

java -jar openapi-generator-cli-4.3.1.jar generate -i camelCase.yaml -g csharp -o camelCase
java -jar openapi-generator-cli-4.3.1.jar generate -i pascalCase.yaml -g csharp -o pascalCase

Steps to reproduce

• Execute above two command lines
• Compare the camelCase and pascalCase variants of src/Org.OpenAPITools/Models/Animal.cs and src/Org.OpenAPITools/Models/Cat.cs
• See the CamelCase variant of Animal having consistent JsonConverter-attribute on the class and DataMember-attribute on the property ClassName, but not having a default value for Cat's constructor parameter className (default(string))
• See the PascalCase variant of Animal having inconsistent JsonConverter-attribute on the class and Datamember-attribute on the property ClassName, but having a correct default value for Cat's constructor parameter className ("Cat")

Related issues/PRs

#3308

#5680: Related because forcing the JsonConverter attribute to camelCase and because of the comment:

Right now if this discriminator isn't camelCased it causes things to not deserialize properly when receiving them from an API (since the json is expected to come back camelcased)

Suggest a fix

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[tomelfring]

Did some debugging on the default value, found a bug in the generator. I've opened PR #6232 for that.

The inconsistency of the attributes is caused by #5680 which forces the JsonConverter converterParameters to be camelCase. I don't know how to fix it, as that PR comments that PascalCase is not in line with the JSON standard.