Nullable enum does not populate nullable

[fairking]

The following class:

public class BookingQueryForm
{
    public BookingStatusEnum? Status { get; set; }
}

populates the following schema:

"BookingQueryForm": {
        "required": [
        ],
        "type": "object",
        "properties": {
          "status": {
            "$ref": "#/components/schemas/BookingStatusEnum"
          }
        },
        "additionalProperties": false
      },
      "BookingStatusEnum": {
        "enum": [
          "Booked",
          "Waiting",
          "Cancelled"
        ],
        "type": "string"
      },

Please note that the status property is not nullable while it should be:

 "properties": {
          "status": {
            "$ref": "#/components/schemas/BookingStatusEnum",
            "nullable": true
          }
        },

Related issue: RicoSuter/NSwag#4397

[kjkrum]

I noticed this while digging into a different problem with enum properties. It may have something to do with how enum nullability is defined in the OpenAPI spec. In languages like C#, nullability is a property of the reference. In OpenAPI, nullability of an enum is a matter of null being a member of the enum. You can use an ISchemaFilter to set Nullable and add OpenApiNull to the Enum property of enum schemas. I tried this and it didn't solve my other problem, but I haven't tried generating an NSwag client against the resulting schema.

[martincostello]

To make issue tracking a bit less overwhelming for the new maintainers (see #2778), I've created a new tracking issue to roll-up various nullability issues here: #2793.

We'll refer back to this issue from there and include it as part of resolving that issue, but I'm going to close this one to help prune the backlog.