[BUG] [csharp-netcore] Constructor accepts default null values for strings but no null check in code

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

See the example below. The generated constructor has the signature public Capability(string dlRate = default(string), string ulRate = default(string)). If application calls this constructor with only dlRate, ulRate will be set to null. However, within the class UlRate property is not nullable: public string UlRate { get; set; }

In Validate() method, we have this code: if (false == regexUlRate.Match(this.UlRate).Success)
This creates a runtime exception when UlRate is null.

openapi-generator version

openapi-generator-cli-6.5.0-20230329.141901-70.jar from https://oss.sonatype.org/content/repositories/snapshots/org/openapitools/openapi-generator-cli/6.5.0-SNAPSHOT/

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  version: '1.0.0'
  title: 'Regex issue'
paths: {}
components:
  schemas:
    Capability:
      properties:
        dlRate:
          $ref: '#/components/schemas/BitRate'
        ulRate:
          $ref: '#/components/schemas/BitRate'
    BitRate:
      type: string
      pattern: '^\d+(\.\d+)? (bps|Kbps|Mbps|Gbps|Tbps)$'

Generation Details

java -jar lib/openapi-generator-cli.jar generate -i RegExp.yaml -g csharp-netcore -o RegExp --additional-properties=targetFramework=net7.0 --additional-properties=packageName=Com.Example --skip-validate-spec --openapi-normalizer SIMPLIFY_BOOLEAN_ENUM=true,SIMPLIFY_ANYOF_STRING_AND_ENUM_STRING=true,REMOVE_ANYOF_ONEOF_AND_KEEP_PROPERTIES_ONLY=true,ADD_UNSIGNED_TO_INTEGER_WITH_INVALID_MAX_VALUE=true,SIMPLIFY_ONEOF_ANYOF=true

Steps to reproduce

As above

Related issues/PRs

#15101 is also a C# regex issue

Suggest a fix

If statement can check for non-null first:
if (this.UlRate != null && false == regexUlRate.Match(this.UlRate).Success)

Start by running the provided generation command with RegExp.yaml and inspect the generated Capability constructor and Validate() method. Reproduce the case where only dlRate is supplied; done means validation no longer throws when the optional ulRate value is null, with the generated output or a regression test demonstrating the behavior.