[BUG] [csharp] EmitDefault false should allow to set null value for the nullable property type.

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Description

As per open API specification there are two ways to define the type which can be set to null. When EmitDefault is set to false in config file while generating the sdk, nullable property should not have attribute EmitDeafult = false
Here is the two case in which nullable property has two different behavior.

In case1 csharp does not apply EmitDefault fasle to the nullable property which is perfectly fine, where as in case2 it is unable to recognize the AssignesServer as nullable property and set the EmitDefault : false.

First way to define nullable property. (generator works as expected.)
Case1:-

"ConfigChangeDetails": 
{
      "description": "An array of relationships to serverConfigChangeDetail resources.",
      "type": "array",
      "items": {
        "$ref": "#/components/schemas/server.ConfigChangeDetail.Relationship"
      },
      "nullable": true,
      "readOnly": true,
      "x-since": "2017-10-02"
  }

for the above definition the generator is able to generate the perfect code as per the definition, with EmitDefault = true so that null can also be set.

The issue exist is the below i.e second way to define nullable property

Case2:- (generator does not work as expected.)

"AssignedServer": {
      "$ref": "#/components/schemas/compute.Physical.Relationship",
      "description": "A reference to a computePhysical resource.\nWhen the $expand query parameter is specified, the referenced resource is returned inline.",
      "x-since": "2017-10-02"
}

"compute.Physical.Relationship": {
        "description": "A relationship to the 'compute.Physical' resource, or the expanded 'compute.Physical' resource, or the 'null' value.",
        "x-one-of-name": "compute.Physical.Relationship",
        "oneOf": [
          {
            "type": "null"
          },
          {
            "$ref": "#/components/schemas/mo.MoRef"
          },
          {
            "$ref": "#/components/schemas/compute.Physical"
          }
        ],
        "discriminator": {
          "propertyName": "ClassId"
        }
      }

The above property AssignedServer can also be set to null.
But the generator generates the code with EmitDefault = false which leads to issue, so the AssignedServer cannot be set to null.
generator should recognize such property as nullable property.

openapi-generator version

Latest Master

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Set the EmitDeafult : false in config file.
In the open API document define the property with oneOf which can be accept null or an object.
generate the code and check the EmitDeaflt attribute, it will never allow to set this to null, where as null is valid value for that property.

Related issues/PRs

Suggest a fix

generator needs to parse the property as nullable property.

Start by reproducing the C# generation case with EmitDefault set to false and the supplied oneOf schema containing a null type. Trace how the generator identifies nullable properties and emits the EmitDefault attribute; done means AssignedServer is recognized as nullable and the generated property can be set to null without the incorrect attribute.