[BUG][C#] Generated C# client does not produce nullable response types

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?0
• 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

When using the OpenAPI Generator CLI (version 7.10.0) to generate a C# client with the nullableReferenceTypes=true option, methods returning a nullable schema do not generate nullable response types (e.g., Task<MyType?>).

openapi-generator version

7.10.0

OpenAPI declaration file content or url

Example.yaml or Example2.yaml

Generation Details

My Environment:

• Windows 11 Enterprise
• openjdk version "23.0.1" 2024-10-15

I installed openapi-generator-cli via:
npm install -g @openapitools/openapi-generator-cli

Configuration file:
config.yaml

Steps to reproduce

In my C:\Temp folder I have only Example.yaml and config.yaml files. I'm running next command to generate api in Console:
openapi-generator-cli generate -c config.yaml

As a result I have C:\Temp\output-directory\src\Org.OpenAPITools\Api\DefaultApi.cs with next code:
public async System.Threading.Tasks.Task<MyType> ExamplePostAsync(...
You can find it in attachment DefaultApi.cs.txt.

However, I expected to have
public async System.Threading.Tasks.Task<MyType?> ExamplePostAsync(...

Related issues/PRs

Suggest a fix

Ensure that the C# generator respects the nullableReferenceTypes=true configuration and generates nullable response types (Task<MyType?>) for schemas defined as nullable: true or oneOf with null.

Reproduce the issue with Example.yaml and config.yaml using the openapi-generator-cli command, then compare the generated DefaultApi.cs output with the expected Task<MyType?> signature. Trace the C# generator handling of nullableReferenceTypes=true and nullable schemas, and consider the issue done when the generated response types preserve nullability for the reported cases.