[BUG][CSHARP-NETCORE] OneOf generator for not nullable primitives generates invalid code.

Created by: jafin

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
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Description

A template that contains oneOf logic for a not nullable primitives generates invalid code.

openapi-generator version

5.4 and ff7c2bdb

OpenAPI declaration file content or url

Part of schema relating to this bug report:

PolymorphicProperty:
  oneOf:
  - type: boolean
  - type: string
  - type: object
  - type: array
     items:
  $ref: '#/components/schemas/StringArrayItem'
StringArrayItem:
  type: string
  format: string

Full schema: https://gist.github.com/jafin/5b06f1a09e8a8c983bf25d61c47c1f4e

Generation Details

Checking non nullable primitive for null.

existing (compile error)

public PolymorphicProperty(bool actualInstance)
{
	//...
	this.ActualInstance = actualInstance ?? throw new ArgumentException("Invalid instance found. Must not be null.");
}

for primitives should not have the conditional check, i.e.

public PolymorphicProperty(bool actualInstance)
{
	//...
	this.ActualInstance = actualInstance;
}

XML Comments should be escaped to accomodate to Generic <> Types

existing

/// <summary>
/// Initializes a new instance of the <see cref="PolymorphicProperty" /> class
/// with the <see cref="List<string>" /> class
/// </summary>
/// <param name="actualInstance">An instance of List<string>.</param>

better

/// <summary>
/// Initializes a new instance of the <see cref="PolymorphicProperty" /> class
/// with the <see cref="List&lt;string&gt;" /> class
/// </summary>
/// <param name="actualInstance">An instance of List&lt;string&gt;.</param>

Getter for Generic Type has an invalid method name

existing (won't compile)

public List<string> GetList<string>()

should maybe be:

public List<string> GetListString()

Steps to reproduce

Generate csharp code:

java -jar openapi-generator-cli.jar generate -i firefly-iii-1.5.5.yaml -g csharp-netcore --library httpclient